SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK COPOLYMERS FOR PROTON EXCHANGE MEMBRANE
FUEL CELLS (PEMFC)
by
Hang Wang
Dissertation submitted to the Faculty of the
Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY in
Macromolecular Science and Engineering
Dr. James E. McGrath, Chairman Dr. Judy Riffle
Dr. Timothy Long Dr. John G. Dillard Dr. Richey M Davis
Defense date: December 15th 2006 Blacksburg, Virginia
Keywords: Proton Exchange Membranes, Fuel Cells, Multiblock Copolymers,
Morphology, Poly(p-phenylene), Poly(2,5-benzophenone), Ni (0)-catalyzed coupling reaction, Disulfonated Poly(arylene ether sulfones)
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SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK COPOLYMERS FOR PROTON EXCHANGE MEMBRANE
FUEL CELLS (PEMFC)
by
Hang Wang
Committee Chairman: Dr. James E. McGrath
Macromolecules and Interfaces Institute
ABSTRACT
Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are
promising proton exchange membrane (PEM) materials due to their ability to form
various morphological structures which enhance transport.
Four arylene chlorides monomers (2,5-Dichlorobenzophenone and its derivatives)
were first successfully synthesized from aluminum chloride-catalyzed, Friedel-Crafts
acylation of benzene and various aromatic compounds with 2,5-dichlorobenzoyl chloride.
These monomers were then polymerized via Ni (0)-catalyzed coupling reaction to form
various high molecular weight substituted poly(2,5-benzophenone)s. Great care must be
taken to achieve anhydrous and inert conditions during the reaction.
A series of poly(2,5-benzophenone) activated aryl fluoride telechelic oligomers
with different block molecular weights were then successfully synthesized by Ni (0)-
catalyzed coupling of 2,5-dichloro-benzophenone and the end-capping agent 4-chloro-4'-
fluorobenzophenone or 4-chlorophenly-4’-fluorophenyl sulfone. The molecular weights
of these oligomers were readily controlled by altering the amount of end-capping agent.
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These telechelic oligomers (hydrophobic) were then copolymerized with phenoxide
terminated disulfonated poly (arylene ether sulfone)s (hydrophilic) by nucleophilic
aromatic substitution to form novel hydrophilic-hydrophobic multiblock copolymers.
A series of novel multiblock copolymers with number average block lengths
ranging from 3,000 to 10,000 g/mol were successfully synthesized. Two separate Tgs
were observed via DSC in the transparent multiblock copolymer films when each block
length was longer than 6,000 g/mol (6k). Tapping mode atomic force microscopy (AFM)
also showed clear nanophase separation between the hydrophilic and hydrophobic
domains and the influence of block length, as one increased from 6k to 10k. Transparent
and creasable films were solvent-cast and exhibited good proton conductivity and low
water uptake.
These PAES-PBP multiblock copolymers also showed much less relative
humidity (RH) dependence than random sulfonated aromatic copolymers BPSH 35 in
proton conductivity, with values that were almost the same as Nafion with decreasing
RHs. This phenomenon lies in the fact that this multiblock copolymer possesses a unique
co-continuous nanophase separated morphology, as confirmed by AFM and DSC data.
Since this unique co-continuous morphology (interconnected channels and networks)
dramatically facilitates the proton transport (increase the diffusion coefficient of water),
improved proton conductivity under partially hydrated conditions becomes feasible.
These multiblock copolymers are therefore considered to be very promising candidates
for high temperature proton exchange membranes in fuel cells.
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Dedicated to my parents, Peiqi Zhang and Zaisong Wang for their never-ending love, support and encouragement
v
Acknowledgements
I would like to take this opportunity to express my sincere gratitude to my
research advisor, Dr. James E. McGrath, for his encouragement, guidance and inspiration
throughout my graduate career. His knowledge, patience, and generosity toward his
students make him an exceptional advisor and person. I have exceedingly benefited from
his vast knowledge, lasting enthusiasm, and exceptional personality. I would also like to
thank the members of my advisory committee, Dr. Timothy E. Long, Dr. John G. Dillard,
Dr. Dr. Richey M Davis and Dr. Judy S. Riffle for their support.
The staffs in the Macromolecules and Interfaces Institute as well as in the
Chemistry Department have been very helpful in my success. Mrs. Laurie Good, Mrs.
Millie Ryan, and Mrs. Angie Flynn have been of tremendous assistance in all the little
details that really make the department such a great place to work.
I would like to especially acknowledge Mr. Tom Glass (NMR), Mr. Anand S.
Badami (Atom Force Microscopy), Juan Yang (GPC) and Mr. Ahbishek Roy (Proton
Conductivity Measurement) for their valuable assistance.
Many valuable discussions with my lab mates and post-doctoral researchers in the
McGrath research group contributed to my success. For this, I thank: Yanxiang Li, Hae-
seung Lee, Dr. William Harrison, Dr. Melinda Hill, Dr. Brian Einsla, Dr.Kent Wiles, Dr.
Guangyu Fang, Natalie Arnet, Xiang Yu, Rachael Hopp, Dr. Zhongbiao Zhang and Dr.
Hossein Ghassemi. I am grateful to each of them for their patience and for sharing their
knowledge with me.
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My friends, Ran Miao, Danny Hsu, Qi Guo, Yiqun Zhang and Suolong Ni have
been a very important part of my life and I hope that our friendships will continue
throughout the rest of our lives.
My best friend and fiancé, Juanjuan Han has always been there for me. I could
not have made it without her love and understanding. I would also like to thank my
entire family for their love and assistance that have aided me to this point of my life.
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SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK COPOLYMERS FOR PROTON EXCHANGE
MEMBRANE FUEL CELLS (PEMFC)
Table of Contents
Chapter 1............................................................................................................................. 1 1 Literature Review ....................................................................................................... 1 1.1 Introduction............................................................................................................. 1
1.1.1 What is a Fuel Cell?.................................................................................... 2 1.1.2 Proton Exchange Membrane Fuel Cells (PEMFC)..................................... 4 1.1.3 Proton Exchange Membranes ..................................................................... 7 1.1.4 Important Parameters .................................................................................. 9
1.1.4.1 Proton Conductivity ................................................................................ 9 1.1.4.2 Water uptake ......................................................................................... 10 1.1.4.3 Ion Exchange Capacity--IEC ................................................................ 11 1.1.4.4 Methanol Permeability.......................................................................... 12 1.1.4.5 Electro-Osmotic Drag ........................................................................... 12
1.2 Perfluorinated Copolymers for PEM .................................................................... 13 1.3 Non-Fluorinated Polymer Materials for PEM ...................................................... 18
1.3.1 Polybenzimidazole.................................................................................... 20 1.3.1.1 Acid Doping.......................................................................................... 20 1.3.1.2 Direct Sulfonation of Polymeric Backbone .......................................... 21 1.3.1.3 Synthesis from Sulfonated Monomer ................................................... 22 1.3.1.4 Grafting of a Functional Group............................................................. 22
1.3.2 Poly(phenylquinoxalines) ......................................................................... 23 1.3.3 Poly(phenylene oxides)............................................................................. 24 1.3.4 Poly(phenylene)s and Derivatives ............................................................ 26 1.3.5 Poly(phenylene sulfide) ............................................................................ 28 1.3.6 Poly(arylene ether sulfones) ..................................................................... 29
1.3.6.1 Post Sulfonated Copolymers................................................................. 30 1.3.6.2 Synthesis of copolymers from sulfonated monomers........................... 33 1.3.6.3 Processing and Morphological Issues................................................... 37
1.3.7 Poly(arylene ether ketone) Copolymers.................................................... 39 1.3.8 Poly(phosphazene)s .................................................................................. 45
1.3.8.1 Post Sulfonated Copolymers................................................................. 45 1.3.9 Polyimides................................................................................................. 46
1.4 Block Copolymers for PEM.................................................................................. 51 1.4.1 Copolymer Architecture............................................................................ 52 1.4.2 Nanophase Separation in Block Copolymers ........................................... 53 1.4.3 Di-Block and Tri-Block Copolymers for PEM......................................... 56 1.4.4 Multiblock copolymers for PEM .............................................................. 60
1.4.4.1 Poly(arylene ether sulfone) Multiblock Copolymers............................ 60 1.4.4.2 Polyimide Multiblock Copolymers....................................................... 66
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1.5 Multiblock Copolymers Based on Substituted Poly(phenylene)s ............................ 69 1.5.1 Ni (0) -Catalyzed Coupling Polymerization Reactions ............................... 70 1.5.2 Poly(p-phenylene) (PPP) and Derivatives ................................................... 72 1.5.3 Multiblock Copolymers Based on Substituted Poly(p-phenylene)s ............ 76
Chapter 2........................................................................................................................... 81 2 EXPERIMENTAL................................................................................................ 81 2.1 Solvent Purification .............................................................................................. 81
2.1.1 N,N-Dimethylacetamide (DMAc) ............................................................ 81 2.1.2 N-Methyl-2-Pyrrolidone or 1-Methyl-2-Pyrrolidone (NMP) ................... 82 2.1.3 Fuming Sulfuric Acid ............................................................................... 82 2.1.4 Toluene ..................................................................................................... 83 2.1.5 Ethanol or Ethyl Alcohol .......................................................................... 83 2.1.6 Methanol or Methyl Alcohol .................................................................... 84 2.1.7 Isopropyl Alcohol or 2-Propanol or Isopropanol...................................... 84 2.1.8 Tetrahydrofuran (THF) ............................................................................. 85 2.1.9 Hydrochloride Acid (HCl) ........................................................................ 85 2.1.10 Benzene..................................................................................................... 85 2.1.11 Cyclohexane.............................................................................................. 86 2.1.12 Acetic Anhydride ...................................................................................... 86
2.2 Reagents and Purification of Monomers............................................................... 87 2.2.1 2,2’-bis(4-hydroxyphenol)propane (Bisphenol A) or (Bis A) .................. 87 2.2.2 4,4’-Biphenol (BP).................................................................................... 87 2.2.3 4,4'-(hexafluoroisopropylidene) diphenol (6F-Bisphenol A) ................... 88 2.2.4 4,4’-Dichlorodiphenyl sulfone (DCDPS) ................................................. 89 2.2.5 Potassium Carbonate (Anhydrous) ........................................................... 89 2.2.6 Chlorosulfonic acid................................................................................... 90 2.2.7 Chlorotrimethylsilane ............................................................................... 90 2.2.8 2,2’-Bipyridyl (bipy)................................................................................. 91 2.2.9 Triphenylphosphine (PPh3) ...................................................................... 91 2.2.10 Dichlorobis(triphenylphosphine)nickel-(II).............................................. 92 2.2.11 Nickel(II) Chloride (NiCl2) ...................................................................... 92 2.2.12 Zn Powder................................................................................................. 92 2.2.13 2,5-dichlorobenzoic acid........................................................................... 93 2.2.14 4-Fluorobenzoyl chloride.......................................................................... 93 2.2.15 4-chlorophenly-4’-fluorophenyl sulfone................................................... 94 2.2.16 Thionyl chloride (SOCl2).......................................................................... 94 2.2.17 Chlorobenzene .......................................................................................... 95 2.2.18 Fluorobenzene........................................................................................... 95 2.2.19 Biphenyl.................................................................................................... 96 2.2.20 Diphenyl ether........................................................................................... 96 2.2.21 Aluminum Chloride (AlCl3) ..................................................................... 97
2.3 Monomer Synthesis .............................................................................................. 98 2.3.1 2,5-Dichlorobenzophenone....................................................................... 98 2.3.2 2,5-Dichloro-4’-fluorobenzophenone ..................................................... 100 2.3.3 2,5-Dichloro-4’-phenylbenzophenone .................................................... 101 2.3.4 2,5-Dichloro-4’-oxyphenylbenzophenone.............................................. 102
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2.3.5 4-Chloro-4'-fluorobenzophenone............................................................ 103 2.3.6 Sodium Salt of 3,3’-Disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS) 105
2.4 Synthesis of Homopolymers from Substituted Poly(phenylene)s via Ni(0)-catalyzed Coupling Reaction .......................................................................................................... 107
2.4.1 Poly(2,5-benzophenone) ......................................................................... 107 2.4.2 Poly(4’-fluoro-2,5-benzophenone) ......................................................... 107 2.4.3 Poly(4’-phenyl-2,5-benzophenone) ........................................................ 108 2.4.4 Poly(4’-oxyphenyl-2,5-benzophenone) .................................................. 109
2.5 Telechelic Oligomer Synthesis ............................................................................... 110 2.5.1 Synthesis of Telechelic Poly(2,5-benzophenone) (PBP) Oligomers ...... 110 2.5.2 Synthesis of Telechelic Poly(4’-oxyphenyl-2,5-benzophenone) (PPBP) Oligomers................................................................................................................ 110 2.5.3 Synthesis of Telechelic Disulfonated Poly(arylene ether sulfone) (PAES) Oligomers................................................................................................................ 111
2.6 Synthesis of Multiblock Copolymers...................................................................... 112 2.6.1 Synthesis of PAES-PBP Multiblock Copolymers .................................. 112 2.6.2 Synthesis of PPAES-PBP Multiblock Copolymers ................................ 112
2.7 Characterization ...................................................................................................... 113 2.7.1 Nuclear Magnetic Resonance (NMR) Spectroscopy .............................. 113 2.7.2 Fourier Transform Infrared (FTIR) Spectroscopy .................................. 113 2.7.3 Gel Permeation Chromatography (GPC) ................................................ 114 2.7.4 Intrinsic Viscosity Determinations ([ ]) ............................................... 114 2.7.5 Thermogravimetric Analysis (TGA)....................................................... 115 2.7.6 Differential Scanning Calorimetry (DSC) .............................................. 115 2.7.7 Elemental Analysis ................................................................................. 116 2.7.8 Non-Aqueous Potentiometric Titration................................................... 116 2.7.9 Film Preparation...................................................................................... 117 2.7.10 Water Uptake .......................................................................................... 117 2.7.11 Conductivity Measurements ................................................................... 118 2.7.12 Ion Exchange Capacity (IEC) ................................................................. 119 2.7.13 UV-visible Absorption Spectroscopy (UV-vis)...................................... 119 2.7.14 Atomic Force Microscopy (AFM).......................................................... 121
Chapter 3......................................................................................................................... 122 3 Results and Discussion ........................................................................................... 122 3.1 Introduction............................................................................................................. 122 3.2 Synthesis of Substituted Poly(p-phenylene)s Homopolymers................................ 126
3.2.1 Monomer Synthesis ................................................................................ 128 3.2.2 Polymer Synthesis................................................................................... 131 3.2.3 Polymer Characterization........................................................................ 135
3.2.3.1 1H and 13C NMR Spectroscopy......................................................... 136 3.2.3.2 Fourier Transform Infrared Spectroscopy (FTIR) .............................. 138 3.2.3.3 Intrinsic Viscosity [η] (IV) ................................................................. 139 3.2.3.4 Gel Permeation Chromatography (GPC) ............................................ 141 3.2.3.5 Differential Scanning Calorimetry (DSC) .......................................... 142 3.2.3.6 Thermogravimetric Analysis (TGA)................................................... 143
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3.2.3.7 Ultraviolet-visible Spectroscopy (UV/ VIS)....................................... 145 3.3 Disulfonated Poly (arylene ether sulfone) Copolymers (BPSH series) .................. 148
3.3.1 Synthesis of Disulfonated Poly (arylene ether sulfone) Copolymers (BPSH) 148 3.3.2 Characterization of BPSH copolymers ................................................... 150
3.4 Multiblock Copolymers .......................................................................................... 155 3.4.1 Synthesis of poly (2,5-benzophenone) (PBP) telechelic oligomers........ 156 3.4.2 Synthesis of disulfonated poly(arylene ether sulfone) (PAES) telechelic oligomers................................................................................................................. 166 3.4.3 Synthesis of PAES-PBP Multiblock Copolymers .................................. 172 3.4.4 Characterization of Multiblock Copolymers .......................................... 173
3.4.4.1 1H and 13C NMR Spectroscopy......................................................... 173 3.4.4.2 Fourier Transform Infrared Spectroscopy (FT-IR)............................. 176 3.4.4.3 Gel Permeation Chromatography (GPC) and Intrinsic Viscosity [η] (IV) 178 3.4.4.4 Differential Scanning Calorimetry (DSC) .......................................... 181 3.4.4.5 Thermogravimetric Analysis (TGA)................................................... 183 3.4.4.6 Atomic force microscopy (AFM) ....................................................... 186 3.4.4.7 X-ray Photoelectron Spectroscopy (XPS) .......................................... 188 3.4.4.8 Proton conductivity and water uptake................................................. 189
Chapter 4......................................................................................................................... 195 4 Multiblock Copolymers of Poly (2,5-benzophenone) and Disulfonated Poly (arylene ether sulfone) for Proton Exchange Membranes. I. Synthesis and Characterization………………………………………………………………………...195 Abstract………………………………………………………...……………………….196 INTRODUCTION .......................................................................................................... 197 EXPERIMENTAL.......................................................................................................... 200
Materials. ................................................................................................................ 200 Characterization ...................................................................................................... 200 Water Uptake, Ion Exchange Capacity and Proton Conductivity........................... 202 Monomer synthesis ................................................................................................. 203 Synthesis of telechelic poly(2,5-benzophenone) (PBP) oligomers......................... 203 Synthesis of PAES telechelic oligomers and PAES-PBP multiblock copolymers. 204 Membrane preparation ............................................................................................ 205
RESULTS AND DISCUSSION………………………………………………………..206 Synthesis of Monomers and Oligomers.................................................................. 206 Synthesis of PAES-PBP Multiblock Copolymers .................................................. 214 Morphology............................................................................................................. 216 Ion exchange capacity, Water uptake and Proton conductivity.............................. 218
CONCLUSIONS............................................................................................................. 220 ACKNOWLEDGEMENTS............................................................................................ 220 Chapter 5......................................................................................................................... 221 5 Summary and Conclusions ................................................................................. 221 Chapter 6......................................................................................................................... 224 6 Suggested Future Research ................................................................................. 224 Reference: ....................................................................................................................... 227
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Table of Figures
Figure 1 Membrane electrode assembly7............................................................................ 6 Figure 2 Fuel cell stack made up of flow field plates (or bipolar plates) and MEAs
(shown in the insert) 7 ................................................................................................. 7 Figure 3Typical cell geometry for an in-plane proton conductivity measurement........... 10 Figure 4 Chemical Structure of Nafion ......................................................................... 14 Figure 5 Conductivity of Nafion 117 membranes at the indicated relative humidities for
temperature increasing from 80 to 160oC 37. ............................................................ 16 Figure 6 Development of proton-conducting membranes by the modification of
thermostable polymers.41 .......................................................................................... 19 Figure 7 Synthesis route to sulfonated (stabilized) polybenzimidazole46......................... 20 Figure 8 Synthesis route to benzylsulfonated polybenzimidazole via polyanion
formation68 ................................................................................................................ 23 Figure 9 Sulfonated poly(phenyl quinoxaline) ................................................................. 24 Figure 10 (a) Sulfonated poly(2,6-disubstituted-1,4-phenylene oxide); (b) reaction
scheme for the sulfonation of poly(3–bromo-2,6-diphenyl-4-phenylene oxide)...... 25 Figure 11 Chemical structures of PEEK, PPBP, and their sulfonated polymers76 ........... 27 Figure 12 Diels-Alder Polymerization of 1,4-Bis(2,4,5-
triphenylcyclopentadienone)benzene and di(ethynyl)benzene78 .............................. 27 Figure 13 Synthesis of poly(phenylenesulfide sulfonic acid) via a poly(sulfonium)
cation.81 ..................................................................................................................... 29 Figure 14 (a) Poly(arylene ether sulfone); (b) Sulfonic Acid Group in Post-Sulfonation
(Activated Ring); (c) Sulfonic Acid Group in Direct Copolymerization (Deactivated Ring) ......................................................................................................................... 30
Figure 15 Metalation Route to Sulfonated Polysulfones85 ............................................... 32 Figure 16 Chemical Structure of Sulfophenylated Polysulfones86 ................................... 33 Figure 17 Direct polymerization synthesis of random sulfonated poly(arylene ether
sulfone) copolymers 89. ............................................................................................. 34 Figure 18 Atomic Force Micrographs of BPSH-40 and Nafion 117 ................................ 35 Figure 19 Influence of the degree of sulfonation on the water uptake of sulfonated
poly(arylene ether sulfone) copolymers.95................................................................ 36 Figure 20 TM-AFM phase image of BPSH-45 in (a) Regime1, (b) Regime2, and (c)
Regime3; Hydro-thermal treatment temperatures of (a), (b), and (c) were 30, 80, and 120 oC, respectively, phase angle: 30o Scan size: 500 nm; darker features represent the hydrophilic phase while lighter features represent the hydrophobic phase102 .... 38
Figure 21 Structures of representative membranes of the poly(ether ketone) family....... 40 Figure 22 (a) Conductivity, at 100oC, as a function of relative humidity for Nafion 117
and S-PEEK-2.48 membranes. The conductivity of g-zirconium sulfophenylphosphonate is reported for comparison; (b) Conductivity of Nafion 117 and S-PEEK-2.48 at 75% r.h. for temperature increasing from 80 to 160oC. The conductivity of S-PEEK-1.6 is reported for comparison. 37 .................................... 41
Figure 23 Schematic representation of the microstructures of NAFION and a sulfonated polyetherketone, illustrating the less pronounced hydrophobic/hydrophilic separation of the latter. 109......................................................................................... 42
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Figure 24 Synthesis of sulfonated poly(aryl ether ether ketone ketone)s111 ..................... 43 Figure 25 Synthesis of highly fluorinated poly(arylene ether) copolymers containing
sulfonic acid groups .................................................................................................. 44 Figure 26 The repeat unit of poly[bis(3-methylphenoxy)phosphazene].......................... 46 Figure 27 Structures of phthalic model A and naphthalenic model B imides.124 ............. 47 Figure 28 Synthesis of sequenced sulfonated copolyimides124......................................... 49 Figure 29 Naphthalenic sulfonated polyimides ............................................................... 50 Figure 30 Illustration of copolymer architectures: III: di-block; IV: tri-block; V: multi-
block; VI: graded block; VII: four arm star block. ................................................... 52 Figure 31 Morphological phase diagram for PS-PI diblock copolymers ......................... 55 Figure 32 Preparation of Fluorine-Containing Block Copolymers by Chain Transfer
Emulsion Polymerization and ATRP 155................................................................... 58 Figure 33 TEM of partially sulfonated P(VDF-co-HFP)-b-PS block copolymer (IEC =
0.89 meq/g) ............................................................................................................... 58 Figure 34 Chemical Structure of Sulfonated SEBS Block Copolymer ............................ 59 Figure 35 Synthetic scheme of Sulfonated–fluorinated poly(arylene ether) multiblocks 61 Figure 36 AFM taping mode phase images of a Sulfonated–fluorinated poly(arylene ether)
multiblock ................................................................................................................. 62 Figure 37 Synthesis of SPSF-b-PVDF.............................................................................. 63 Figure 38 Dependence of proton conductivity on IEC for SPSF (diamond) and SPSF-b-
PVDF (square) membranes. (30 oC, 95% relative humidity).................................... 64 Figure 39 TEM micrographs of polymer membranes. (a): SPSF (IEC: 1.55); (b): SPSF-b-
PVDF (IEC:1.62); (c): SPFSF (IEC: 0.83); (d): SPSF-b-PVDF (IEC: 0.78) ........... 65 Figure 40 Synthesis of sequenced sulfonated naphthalenic polyimide block copolymers
.................................................................................................................................. .67 Figure 41 Ionic conductivity vs. ionic block length for BDSA/NTDA/ODA copolymers
with X/Y = 30/70. ..................................................................................................... 68 Figure 42 Reaction Mechanism Proposed by Colon for the Ni(0)-Catalyzed
Polymerization of Arylene Chlorides ....................................................................... 70 Figure 43 Alkyl-substituted Poly(2,5-benzophenone)s .................................................... 73 Figure 44 Structures of 3-benzoyl-2,5-dichlorothiophene (M1), 2,5-dichloro-3-(2’-
thiophenecarbonyl)thiophene (M2), 3-benzenesulfonyl-2,5-dichlorothiophene (M3), 2,5-dichlorobenzophenone (M4), and 2-benzenesulfonyl-1,4-dichlorobenzene (M5).................................................................................................................................... 76
Figure 45 Synthesis of poly(4’-methyl-2,5-benzophenone) telechelics. .......................... 77 Figure 46 Synthesis of Hydrophilic-Hydrophobic Multiblock copolymers164 ................. 79 Figure 47 Synthesis scheme of substituted 2,5-dichlorobenzophenone monomers ......... 99 Figure 48 Synthesis scheme for 4-Chloro-4'-fluorobenzophenone ................................ 104 Figure 49 Synthesis scheme for 3,3’-Disulfonated-4,4’-dichlorodiphenyl sulfone
(SDCDPS) and its salt form.................................................................................... 106 Figure 50 Schematic of a four-point membrane proton conductivity cell. ..................... 119 Figure 51 Chemical structure of disulfonated poly(arylene ether sulfone) copolymers
where the letter x in abbreviation refers to the sulfonation percentage of a disulfonated monomer. ........................................................................................... 123
Figure 52 Synthesis of substituted 2,5-Dichlorobenzophenones by Friedel-Crafts Reactions................................................................................................................. 129
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Figure 53 Proton NMR spectrum of 2,5-dichlorobenzophenone.................................... 130 Figure 54 Proton NMR spectrum of 2,5-dichloro-4’-phenyl-benzophenone ................. 130 Figure 55 Proton NMR spectrum of 2,5-dichloro-4’-oxyphenyl-benzophenone ........... 131 Figure 56 Synthesis of substituted poly(2,5-benzophenone)s by Ni (0)-catalyzed coupling
reaction.................................................................................................................... 134 Figure 57 Proton NMR of Poly (2,5-benzophenone) in CDCl3...................................... 135 Figure 58 13C NMR of Poly (2,5-benzophenone) in CDCl3 ........................................... 136 Figure 59 Poly (2,5-benzophenone)s structures.............................................................. 137 Figure 60 FTIR spectrum of poly (2,5-benzophenone)s................................................. 138 Figure 61 Substituted poly (2,5-benzophenone)s P1-P4 structures ................................ 141 Figure 62 TGA thermogram for poly(2,5-benzophenenone) in N2 ................................ 145 Figure 63 UV-vis spectrum of poly(2,5-benzophenone) in CHCl3 ................................ 147 Figure 64 Synthesis of “BPSH” series random copolymers via direct copolymerization
................................................................................................................................. 149 Figure 65 Proton NMR spectrum of BPSH 35 in d6-DMSO.......................................... 151 Figure 66 Synthetic scheme of fluorine terminated poly(2,5-benzophenone) oligomers.
................................................................................................................................. 157 Figure 67 13C NMR spectrum of a fluorine terminated poly(2,5-benzophenone) oligomer
in CDCl3 ................................................................................................................. 158 Figure 68 19 F NMR of a fluorine terminated poly(2,5-benzophenone) oligomer in CDCl3
................................................................................................................................. 159 Figure 69 Double logarithmic plot of [η] against Mn of fluorine terminated poly(2,5-
benzophenone) oligomers ....................................................................................... 160 Figure 70 UV-vis spectra of a series of poly(2,5-benzophenone) oligomers ................. 162 Figure 71 Synthetic scheme of fluorophenyl sulfone terminated PBP oligomers .......... 164 Figure 72 19 F NMR of a fluorophenyl sulfone-terminated and fluorophenyl ketone-
terminated PBP telechelic oligomers in CDCl3 ...................................................... 165 Figure 73 Proton NMR of a fluorophenyl sulfone-terminated PBP oligomer with target
Mn of 6,000 g/mol .................................................................................................. 166 Figure 74 Synthetic scheme for hydroxy terminated disulfonated poly(arylene ether
sulfone) oligomers .................................................................................................. 167 Figure 75 2D-COSY spectrum (COrrelation SpectroscopY) of a phenol terminated
disulfonated poly(arylene ether sulfone) oligomer ................................................. 168 Figure 76 Proton NMR spectrum of a phenol terminated disulfonated poly(arylene ether
sulfone) oligomer .................................................................................................... 169 Figure 77 13 C NMR spectrum of a phenol terminated disulfonated poly(arylene ether
sulfone) oligomer .................................................................................................... 170 Figure 78 Synthetic scheme of a PAES-PBP hydrophobic-hydrophilic multiblock
copolymer ............................................................................................................... 172 Figure 79 Proton NMR of PAES-PBP hydrophobic-hydrophilic multiblock copolymer
with block lengths of approximately 6000 g/mol ................................................... 174 Figure 80 13C NMR spectra of a PAES-PBP multiblock copolymer (a), a PBP oligomer
(b) and a PAES oligomer (c)................................................................................... 175 Figure 81 FT-IR spectrum of a PAES-PBP multiblock copolymer................................ 177 Figure 82 Comparison of neutral and ion-containing polymers with different ion
concentration........................................................................................................... 179
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Figure 83 Effect of ion strength on the shape of a polyelectrolyte molecular in solution................................................................................................................................. 180
Figure 84 DSC thermogram of PAES-PBP hydrophobic-hydrophilic multiblock copolymer with block lengths of approximately 6000 g/mol showing two Tgs………………………………………………………………………….……...182
Figure 85 TGA thermogram of a PAES-PBP multiblock copolymer, a BPS 40 random copolymer and a PBP homopolymer………………………………….…………..185
Figure 86 Tapping mode AFM phase images of (a) a PAES-6K-PBP-6K multiblock copolymer, and (b) a PAES-10K-PBP-10K multiblock copolymer with block lengths of approximately 6000 and 10,000 g/mol, respectively.(Scan sizes are 500 nm; Z ranges for phase images are 10 o)………………………………..…………188
Figure 87 Comparison of water uptake of BPSH 35, Nafion, and PAES-PBP multiblock copolymers……………………………………………………………..………….191
Figure 88 Influence of relative humidity on proton conductivity for Nafion 117, BPSH 35 and multiblock copolymer at 80 oC……………………………………………….193
Figure 89 13C NMR spectrum of a fluorine terminated poly(2,5-benzophenone) oligomer in CDCl3. ……………………………………………………………..…………..208
Figure 90 Double logarithmic plot of [η] against Mn of fluorine terminated poly(2,5-benzophenone) oligomers. ………………………………………………………..210
Figure 91 Proton NMR spectrum of a phenol terminated disulfonated poly(arylene ether sulfone) oligomer. ……………………………………………………………..….213
Figure 92 DSC thermogram of PPP-PAES hydrophobic-hydrophilic multiblock copolymer with block lengths of approximately 6000 g/mol showing two Tgs…..217
Figure 93 Tapping mode AFM phase images of (a) a PAES-6K-PBP-6K multiblock copolymer, and (b) a PAES-10K-PBP-10K multiblock copolymer with block lengths of approximately 6000 and 10,000 g/mol, respectively.(Scan sizes are 500 nm; Z ranges for phase images are 10 o) . …………………………………………218
Figure 94 Synthetic scheme of PAES-PBP segmented copolymers……………………225
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Table of Schemes
Scheme 1 Synthesis of fluorine terminated poly(2,5-benzophenone) oligomers………207 Scheme 2 Synthesis of hydroxy terminated disulfonated poly(arylene ether sulfone)
oligomers………………………………………………………………………….212 Scheme 3 Synthesis of a PBP-PAES hydrophobic-hydrophilic multiblock
copolymer…………………………………………………………………………215
xvi
Table of Tables
Table 1 Properties of substituted poly(2,5-benzophenone)s……………………………140 Table 2 Degree of sulfonations and intrinsic viscosities of BPSH series of disulfonated
copolymers………………………………………………………………………...152 Table 3 Influence of degree of disulfonation on several properties of BPSH
copolymers..……………………………………………………………………….153 Table 4 Characterization of fluorine terminated poly(2,5-benzophenone)
oligomers………………………………………………………………………….161 Table 5 Summary of UV-vis λmax of a series of poly(2,5-benzophenone)
oligomers………………………………………………………………………….163 Table 6 Properties of hydroxy terminated poly(arylene ether sulfone) oligomers……..171 Table 7 Molecular Weight Data of PAES-PBP Multiblock Copolymers……………....181 Table 8 DSC and TGA results for PAES-PBP multiblock copolymers………………..184 Table 9 XPS data of a PAES-PBP multiblock copolymer……………………………..189 Table 10 Characterization of PPP-PAES hydrophobic-hydrophilic multiblock
copolymers………………………………………………………………………...192 Table 11 Characterization of fluorine terminated poly(2,5-benzophenone)
oligomers………………………………………………………………………….211 Table 12 Properties of hydroxy terminated poly(arylene ether sulfone) oligomers……213 Table 13 Characterization of PPP-PAES hydrophobic-hydrophilic multiblock
copolymer…………………………………………………………………………215
1
Chapter 1
1 Literature Review
1.1 Introduction
The whole world is facing a rapid depletion of natural resources and serious
global environmental problems.1 Renewable and environmentally friendly energy
sources will be essential for an ever-changing and populous planet. Solar, hydro, and
wind power systems have been employed to complement current electric power sources.
Arguably, the most attractive alternative energy sources are fuel cells. Hydrogen may be
used to produce energy by combining with oxygen in a fuel cell. Hydrogen is the
cleanest, most sustainable and renewable energy carrier.
Fuel cells are viable, renewable, and environmentally friendly energy sources
since the only byproduct is water. 2-5 The recent success of fuel-cell-powered
demonstration vehicles using the proton exchange membrane fuel cell (PEMFC)
developed by Ballard Power Systems, DaimlerChrysler, GM, Nissan and many others,
suggests that fuel cells are beginning finally to come of age.
This literature review will focus on the synthesis of polymeric materials that have
attached ion-conducting groups for proton exchange membranes. First of all, fuel cell
and the basics of proton exchange membrane will be introduced. Then, state-of-the-art
Nafion® and its commercially available perfluorosulfonic acid relatives will be discussed.
The large amount of recent literature dealing with non-fluorinated polymer materials for
PEMFC will be reviewed. Finally, synthetic strategies for obtaining block or multiblock
2
copolymers with controlled molecular structure, which afford nanophase separation of
ionic and hydrophobic domains, will be reviewed. This research thesis is directed
towards whether multiblock copolymers for proton exchange membrane can generate
superior membranes.
1.1.1 What is a Fuel Cell?
As early as 1839, William Grove discovered the basic operating principle of fuel
cells by reversing water electrolysis to generate electricity from hydrogen and oxygen.6
The principle that he discovered remains unchanged today.
A fuel cell is an electrochemical “device” that continuously converts chemical
energy into electric energy (and some heat) for as long as fuel and oxidant are supplied.
Fuel cells therefore bear similarities both to batteries, with which they share the
electrochemical nature of the power generation process, and to engines which — unlike
batteries — will work continuously consuming a fuel of some sort.
Unlike engines or batteries, a fuel cell does not need recharging, it operates
quietly and efficiently, and — when hydrogen is used as fuel — it generates only power
and water. Thus, it is a so-called zero emission engine. Thermodynamically, the most
striking difference is that thermal engines are limited by the Carnot efficiency while fuel
cells are not.
Fuel cells are expected to eventually come into widespread commercial use
through three main applications: transportation, stationary power generation, and portable
applications. Fuel cells are in fact rather different for each of these three sectors.
3
In the transportation sector, fuel cells are probably the most serious contenders to
compete with internal combustion engines (ICEs). They are highly efficient because they
are electrochemical rather than thermal engines. Hence, they can help to reduce the
consumption of primary energy and the emission of CO2. What makes hydrogen fuel
cells most attractive for transport applications is the fact that they emit zero or ultralow
emissions. This is what mainly inspired automotive companies and other fuel cell
developers in the 1980s and 1990s to start developing fuel-cell-powered cars and buses.
Stationary power generation is viewed as the leading market for fuel cell
technology other than buses. The reduction of CO2 emissions is an important argument
for the use of fuel cells in small stationary power systems, particularly in combined heat
and power generation (CHP). In fact, fuel cells are currently the only practical engines
for micro-CHP systems in the domestic environment (5–10 kW). The higher capital
investment for a CHP system would be offset against savings in domestic energy supplies
and — in more remote locations — against power distribution cost and complexity. In the
50- to 500-kW range, CHP systems will have to compete with spark or compression
ignition engines modified to run on natural gas. So far, several hundred 200-kW
phosphoric acid fuel cell plants manufactured by UTC now have UTC fuel cells and have
been installed worldwide.
The portable market is less well defined, but Plug Power also sells stationary units
for quiet fuel cell power generation may be in the 1-kW portable range and possibly, as
ancillary supply in cars, so-called auxiliary power units (APUs). They are much lighter
and have higher power density than batteries. The term “portable fuel cells” often
includes grid-independent applications such as camping, yachting, and traffic monitoring.
4
In addition, the choice of fuel is not the only way in which these applications vary.
Different fuel cells based on H2 or methanol may be needed for each sub-sector in the
portable market.
1.1.2 Proton Exchange Membrane Fuel Cells (PEMFC)
Principally, five fuel cell systems are considered to be significant, each with its
distinct electrochemical reaction and operation requirements. They are generally
classified by the electrolyte used: phosphoric acid fuel cells (PAFC), molten carbonate
fuel cells (MCFC), solid oxide fuel cells (SOFC), alkaline fuel cells (AFC), and proton
exchange membrane fuel cells (PEMFC). The proton exchange membrane fuel cell,
PEMFC, stands out because of its simplicity and high power density, which make it the
only fuel cell type currently being considered for powering passenger cars. The PEMFC
is also being developed for stationary and portable power generation.
The smallest building block of a PEM fuel cell is the membrane electrode
assembly or MEA, as shown in Figure 1. Consisting of two electrodes, anode and
cathode, and the polymer electrolyte, it essentially forms the basic fuel cell. Thin gas-
porous electrodes featuring noble metal (usually Pt and its alloy) layers (several microns
to several tens of microns) on either side of the membrane contain all the necessary
electrocatalysis, which drives the electrochemical power generation process.
5
The electrochemical reactions of proton exchange membrane fuel cells using
hydrogen and oxygen gas are:
Anode: H2 → 2H+ + 2e- Equation 1 Cathode: ½ O2 + 2H+ + 2e- → H2O Equation 2
Overall: H2 + ½ O2 → H2O Equation 3
Theory predicts a maximum of 1.23 voltage could be generated per single fuel
cell.6 The gas diffusion layer or electrode substrate (or electrode backing material) at the
anode allows hydrogen to reach the reactive zone within the electrode. Upon reacting,
hydrated protons migrate through the ion conducting membrane, and electrons are
conducted through the substrate layer and, ultimately, to the electric terminals of the
multi fuel cell stack. The anode substrate therefore has to be gas porous as well as
electronically conducting. Not all of the chemical energy supplied to the MEA by the
reactants is converted into electric power and heat will also be generated somewhere
inside the MEA. Hence, the gas porous substrate also acts as a heat conductor to remove
heat from the reactive zones of the MEA.
At the cathode, the functions of the substrate become even more complex.
Product water is formed at the cathode. Should this water exit from the electrode in
liquid form (as it usually does if the reactants are saturated with water vapor), there is a
risk of liquid blocking the pores within the substrate and, consequently, gas access to the
reactive zone, which produces flooding. This poses a serious performance problem
because for economic reasons the oxidant used in most practical applications is not pure
oxygen but air. Therefore, 80% of the gas present within the cathode is inert — with all
6
associated boundary and stagnant layer problems. Fuel cell operation will therefore result
in a depletion of oxygen towards the active cathode catalyst.
Figure 1 Membrane Electrode Assembly7
The membrane acts as a proton conductor. This requires the membrane to be well
humidified because the proton conduction process relies on the hydration of the
membrane for transport. As a consequence, an additional water flux from anode to
cathode is present and is associated with the migration of protons (electro-osmotic drag).
Since this will eventually lead to a depletion of water from the anode interface of the
membrane, humidity is often provided with the anode gas by pre-humidifying the
reactant.
7
Figure 2 Fuel cell stack made up of flow field plates (or bipolar plates) and MEAs (shown in the
insert) 7
The MEA is typically located between a pair of current collector plates with
machined flow fields for distributing fuel and oxidant to the anode and cathode,
respectively, as shown in Figure 2. A water jacket for cooling is often placed at the back
of each reactant flow field followed by currently a metallic current collector plate. The
cell can also contain a humidification section for the reactant gases, which are kept close
to their saturation level in order to prevent dehydration of the membrane electrolyte.
1.1.3 Proton Exchange Membranes
The key component in a proton exchange membrane fuel cell (PEMFC), the
proton exchange membrane (PEM), performs two basic, essential functions in PEMFCs:
(1) a separator to prevent mixing of the fuel (i.e., hydrogen gas, methanol, etc.) and the
8
oxidant (i.e., pure oxygen or air), and (2) an electrolyte for transporting protons from the
anode to the cathode 8.
Desirable properties of the proton exchange membrane in fuel cells include 9:
(1) High proton conductivity;
(2) Low electronic conductivity;
(3) Low permeability to fuel and oxidants;
(4) Low water transport through diffusion and electro-osmosis;
(5) Oxidative and hydrolytic stability;
(6) Low swelling stresses;
(7) Good mechanical properties both in the dry and hydrated state;
(8) Affordable cost;
(9) Capability of fabrication into membrane electrode assemblies (MEAs).
In addition, proton exchange membranes must be fabricated into a membrane
electrode assembly (MEA) as outlined in Figure 1. Therefore, the ease of membrane
electrode assembly fabrication and the resulting properties of the MEA must be
considered. Current work in the area of fabricating MEAs from novel polymeric
membranes has focused on the electrode-membrane interface and the problems of having
dissimilar ion conducting copolymers in the membrane and the electrode.10, 11 Moreover,
ion conducting polymers that are compatible for use in the catalyst layer, in concert with
novel polymer membranes are also an emerging area of research.
9
1.1.4 Important Parameters
Several important parameters need to be defined before beginning detailed
discussion of various proton exchange membrane candidates. Those parameters are
extremely useful when it comes to comparison between different polymeric materials.
1.1.4.1 Proton Conductivity
How does one measure proton conductivity? Scientists at Los Alamos National
Labs (LANL) have devised a facile method for determining the conductivity of proton
exchange membranes using electrochemical impedance spectroscopy and a simple cell
that allows equilibration in a variety of environments.12 This method measures protonic
conductivity in the plane of the membrane as opposed to through the plane (as in a fuel
cell), and thus works well as an initial screening test. Through-plane conductivity
measurements13-15 are considered more difficult experimentally than in-plane
measurements because the measured membrane resistances are small in this geometry
and interfacial resistances may play a very significant role.
Figure 3 Typical cell geometry for an in-plane proton conductivity measurement.
10
While the through-plane technique is more analogous to actual proton conduction
in a fuel cell, the resistance across a thin membrane is usually too small to be measured
accurately. For this reason, the in-plane technique is more commonly used. A typical cell
for in-plane proton conductivity measurements is shown in Figure 3. In this case, the cell
geometry was chosen to minimize the resistance of the cell so that the measured
resistance was due primarily to the membrane resistance.16
1.1.4.2 Water uptake
Water uptake is another important parameter in determining the ultimate
performance of PEM materials. For most current conducting polymeric materials, water
is needed as the mobile phase to facilitate proton conductivity. Absorbed water also
affects the mechanical properties of the membrane by acting partially as a plasticizer,
which may lower the Tg and modulus of the membrane. Also, swelling and degradation
of the mechanical properties of the membrane may become serious problems in humid
environments or during cyclic operations. Water uptake is usually reported as a mass
fraction, mass percent, or a lambda value, where lambda (λ) equals the number of water
molecules absorbed per acid site.
There has been extensive research on types of water in the area of hydrogel, and
the three types of water are generally described as nonfreezing water (Tg ~ 130 oC17, 18),
freezable loosely bound water, and free water. 18 Nonfreezing water is strongly
associated with the acid sites on the polymer chain, and causes depression of the polymer
Tg (plasticization), but does not participate in the melting at 0 °C. Freezable loosely
bound water is loosely associated with the copolymer. Freezable loosely bound water has
11
a broad melting centered around 0 °C. Free water is not associated with the polymer
chain, and behaves like bulk water. It shows a sharp melting endothermic at 0 °C in DSC.
1.1.4.3 Ion Exchange Capacity--IEC
Both conductivity and water uptake rely heavily on the concentration of ion
conducting units in the polymer membrane. The ion content is characterized by the
molar equivalents of ion conductor per mass of dry membrane and is expressed as
equivalent weight (EW) with units of grams of polymer per equivalent or ion exchange
capacity (IEC) with units of milli-equivalents per gram (meq/g or mmol/g) of polymer
(EW = 1000/IEC). (Usually expressed on a weight basis)
IEC is a very important property because it allows for a more accurate
comparison of materials than does the degree of sulfonation. IEC can be determined
from NMR by integrating known signals due to the sulfonated and non-sulfonated repeat
units. However, titration of the acidic groups can often be a more precise method of
determining IEC. Titration of sulfonic acid groups is typically accomplished by one of
two methods. The first is a potentiometric titration of the polymer in an organic solvent,
such as dimethyl acetamide, with a soluble base, such as tetramethyl ammonium
hydroxide. When this method is used, a “blank” titration of the solvent should be
performed. The second common method is aqueous titration with sodium hydroxide. To
use this method, the acidic sites on the polymer must first be dissociated using a weak
base such as sodium sulfate. This method can sometimes result in lower-than-theoretical
IEC values, possibly due to incomplete dissociation of the sulfonic acid sites. While it is
desirable to maximize the conductivity of the membrane by increasing its ion content
12
(decreasing equivalent weight), excessively swelling with water and decreased
mechanical integrity and durability must be considered.
1.1.4.4 Methanol Permeability
Methanol permeability of the membrane is an important factor in Direct Methanol
Fuel Cells (DMFCs) because it leads to undesirable transport of methanol across the
membrane. This has two important consequences: 1) the fuel is wasted instead of being
usefully oxidized at the anode, and 2) the oxidation of methanol at the cathode results in a
backward flux of protons and electrons, and lowers the overall efficiency of the cell.
The diffusion of methanol across a membrane can be measured by a diffusion
experiment in which the membrane separates two compartments: one rich in methanol
and the other rich in water. This method is fairly simple; the main equipment
requirements are a methanol pump, a refractive index detector, and a desktop computer.
In a working fuel cell, the movement of protons and water through the membrane
complicates the measurement of methanol permeation, but in-situ measurements of
methanol crossover current are possible with fuel cell hardware.10
1.1.4.5 Electro-Osmotic Drag
When protons are transported across the PEM, they are usually hydrated by a
certain number of water molecules. The number of water molecules per proton is defined
as the electroosmotic drag coefficient.19, 20 Electro-osmotic drag is an important factor in
PEMFCs because as protons are conducted across the membrane, water is transported
13
from the anode to the cathode. The amount of water in the electrodes has a large
influence on the electrode performance. If there is too much water, flooding will occur
on the cathode side, which results in decreased gas diffusion to the reactive sites. If there
is too little water, the electrode will become too dry and the proton conductivity will be
restricted. The electro-osmotic drag coefficient can be measured during fuel cell
operation.21 The electro-osmotic drag has also been correlated to the states of water in the
membrane.
1.2 Perfluorinated Copolymers for PEM
Perfluorinated sulfonic acid copolymers are the most widely studied and applied
materials in proton exchange membrane (PEM) fuel cell investigations. The
perfluorinated copolymers have demonstrated the most superior properties for PEM up to
now.22, 23 Perfluorinated copolymers (PFSA) are well known for their exceptionally high
chemical and thermal stabilities. Several studies have confirmed the chemical stability of
perfluorinated polymers at less than 100 oC in high humidity environment, including
strong bases, strong oxidizing and reducing agents. These copolymers are chemically
stable against the oxidative conditions of a fuel cell due to the strong carbon-fluorine
bonds24, which are approximately 4 kcal stronger than aliphatic carbon-hydrogen bonds25.
The development of perfluorinated polymers increased confidence in the application of
PEMFCs after the early limit use of crosslinked sulfonated polystyrene (essentially ion
exchange resins).
Nafion® is produced and marketed by E.I. DuPont de Nemours and was
developed for electrochemical chlorine production and for space exploration
14
applications.26 These fully fluorinated electrolytes have been the focus of several books
and reviews in PEMFC literature.27-29 Nafion is a free radical initiated random
copolymer of a crystallizable hydrophobic tetrafluoroethylene (TFE) backbone sequence
(~ 87 moles %) with a comonomer which ultimately has pendant side chains of
perfluorinated vinyl ethers terminated by perfluorosulfonic acid groups. The proposed
structure of Nafion® is shown in Figure 4.
Figure 4. Chemical Structure of Nafion®
**x and y represent molar compositions and do not imply a sequence length.
Nafion 1100 EW in thicknesses of 2, 5, and 7 mils (1 mil equals 25.4 microns)
(Nafion 112, 115, and 117, respectively) is currently widely available. This equivalent
weight provides high protonic conductivity and moderate swelling in water, which seems
to suit many current applications and research efforts. Modest retention of a
semicrystalline morphology at this composition is no doubt important to Nafion’s
mechanical strength. The thinner membranes are generally applied to hydrogen/air
applications to minimize Ohmic losses, while thicker membranes are employed for direct
methanol fuel cells (DMFC) to reduce methanol crossover in direct methanol fuel cell
(DMFC) systems.
Unsaturated perfluoroalkyl sulfonyl fluoride and their derivatives are believed to
be the starting comonomers for preparing perfluorosulfonic membranes by DuPont.
Nafion is prepared via the copolymerization of variable amounts of the unsaturated
CF2 CF2 CF CF2
OCF2 CF O(CF2)2 SO3-H+
CF3
x y
z
nCF2 CF2 CF CF2
OCF2 CF O(CF2)2 SO3-H+
CF3
x y
z
n
15
perfluoroalkyl sulfonyl fluoride with tetrafluoroethylene.30-35 There have been no
detailed literature reports of Nafion’s synthesis and processing, but it is generally
believed that the copolymer is then extruded in the melt processible sulfonyl fluoride
form into a membrane, which is later converted from the sulfonyl fluoride via base
catalyzed hydrolysis and acidification to the sulfonic acid functionality. It seems unlikely
that the sulfonyl fluoride containing precursor unit in the copolymer would self propagate.
Thus, the length of the comonomer sequence (y) is likely only one unit.
These membranes exhibit a protonic conductivity as high as 0.1 S⋅cm-1 in liquid
water under fully hydrated conditions. For a membrane thickness of, 175µm (Nafion
117), this conductivity corresponds to a real resistance of 0.2 ohm (Ω)⋅cm2, i.e., a voltage
loss of about 150mV at a practical current density of 750 mA⋅cm-2. Under relative low
current density, a lifetime of over 60,000 hours under fuel cell conditions (80% relative
humidity at 80oC) has been reported with commercial Nafion membranes 36.
However, Nafion membranes are limited in their temperature range of operation
to around 80oC. Figure 5 shows the protonic conductivity of Nafion 117 membranes at
the indicated relative humidities for temperature increasing from 80 to 160oC. At
different relative humidities there was a dramatic decrease in conductivity when
temperatures reached certain values. Besides, the decrease was shifted to higher
temperature when lowered the relative humidity. Furthermore, it was found that the
conductivity decrease was not reversible since the initial value was not recovered when
the temperature was again decreased below 110 oC. Alberti et al 37 believe this is due to
the excessive swelling of the polymer, which leads to the decrease of the effective contact
area with the electrode. However, the Virginia Tech group perhaps was the first one to
16
point out that the glass transition temperatures (Tg) or at least of α-relaxations in Nafion
hydrated Nafion 117 membrane were actually reached and thus some dramatic
morphological changes happened and ruined the transport of protons. 37 (Water is well
known as a plasticizer to lower the glass transition temperature of an ion containing
polymer.) As a matter of fact, a major loss of conductivity of hydrated Nafion 117
membrane shifted to higher temperature (from around 120 oC to 160 oC) as relative
humidity decreased from 100% to 34%.37
Figure 5 Conductivity of Nafion 117 membranes at the indicated relative humidities for temperature
increasing from 80 to 160oC 37.
Recently, Moore et al. 29, 38 report the existence of α and β relaxations in Nafion.
The β-relaxation is often related to chemical characteristics of the molecular structure,
like reorientational or vibrational motions of the highly flexible side chain with
17
perfluorosulfonic acid groups. The α relaxation, on the other hand, denotes the complete
disruption of the ionic domains. More recently, the glass transition temperature (β-
relaxation temperature) of pure H+ form Nafion was reported to be approximately -20 oC
by dynamic mechanical analysis (DMA) data.29, 39 More importantly, the presence of α
and β relaxations in Nafion is good evidence of nanophase separation in this ion
containing polymer which has the highly hydrophobic tetrafluoroethylene backbone.
Other drawbacks include low conductivity at low water contents, relative low
mechanical strength, poor barrier properties to methanol, allowing methanol crossover
from the anode to the cathode in a DMFC, high osmotic drag, which makes water
management difficult at high current densities. These features are also common to other
perfluorinated membranes, such as those that are or were produced by Dow, Asahi Glass,
Asahi Chemical and 3M.40 Finally, the high cost of Nafion is currently incommensurate
with potential mass markets such as vehicles.
18
1.3 Non-Fluorinated Polymer Materials for PEM
Consequently, much effort has focused on the development of alternative proton
exchange membranes for PEM fuel cells and in DMFC, in particular with the aim of
increasing the temperature of operation of the fuel cell. An increase in temperature is
attractive for a number of reasons: (a) improved tolerance of the electrodes to carbon
monoxide, which enables the use of hydrogen produced by reforming of natural gas,
methanol or gasoline; (b) simplification of the cooling system; (c) possible use of
cogenerated heat; (d ) increased proton conductivity; and (e) in DMFC, improved kinetics
of the methanol oxidation reaction at the anode, and at the cathode.
However, an increase in temperature also has some restraints. The majority of
proton-conducting polymer materials proton conduction is water-assisted, and they
exhibit the highest proton conductivity when fully hydrated. This requires humidification
of gas feeds before entering the fuel cell and the application of pressure to maintain
adequate relative humidity.
There are few non-fluorinated membrane materials appropriate for fuel cell
application at temperatures above 80oC. These polymers are normally made up of
polyaromatic or polyheterocyclic repeat units, and examples include polysulfones (PSU),
poly(ether sulfone) (PES), poly(ether ketone)s (PEK), poly(phenyl quinoxaline) (PPQ),
and polybenzimidazole (PBI). Developed for high-temperature applications, the thermal
stability of these types of polymers is well documented. However, they are electrically
insulating until modified. Several methods for the preparation of thermally stable proton-
conducting polymers have been developed that include acid or base doping of a
thermostable polymer, direct sulfonation of a polymer backbone, grafting of a sulfonated
19
or phosphonated functional group on to a polymer main chain, graft polymerization
followed by sulfonation of the graft component, and total synthesis from monomer
building blocks (Figure 6). These methods will be reviewed in details with respect to
various polymeric materials. In addition, the proton-conducting properties may be
entirely conferred or enhanced by the addition of an inorganic proton conductor41; see the
review by Casciola and Alberti42 for details. The approach chosen depends on the
particular properties and chemical reactivity of the polymer concerned. Recent progress
in the area of non-fluorinated proton conducting membranes includes a detailed review
by Rozière & Jones41 and a survey early Savadogo43.
Figure 6. Development of proton-conducting membranes by the modification of thermostable
polymers.41
20
1.3.1 Polybenzimidazole
Aromatic polybenzimidazoles are well known as highly thermostable materials
with glass transition temperatures as high as 430°C.44 Due to its commercial availability,
the most widely used one is poly[2,2”-(m-phenylene)-5,5’-bibenzimidazole] or PBI as
shown in Figure 7. 45
Figure 7. Synthesis route to sulfonated (stabilized) polybenzimidazole46
1.3.1.1 Acid Doping
PBI has very low proton conductivity (∼10-7 S cm-1) because it is able to take up
small amounts of water47, 48. Nevertheless, after treatment with sulfuric acid or especially
phosphoric acid, its conductivity can increase significantly. The original acid uptake
21
was 5 mol H3PO4 per PBI repeat unit, far beyond the quantity for other systems. This
treatment is also called as “Doping” 49-51. The reason for this dramatic difference lies in
the fact that PBI readily forms complexes with organic and inorganic acids due to its
basicity. The properties of such doped membranes and their application in PEM fuel
cells have been studied extensively since 199449, 52, 53,54,55-57culminating in the production
by Celanese Ventures of MEAs based on phosphoric acid-doped PBI. These membranes
have some distinct features such as low permeability to methanol and low electroosmotic
drag, which is extremely suitable for the direct methanol fuel cell and high-temperature
operation58. Various acid and different methods are used in the formation of PBI-acid
complexes and, eventually, a homogeneous polymer electrolyte is formed by dissolution
of the acid in the PBI matrix. The conductivity depends on the level of doping, which is
often expressed as the number of H3PO4 molecules per PBI matrix. For membranes with
4-5 H3PO4/PBI, the conductivity is >10-3 S cm-1 at 25°C and >3×10-2 S cm-1 at 190°C52.
More recently, Benicewicz et al, 59 reported a sol-gel process which allows very high
levels of H3PO4 to be achieved by direct casting of the PPA(poly phosphoric acid)-
polymerization solution, without isolation or re-dissolving the polymers. The phosphoric
acid (PA)-doped polybenzimidazole (PBI) films produced in this way can be operated in
a fuel cell at temperatures above 150 °C without any feed gas humidification or pressure
requirements for much more than 1000 h.
1.3.1.2 Direct Sulfonation of Polymeric Backbone
Sulfonated polybenzimidazole can be made by heating sulfuric acid –doped PBI
at 450-500°C for a short period of time (Figure 7), leading to levels of substitution up to
22
0.6 sulfonic acid groups/PBI46,60, 61. However this kind of treatment seems to bring no
benefits at all. The conductivity is barely higher than that of non-substituted PBI and the
membrane can not be processed further because solubility in solvent like
dimethylacetamide is lost. The thermally initiated sulfonation leads to crosslinking,
either by strong hydrogen bonding or through sulfone linkages, or degradation.
1.3.1.3 Synthesis from Sulfonated Monomer
Sulfonated polybenzimidazole can also be prepared from pre-sulfonated monomer
units, for example, by the polycondensation of 2-sulfoterephthalic acid with 1,2,4,5-
tetraaminobenzene tetrahydrochloride at 190 °C 62, 63 . But the number of pendant
sulfonic acid groups should be controlled in a low range to avoid solubility in water.
1.3.1.4 Grafting of a Functional Group
Another approach of making sulfonated polybenzimidazole involves a hydrogen
abstraction with LiH, followed by reaction of the PBI polyanion with a functionalized
grafting species, which includes both sulfonated and carboxymethyl groups (Figure 8). It
is actually a functional group grafting reaction.64-67 The advantage of this method is that
the chemical stability of PBI can be improved by introducing less reactive groups directly
into the imidazole ring68. Furthermore, the degree of sulfonation can be controlled by
limiting the number of –NH sites ionized or limiting the ratio of the sulfonated side chain
group to PBI. Control of the number and location of ionic groups is critical to a
systematic study of the PEM properties.
23
Figure 8. Synthesis route to benzylsulfonated polybenzimidazole via polyanion formation68
As the degree of sulfonation increases, solubility and textural properties of PBI
improve significantly. The hydration number is ∼7 water molecules per sulfonic group,
which is much lower than 12 of Nafion. The conductivity was measured at 25°C in
aqueous phosphoric acid and the membranes were allowed to equilibrate for 8 h before
measurement. Compared with the low conductivity of non-grafted PBI (∼10-5 S cm-1),
grafted PBI displays fairly high conductivity in the range of 3×10-3 to 2 ×10-2 S cm-1.68
1.3.2 Poly(phenylquinoxalines)
Poly(phenylquinoxaline) or PPQ as shown in Figure 9, can be sulfonated as a cast
film by briefly treating sulfuric acid-doped membranes at high temperature69, just like
24
immersion/thermal treatment for polybenzimidazoles above. In contrast to low
conductivity of polybenzimidazoles, sPPQ exhibits proton conductivity up to 0.1 S cm-1.
Figure 9. Sulfonated poly(phenyl quinoxaline)
Sulfonation of PPQs was carried out usually by dissolving PPQ in chlorosulfonic
acid and the site of sulfonation is ortho to the electron donating ether linkage 70. The
performance of MEAs prepared using sPPQs in a hydrogen/air fuel cell at a current
density of 0.538 A cm-2 at 70 °C is in the range of 0.50 V to 0.66V.69 However, the
average MEA lifetime was only 350h, which is possibly due to increased permeability
and/or membrane embrittlement. In fact, no details of MEA fabrication have been
published.
1.3.3 Poly(phenylene oxides)
Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is an important material for the
molding and preparation of membranes because its excellent film and membrane-forming
properties, as well as good thermal and chemical stability.
Sulfonation of PPO was accomplished either in a chloroform solution with
chlorosulfonic acid 71, or in a 1,2-dichlorethane 72. Then the sulfonated product was
precipitated after the addition of a certain amount of chlorosulfonic acid and could
25
subsequently be easily isolated. The SPPO has shown good thermal stability and
resistance against aqueous solutions of strong acids and bases and oxidation agents. 73-
75(Note benzylic methyl oxidizes easily)
Sulfonated poly(2,6-diphenyl-4-phenylene oxide) or P3O (Figure 10a) is made by
oxidative coupling of 2,6-diphenylphenol followed by directly sulfonation by
chlorosulfonic acid. Ion exchange capacity (IEC) values were high (2.22 meq g-1) but
mechanical properties are poor and life time was limited to 500h, owing to a reported
combination of physical and chemical degradation.71 Bromide or cyanide was introduced
to the central aryl group, as shown in Figure 10b aiming to deactivate or improve
chemical stability of P3O. However, the fuel cell lifetimes were still 450-500h.
(a) (b)
Figure 10. (a) Sulfonated poly(2,6-disubstituted-1,4-phenylene oxide); (b) reaction scheme
for the sulfonation of poly(3–bromo-2,6-diphenyl-4-phenylene oxide).
26
1.3.4 Poly(phenylene)s and Derivatives
Poly(p-phenylene) (PPP) and its derivatives are a promising class of high-
performance polymers because of their excellent thermal and mechanical properties.
The sulfonated poly(p-phenylene) derivatives were synthesized76 and later by
Ghassemi and McGrath77 via Ni (0)-catalyzed coupling polymerization. Concentrated or
fuming sulfuric acid was utilized at room temperature to introduce sulfonic acid moieties
to the aromatic side group. Although the sulfonated polymers were not good film
formers, they exhibited high proton conductivity in the range of 0.06-0.11 S⋅cm-1. To
obtain good films, approaches such as polymer blending and multiblock copolymers were
also studied.
Sulfonation of poly(4-phenoxybenzoyl-1,4-phenylene) (PPBP) was also
reported76 and sulfuric acid was utilized to minimize degradation of the polymer by
chlorosulfonic acid or fuming sulfuric acid. At same IEC, sulfonated PPBP showed
much higher and more stable proton conductivity than similar sulfonated PEEK (Figure
11).
Recently, ionomeric poly(phenylene) prepared by Diels-Alder polymerization
first reported by Harris and Stille78, was reported by Fujimoto et al.79 Post-sulfonation of
this high molecular weight and thermochemically stable poly(phenylene) with
chlorosulfonic acid resulted in homogeneous polyelectrolytes with controllable ion
content (IEC = 0.98-2.2 mequiv/g). Fuel cell relevant properties such as high proton
conductivity (123 mS/cm), chemical/thermal stability, and film toughness suggest that
this polyelectrolyte material shows promise as a potential candidate for PEMFCs.
However, economic and availability of the monomer may be questionable.
27
Figure 11 Chemical structures of PEEK, PPBP, and their sulfonated polymers76
Figure 12 Diels-Alder Polymerization of 1,4-Bis(2,4,5-triphenylcyclopentadienone)benzene and
di(ethynyl)benzene78
28
The proton transport properties of a series of sulfonated poly(phenylene)s were
found to strongly correlate to the ion exchange capacity of the polymer.80 In general,
these materials have minimal methanol crossover while maintaining high proton
conductivity, which is necessary for efficient operation of fuel cells powered by liquid
fuels. Proton conductivity in addition to methanol permeability were compared to Nafion
as a function of ion exchange capacity and it was found that the transport in Nafion
membranes was much higher than that in the sulfonated poly(phenylene)s for a given ion
exchange capacity.
1.3.5 Poly(phenylene sulfide)
Poly(phenylene sulfide) or PPS can be directly sulfonated by concentrated
sulfuric acid (Figure 13)81. A polysulfonium cation in 10% SO3/H2SO4 can give a higher
degree of sulfonation and sulfonated PPS can reach proton conductivity of 2.5×10 -3 S/cm.
However, the system became water soluble at a degree of sulfonation > 30%. Here, the
strong electron withdrawing property of the sulfonation group in the main chain
suppresses the crosslinking reaction and promotes sulfonation at 120 °C. The poly
(phenylene sulfide sulfonic acid) is formed after demethylation and acidification
treatment.
29
Figure 13 Synthesis of poly(phenylenesulfide sulfonic acid) via a poly(sulfonium) cation.81
1.3.6 Poly(arylene ether sulfones)
Poly(arylene ether sulfone)s (PES) are known for their excellent thermal and
mechanical properties (Figure 14).82 The basic repeat units in this family of polymers
consist of phenyl rings separated by alternate ether and sulfone (-SO2-) linkages. The
aromatic ether (bisphenol) part provides flexibility, whereas the sulfone group is stable
with respect to oxidation and reduction. The sulfonation of poly (arylene ether sulfone)s
can be accomplished either by simply sulfonation of polymer ( or so called “post-
sulfonation”) or direct copolymerization of sulfonated comonomer. One big difference
between these two methods is that the sites of sulfonic acid in post-sulfonation and direct
copolymerization, as shown in Figure 14 b and c.
30
S
O
O
O
n
S
O
O
OXO
n
S
O
O
OXO
n
SO3H
SO3H
HO3S
Activated Ring
Deactivated Ring
(a)
(b)
(c)
Figure 14 (a) Poly(arylene ether sulfone); (b) Sulfonic Acid Group in Post-Sulfonation (Activated
Ring); (c) Sulfonic Acid Group in Direct Copolymerization (Deactivated Ring)
1.3.6.1 Post Sulfonated Copolymers
Poly (arylene ether sulfone)s can be directly sulfonated by various sulfonation
reagents and these modification reactions have been investigated intensively. 83,84
Noshay and Robeson83 developed a mild sulfonation procedure for the commercially
available bisphenol-A-based poly(ether sulfone) and since then various sulfonating agent
such as chlorosulfonic acid and sulfur trioxide-triethyl phosphate complex have been
employed. A systematic study of the effect of sulfonating agent was undertaken by
Genova-Dimitrova et al. 84 They found chlorosulfonic acid, a strong sulfonating agent,
yielded an inhomogeneous reaction, while mild trimethylsilyl-chlorosulfonate sulfonating
31
agent gave homogeneous reactions. Besides, chlorosulfonic acid also induced chain
cleavages during sulfonation as indicated by viscometric measurements whereas no
polymer degradation or crosslinking was observed with the milder
trimethylsilylchlorosulfonate. However, the degree of sulfonation was limited to 0.85
(85% of the monomer units sulfonated) under short reaction times when the mild
sulfonating agent was employed. Despite its relative low efficiency, the mild sulfonating
agent is still favored because the properties of the resultant sulfonated polymers were
more predictable.
All the above electrophilic substituent reactions locate the sulfonic acid group at
the most activated site ortho to the aromatic ether bond in the bisphenol A part of the
molecule. Ortho site to the sulfone bond in the sulfone unit is also possible for
sulfonation by a series of reactions, as shown in Figure 15, including metalation,
sulfonation by SO2 gas and oxidation.
Kerres 85, et al. invented this method and it turned out the choice of oxidant to
convert sulfonic acid is critical in this synthesis. Because the oxidation step is
accompanied by a loss in ion exchange capacity IEC and this is probably due to the
splitting-off of the sulfonated group during the oxidation process and subsequent
substitution by H. However, by carefully selecting the best suitable oxidation conditions
the loss in IEC can be minimized. Hydrogen peroxide was found to be the best oxidant
due to its good accessibility to all the ionic groups in the polymer, thus minimizing
undesired crosslinking reactions.
32
Figure 15 Metalation Route to Sulfonated Polysulfones85
This method has some advantages over ordinary post-sulfonation procedures: 1) it
can be applied to any polymer that can be lithiated, such as poly (2,6-dimethyl-
paraphenylene ether) (PPO), poly(styrene), poly(vinylthiophene), and
poly(methylphenylphosphazene); 2) the sulfonic acid group is inserted into the more
hydrolysis-stable part of the molecule; 3) it is ecologically less harmful than many
common sulfonation procedures. Therefore, it is an attractive way for controlled
sulfonation for a variety of polymers although it is more expensive.
In fact, this method has been extended to attach a pendant sulfonated phenyl
group on polysulfones via ketone links, as shown in Figure 16. One may think some
phase separation and interesting morphology phenomenal may take place here by the
pendant attachment of the ionic group.
33
Figure 16 Chemical Structure of Sulfophenylated Polysulfones86
1.3.6.2 Synthesis of copolymers from sulfonated monomers
As mentioned before, post-sulfonation has several problems: 1) lack of precise
control over the degree and location of functionalization;2) the possibility of side
reactions, and 3) the possibility of degradation of the polymer backbone.
On the other hand, greater tailoring ability of the polymer can be achieved via
direct synthesis from monomer functionalized with sulfonic groups. Control of position,
number and distribution of sulfonic groups along the polymer backbone could provide
more thermo and hydrolytically stable sulfonated copolymers. Furthermore,
microstructure and important PEM membrane parameters such as conductivity, degree of
34
swelling can be easily tuned by modification of monomers. The difference between sites
of sulfonic acid in post-sulfonation and direct copolymerization are shown in Figure 14.
McGrath’s group in Virginia Tech used one modified procedure to make poly
(aryl ether sulfone)s from sulfonated monomers16, 87-93. As shown in Figure 17, the
nucleophilic substitution condensation polymerization of 4,4’-dichlorodiphenylsulfone,
4,4’-biphenol, and 3,3’-disulfonate-4,4’-dichlorodiphenylsulfone leads to a random
sulfonated poly(arylene ether sulfone) with two sulfonic acid groups per repeat unit, and
in which the sulfonic acid groups are sited on the deactivated sulfone-linked rings.
Figure 17 Direct polymerization synthesis of random sulfonated poly(arylene ether sulfone)
copolymers 89.
The weak base potassium carbonate, K2CO3, instead of a strong base was utilized
here to avoid weighing an exactly stoichiometric amount of strong base and undesirably
hydrolyzing the dihalides to afford deactivated diphenolates94, which would upset the
stoichiometry. Potassium carbonate has high solubility in the reaction medium and the
precise amount of weak base is not extremely critical for achieving high molecular
35
weight, as long as it is in excess. Therefore, this approach makes it fairly easy to control
the molecular weights and the endgroups.
Figure 18 Atomic Force Micrographs of BPSH-40 and Nafion 117
An atomic force microscopy (AFM) tapping mode comparison of the 40%
copolymer with Nafion is shown in Figure 18. The resultant random sulfonated
copolymers displayed a hydrophilic/hydrophobic phase separated morphology that varied
depending on the degree of disulfonation.
The conductivity and water uptake of this series of copolymers also increased
with disulfonation. However, a semi continuous hydrophilic phase was generated and the
membranes swelled dramatically if the degree of disulfonation reached 60 mole %,
resulting in a hydrogel that can not be used as a proton exchange membrane (Figure 19).
1 1 µµmm 700 nm700 nm
Phase Image of BPSH-40 dark regions are sulfonic acid + water(softer portion)
Phase Image of Nafion 117
1 1 µµmm 700 nm700 nm
Phase Image of BPSH-40 dark regions are sulfonic acid + water(softer portion)
Phase Image of Nafion 117
36
Therefore, to obtain a useful proton exchange membrane, one must balance ionic
conductivity with other physical properties.
Figure 19 Influence of the degree of sulfonation on the water uptake of sulfonated
poly(arylene ether sulfone) copolymers.95
BPSH random copolymers exhibited much lower methanol permeability than
Nafion. For example, even though Nafion and BPSH-30 (a poly(arylene ether sulfone)
copolymer with 30 mole% disulfonation) have similar water uptake values, the methanol
permeability of Nafion is much higher (167 x 10-8 cm2/s compared to 36 x 10-8 cm2/s for
BPSH-30).95-97 Recent research has shown that this may be due to a difference in the
“types” of water that are present in each of the membranes. 17, 98 BPSH-30 has a higher
fraction of tightly bound water and a lower fraction of free water than Nafion.
37
1.3.6.3 Processing and Morphological Issues
Processing is a critical factor that affects the proton conductivity and other
electrochemical properties of PEM.99, 100 Kim and McGrath et al.95, 101 studied the effect
of acidification treatment and morphological stability of sulfonated poly (arylene ether
sulfone) copolymer in great detail. Two methods were developed to study how
acidification treatment would influence the proton conductivity and water absorption:
either immersing the solvent-cast membrane in sulfuric acid at 30 °C for 24 h and
washing with water at 30 °C for 24 h (method 1) or immersion in sulfuric acid at 100 °C
for 2 h followed by similar water treatment at 100 °C for 2 h (method 2). The fully
hydrated BPSH membranes treated by method 2 exhibited higher proton conductivity,
greater water absorption, and less temperature dependence on proton conductivity as
compared with the membranes acidified at 30 °C. In contrast, the conductivity and water
absorption of a control perfluorosulfonic acid copolymer (Nafion1135) were invariant
with treatment temperature; however, the conductivity of the Nafion membranes at
elevated temperature was strongly dependent on heating rate or temperature.
Tapping-mode atomic force microscope results also demonstrated that all of the
membranes exposed to high-temperature conditions underwent an irreversible change of
the ionic domain microstructure, resulting in a conductivity decrease. This indicates that
it is likely that observed morphological change in these membranes is at least partially
responsible for dehydration, as was previously hypothesized.101
38
Figure 20 TM-AFM phase image of BPSH-45 in (a) Regime1, (b) Regime2, and (c)
Regime3; Hydro-thermal treatment temperatures of (a), (b), and (c) were 30, 80, and 120
oC, respectively, phase angle: 30o Scan size: 500 nm; darker features represent the
hydrophilic phase while lighter features represent the hydrophobic phase102
Furthermore, Kim102 et al. suggested that the sulfonated poly(arylene ether
sulfone) copolymers have three morphological regimes, which are determined by the
copolymer composition and the hydrothermal treatment temperature of the membrane.
Regime1 has hydrophilic domains that are isolated (closed structure) and both water
uptake and proton conductivity was almost constant with treatment temperature. In
Regime2 hydrophilic domains show increased connectivity (open structure) and water
uptake and proton conductivity monotonically increased. In Regime3 the
hydrophilic/hydrophobic phase domain structure was no longer well defined
(morphology changed and viscoelastic relaxation occurred) and water uptake greatly
increased while the proton conductivity decreased. The TM-AFM phase images of
BPSH-45 in three Regimes are shown in Figure 20.
39
1.3.7 Poly(arylene ether ketone) Copolymers
The poly(ether ketone)s are a family of polyarylenes linked through varying
sequences of ether(E) and ketone (K) units to give ether-rich: PEEK and PEEKK, or
ketone-rich semi-crystalline thermoplastic polymers: PEK and PEKEKK (Figure 21).
The sulfonated poly (ether arylene) family of polymers has probably been more broadly
and extensively studied in recent years than any other systems. For the most readily
commercially available Victrex PEEK, the ether groups offers the ortho position with the
highest reactivity among the four equivalent sites on the hydroquinone unit. Ortho-ether
substitution by sulfonic acid groups can be carried out in concentrated sulfuric acid or
oleum, the extent of sulfonation being a function of the reaction time and temperature and
SO3 concentration. 103,104, 105,106, 107
sPEEK starts to lose weight between 240 and 300°C,76, 105 depending on the rate
of heating and, to a lesser extent, the degree of sulfonation. The sulfur content of the
residue recovered after heating sPEEK at 400°C decreased by 90%, indicating that
thermal decomposition begins by desulfonation76. The Tg increases from 150°C (PEEK)
to around 230°C in sPEEK of 60% sulfonation.
40
Figure 21 Structures of representative membranes of the poly(ether ketone) family.
The proton conductivity of sPEEK depends not only on the degree of sulfonation,
ambient relative humidity and temperature, but also on the thermal history of a given
membrane.76, 108 For example, for the membranes pre-treated in boiling water for 4 h
prior to measurement, the conductivity showed weak temperature dependence whereas
that of a non-treated membrane increased by more than a factor 10 over the same range 76.
Alberti 37 found that the conductivity of non-treated sPEEK membranes shows greater
dependence on the relative humidity than does that of Nafion (Figure 22 a); However, the
differences in conductivity of sPEEK and Nafion 117 level out by either high relative
humidity values or an increasing temperature. Measurements made as a function of
temperature at a relative humidity of 75% (Figure 22 b) show that highly sulfonated
PEEK displays the same conductivity as Nafion at 160°C, whereas that of sPEEK at
41
lower degree of sulfonation 60% is an order of magnitude lower under these conditions.
These data confirmed that conductivity at medium temperatures and high relative
humidity values essentially depends on the concentration of the sulfonic groups while the
effect of the acid strength is evident only at low relative humidity values and vanishes at
75% relative humidity for a temperature of 155°C. These results can be explained by the
fact that interaction between polymer chains decreases with increasing temperature,
favoring greater hydration of the polymer at a given value of the relative humidity.
(a) (b)
Figure 22 (a) Conductivity, at 100oC, as a function of relative humidity for Nafion 117 and
S-PEEK-2.48 membranes. The conductivity of g-zirconium sulfophenylphosphonate is
reported for comparison; (b) Conductivity of Nafion 117 and S-PEEK-2.48 at 75% r.h. for
temperature increasing from 80 to 160oC. The conductivity of S-PEEK-1.6 is reported for
comparison. 37
Kreuer et al.109 reported detailed studies on the transport properties and the
swelling behavior of different sulfonated polyetherketones and compared them with
Nafion. Small angle X-ray scattering (SAXS) results indicate a smaller characteristic
42
separation length with a wider distribution and a larger internal interface between the
hydrophobic and hydrophilic domain for the hydrated sulfonated polyetherketone. As
schematically illustrated in Figure 23, the water filled channels in sulfonated PEEKK are
narrower compared to those in Nafion. They are less separated and more branched with
more dead-end “pockets”. These features correspond to the larger
hydrophilic/hydrophobic interface and, therefore, also to a larger average separation of
neighboring sulfonic acid functional groups.
Figure 23 Schematic representation of the microstructures of NAFION and a sulfonated
polyetherketone, illustrating the less pronounced hydrophobic/hydrophilic separation of
the latter. 109
43
Synthesis of sulfonated poly(aryl ether ether ketone ketone)s statistical
copolymers was reported by Liu & Guiver, as shown in Figure 24.110, 111 Membranes
prepared from the present sulfonated poly(ether ether ketone ketone) copolymers
containing the hexafluoroisopropylidene moiety (SPEEKK-6F) and copolymers
containing the pendant 3,5-ditrifluoromethylphenyl moiety (SPEEKK-6FP) had low
water uptakes and showed proton conductivities >0.01 S/cm at room temperature. 111
Figure 24 Synthesis of sulfonated poly(aryl ether ether ketone ketone)s111
Polymer blends based on sulfonated poly(ether ether ketone) (SPEEK) and
polysulfone bearing benzimidazole side groups have also been prepared by Guiver et
al.112-114 The benzimidazole group tethered to the polysulfone backbone acts as a
medium through the basic nitrogen for transfer of protons between the sulfonic acid
groups of SPEEK. The blend membrane exhibits better performance in PEMFC at 90 oC
and 100 oC compared to the pure SPEEK and Nafion 115 membranes.112 The polymers
44
bearing pendant benzimidazole groups may offer a promising strategy to develop new
membranes that can operate at higher temperatures and low relative humidity.
Guiver115 also reported the synthesis of highly fluorinated poly(arylene ether)s
containing sulfonic acid groups meta to ether linkage for PEMFC. They used potassium
carbonate mediated nucleophilic polycondensation reactions of commercially available
monomers: 2,8-dihydroxynaphthalene-6-sulfonated salt (2,8-DHNS-6),
hexafluorobisphenol A (6F-BPA), and decafluorobiphenyl (DFBP) in dimethylsulfoxide
(DMSO) (Figure 25). The proton conductivities of acid form membrane increased with
sulfonation degree and reached 0.103 S/cm at 90 C. The methanol permeabilities ranged
from 7.4×10−7 to 2.2×10−9 cm/s at 30 C and were much lower than commercial
perfluorinated sulfonated ionomer (Nafion).
Figure 25 Synthesis of highly fluorinated poly(arylene ether) copolymers containing sulfonic acid
groups
45
1.3.8 Poly(phosphazene)s
Sulfonated poly(phosphazene)s may be PEM candidates because of their well-
known chemical and thermal stability116. The variety of different polymer compositions
through the nature of the side chains on the P = N- backbone and polymer crosslinking
onto the -P=N- polymer backbone makes them especially good in controlling the ionic
groups as well as water uptake.
1.3.8.1 Post Sulfonated Copolymers
Allcock et al.116 studied the sulfonation process of polyphosphazenes and
sulfonating agents, including SO3, and concentrated and fuming sulfuric and
chlorosulfonic acids. Additionally, they also describe the sulfonation of
aminophosphazenes with 1,3-propanesulfone.117 Although these materials may not be
useful as PEMs, this study demonstrated a novel technique for synthesizing sulfonated
polyphosphazene materials that may provide more control over sulfonated product than
the strong sulfonating agents approach. However, the glass transition temperatures of
sulfonated polyphosphazenes are low (-10°C) and therefore polymer blends or
crosslinking is necessary.
With respect to crosslinking, Pintauro et al.118 reported proton-exchange
membranes fabricated from poly[bis(3-methylphenoxy)phosphazene] (Figure 26) by first
sulfonating the base polymer with SO3 and then solution-casting thin films. Polymer
crosslinking was carried out by dissolving 15 mol% benzophenone photoinitiator in the
membrane casting solution and then exposing the resulting films after solvent
evaporation to UV light.
46
The methanol diffusivity in the crosslinked polyphosphazene was very low (1.6--
8.5×10-8 cm2/s). Surprisingly, the proton conductivity did not differ between the
crosslinked and non-crosslinked specimens even though their water uptake was different.
Crosslinking usually would accompany reduced conductivity and brittleness in the dry
state. This result indicates that it is possible to reduce water uptake while at the same
time maintain proton conductivity.
Figure 26 The repeat unit of poly[bis(3-methylphenoxy)phosphazene]
1.3.9 Polyimides
Sulfonated polyimides are known for their thermal stability and therefore have
been prepared as proton exchange membranes in fuel cells.119-121 However, their poor
hydrolytic stability is a significant problem. Sulfonated five-membered phthalic
polyimides quickly degrade hydrolytically and chain scission occurs rapidly in water at
80°C.122 Since the six-membered ring of the naphthalic polyimide is much more stable to
hydrolysis, this chemical structure is somewhat better suited for PEM fuel cell
applications.
47
The nature of hydrolysis associated with the sulfonic acid group in phthalic and
naphthalic polyimides has been studied by Genies et al.123, 124, using model compounds
along with IR and NMR spectroscopy. Model compounds of the sulfonic acid containing
phthalic imide (model A) and the sulfonic acid containing naphthalic imide (model B) as
shown in Figure 27, were prepared by a one-step high temperature condensation in m-
cresol. The NMR and IR spectroscopic analyses of model compounds, before and after
aging in water, at temperature representative of fuel cell working temperature, indicated
structural modifications according to imide ring structure. The phthalic model stability
was inferior to 1 h at 80°C whereas the confirmed naphthalenic model one was superior
to 100 h in the same conditions. Moreover, limited hydrolysis of naphthalenic imide
cycle (12%) was also shown. This critical fraction is probably enough to cause some
degradation of polymeric materials.
Figure 27 Structures of phthalic model A and naphthalenic model B imides.124
48
Various random and sequenced sulfonated polyimides from naphthalene-1,4,5,8-
tetracarboxylic dianhydride (NTCD) or 4,4-oxydiphthalic anhydride (OPDA), 4,4’-
diaminobiphenyl-2,2’-disulfonic acid (BDSA), and common non-sulfonated diamine
monomers such as oxydianiline (ODA), were synthesized in several laboratories125, 126.
The incorporation of diamine comonomers adjusts properties such as flexibility and water
uptake by pushing apart the rigid rod backbone of the polymer, thereby creating free
volume for water. This leads to higher water uptakes and therefore higher conductivity,
especially at low humidity.
A synthetic method of two-stage polycondensation of BDSA-NTCD-ODA to
form naphthalenic sulfonated polyimide (sPI) copolymers was first found by Mercier et
al.124 (Figure 28) The molar ratio of BDSA-NTCD in the first stage of polycondensation
gives the length of the ionic sequence, while the subsequent introduction of ODA and the
remaining dianhydride spaces the ionic blocks with hydrophobic sequences. These latter
sequences have some residual flexibility, whereas the phenylene bonds in the sulfonated
moiety confer a glassy character. The degree of sulfonation can be precisely controlled
by regulating the molar ratio of BDA and the unsulfonated diamine, which is 4,4’-
oxydianiliine (ODA) in SPI.
Polymers of various equivalent weight and different ratios (x/y) of sulfonated (x)
to neutral (y) diamine (Figure 29) have been prepared that can be cast into membranes
from m-cresol. 127 The length of the block sequence for a constant equivalent weight has
greater impact on water uptake and conduction properties than does a variation of the
equivalent weight for polymers with the same block length. For membranes swollen in
liquid water, the hydration number increases with the length of the ionic block sequence,
49
but the conductivity decreases (from 1.8 ⋅ 10-2 to 4 ⋅ 10-3 S cm-1 for sPI of equivalent
weight 793 g mol-1 when the length of the ionic block increases from 3 to 9 units). 127
This observation seems in contrast with the general idea that conductivity increases with
the hydration number and suggests that the conductivity also depends on microstructural
changes accompanying the increase in block length.
Figure 28 Synthesis of sequenced sulfonated copolyimides124
50
Figure 29 Naphthalenic sulfonated polyimides
Some polyimide copolymer membranes containing bulky and/or angled
comonomers128-132 have been reported with higher proton conductivities than Nafion,
possibly by increasing the free volume available for water. The larger comonomers
prevent close packing of backbones and result in a more open structure. X-ray diffraction
showed that polymer interchain spacings are higher by ∼0.1-0.4 Å for sulfonated
polyimides than those for the corresponding BDSA-NTCD homopolymers. With more
space between the polymeric chains, more free volume is available for water and which,
therefore, leads to higher water uptake and higher conductivity. Recently, Einsla133, 134 et
al also reported some detailed results on naphthalene dianhydride based polyimide
copolymers. At relatively high ion exchange capacities, the proton conductivities of the
polyimides in water at 30 oC were equivalent to Nafion 1135. Unfortunately, the best
hydrolytic stability achieved was still much lower than Nafion or analogous poly(arylene
ether)s.
51
1.4 Block Copolymers for PEM
In this section reviews synthetic strategies for obtaining block or multiblock
copolymers with controlled molecular structure, which afford nanophase separation of
ionic and hydrophobic domains. Strategies to control the molecular structure of polymers
that bear ionic and hydrophobic segments, and on the influence of phase separation on
selected properties pertinent to fuel cell performance will be highlighted.
Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are
promising materials that can be used as alternate PEMs. This is particularly due to their
ability to form unique morphologies, such as: the spherical, cylinder, and lamellar shape.
It is well known that the membrane morphology is important for the PEM fuel cell
performance, and depends strongly on the water content, and on the type concentration
and distribution of the acidic moieties.27 The unique morphologies of multiblock
copolymers may play an important role in providing good proton transport at low water
contents and high temperatures.135 Compared with random copolymers, multiblock
copolymers are also expected to provide less water swelling and better thermal and
mechanical properties.98, 136, 137
Before detailing the various architectures of proton conducting polymers reported
in the literature, it is necessary to first reviews some basics of copolymer architecture and
nanophase separation in block copolymers
52
1.4.1 Copolymer Architecture
It is unlikely that PEMs will be fabricated from homopolymers of ion bearing
monomers because the ionic exchange capacity (IEC) required will be too high, leading
to problems in processing and dimensional stability in the presence of water.
Copolymers are likely to be more common, i.e. those polymers containing two or more
different monomers. Copolymers can be divided into several classes by the arrangement
of repeating units in the polymer chain, as shown in Figure 30. Random copolymers have
repeating units distributed randomly. Strictly speaking, the majority of random
copolymers reported are not truly random, but “statistical”, which includes all
copolymers for which the sequential distribution of the repeating units follows a specific
statistical law depending on the reactivity of specific monomers and the method of
synthesis. 138 Alternating copolymers possess alternating repeat units along the chain.
Figure 30 Illustration of copolymer architectures: III: di-block; IV: tri-block; V: multi-block; VI:
graded block; VII: four arm star block.
53
Block copolymers are linear polymers which possess one or more uninterrupted
sequences of each repeat unit. Block copolymers can be further sub-categorized as di-
block, tri-blocks, multiblocks, graded blocks and star blocks. Graft copolymers may be
considered as a special case of block copolymers. They are identified as having a graft
chain of a different repeat unit to the main chain.
1.4.2 Nanophase Separation in Block Copolymers
Block copolymers consist of chemically distinct polymer chains covalently linked
to form a single molecule. Owing to their mutual repulsion, dissimilar blocks tend to
segregate into different domains, but because A and B blocks are chemically joined
macroscopic phase separation is prevented. Instead, the system forms a microstructure
with an extensive amount of internal interface between the A- and B-rich regions.139
Although interfacial tension drives the system towards large domain sizes, this is
countered by tension in the stretched copolymers. The actual structure the system form is
governed by the detailed balance between the tension in the A blocks and that in the B
blocks.140 Various microdomain structures are achieved, depending on relative volume
ratio between blocks and chain architecture as well as the persistence lengths of the
respective blocks.141
Leibler142 reported important theoretical studies in the 1980s on the nanophase
separation in block copolymers, expanding earlier work by Molau143. The essential
conclusion is that, at equilibrium, the state of the system is determined by only two
relevant parameters: the polymer chain composition f and the product χN. χ is the Flory-
Huggins interaction parameter and N is the number of monomers along the copolymer
54
backbone. The composition of the A-type segments f =NA/N and N=NA+NB, where NA
and NB are degrees of polymerization of A-type and B-type blocks.
For a symmetric (50/50 vol%) copolymer, f = 0.5, the transition from a disordered
state to one possessing long-range order occurs when χN = (χN)c ≈ 10.5 in the absences of
critical fluctuation. (In polymer melts fluctuation effects are strongly reduced as a result
of screening, due to the interactions of chains with a large number of other chains.144 )
When χN >>(χN)c ≈ 10.5, a strongly segregated symmetric copolymer orders into a
lamellar morphology of alternation A and B blocks. This prediction should also be valid
in multiblock copolymers as long as 1:1 stoichiometry is achieved. Therefore, to obtain a
desired co-continuous microstructure, one must either increase the Flory-Huggins
interaction parameter χ between two monomers or increase the degree of polymerization
(the number of monomers along the copolymer backbone).
The Flory-Huggins interaction parameter χ is a fairly complicated parameter
which contains enthalpy and entropy contributions and is also dependent upon polymer
concentration φ2. Usually it can be expressed in following equation:139, 145
Equation 4
where β1 is sometimes called "the lattice constant of entropic origin"; V1 is the
molar volume of the solvent; δ1 and δ2 are solubility parameters of two monomers.
Therefore, χ is roughly dependent reversely on temperature (1/T, T in Kevins). 146
It is well-established that di- or tri-block copolymers may assemble into a variety
of morphological structures, including spheres, cylinders and lamellae, depending on the
ratio of the molecular weight of the blocks.138, 145 A phase diagram for a series of
poly(styrene-b-isoprene) (PS-PI) diblock copolymers is presented in Figure 31 where χN
55
represents the enthalpic-entropic balance (χ is the Flory-Huggins interaction parameter
and N is the number of monomers along the copolymer backbone).147, 148 As the volume
fraction of polyisoprene (fPI) is changed there is a corresponding change in morphology
of the block copolymer.
The above provides a brief example of how block composition influences the
morphology of the copolymer. However, the effect of sample preparation-casting,
molding, etc.—on the resulting morphology cannot be understated. It is well known that
a morphology observed at low temperature may significantly change upon annealing. In
addition, different morphologies for the same material,149 can be observed in response to
shear forces, solvent, rates of solvent evaporation and film thickness.150
Figure 31 Morphological phase diagram for PS-PI diblock copolymers
56
1.4.3 Di-Block and Tri-Block Copolymers for PEM
While random copolymerization is often an effective means of controlling the
bulk characteristics of a polymer system, block copolymerization potentially offers a
higher level of molecular control. Block copolymers display a nanophase-separated
morphology in which the physicochemical properties of individual block components can
be realized in a single polymer structure.
The most studied, most understood class of block copolymers is the di-block. A
proton conducting analog of a di-block would possess a length of ionic and non-ionic
polymer. Preferentially, the length of the blocks would be controlled in such a way that
structure-morphology relationships could be established both theoretically and
experimentally. However, few papers on the subject are published. This is probably due
to the tendency of ionic block structures being soluble in water. In the case of sulfonic
acid or similar ionic polymers there is a strong tendency for dissolution in water, which is
accentuated by the ability of the polymers to form micelles and other aggregated
structures that expose the ionic regions to the aqueous solution. In principle, the extent of
solubility can be diminished if the non-ionic block is highly hydrophobic, such as a
fluoropolymer—this would also promote phase separation in films.
Main chain fluoro-block copolymers are rare because fluorine-containing
monomers cannot readily be polymerized by living ionic polymerization151 or pseudo-
living radical polymerization.152 One approach to prepare these block copolymers is to
first synthesize halogen-terminated low molecular weight fluoropolymers by means of
telomerization. These low molecular weight fluoropolymers are then used to initiate
atom transfer radical polymerization (ATRP) of a non-fluorinated monomer.
57
For example, telomerization of vinylidene difluoride in the presence of
BrCF2CF2Br provides α,ω-dibrominated poly(vinylidene difluoride) (PVDF), which has
been subsequently used to initiate the ATRP of styrene to form PS-b-PVDF-b-PS triblock
copolymers. 153 PVDF, terminated with trichloromethyl groups, and prepared by
telomerization of VDF, has been used to initiate the ATRP of styrene, methyl
methacrylate, methyl acrylate and butyl acrylate, to form various diblock copolymers. 153,
154 However, the drawback of the telomerization approach is the low molecular weight of
the fluoropolymer segments produced (2,500 g/mol), which are too small, in comparison
to the non-fluorinated segments for the block copolymers to take on fluoropolymer
characteristics.
To address this, chain transfer polymerization of fluoromonomers has been used
to obtain higher molecular weight, halogen-terminated fluoropolymers. 155 For example,
as shown in Figure 32, trichloromethyl-terminated copolymers of vinylidene difluoride
(VDF) and hexafluoropropylene (HFP), possessing molecular weights up to 25 000 g/mol,
were obtained by emulsion polymerization in the presence of chloroform and used to
initiate the ATRP of styrene (St) and methyl methacrylate (MMA) to form a series of
P(VDF-co-HFP)-b-PS and P(VDF-co-HFP)-b-PMMA block copolymers.155
The chain length of both fluorinated block and PS block can be adjusted, and the
polystyrene block sulfonated with acetyl sulfate to provide P(VDF-co-HFP)-b-
poly(styrenesulfonic acid) (P(VDF-co-HFP)-b-PSSA). The IEC of these novel ion-
containing polymers was controlled by two strategies: adjusting the length of the
sulfonated polystyrene block by ATRP or varying the degree of sulfonation of the
polystyrene segments. The conductivity of block copolymer membranes lies in the same
58
order of magnitude of Nafion for a similar IEC.156 Although only preliminary data is
provided, TEM micrographs (Figure 33) clearly show the presence of a network of ionic
channels domains having widths ~ 10 nm. 157
Figure 32 Preparation of Fluorine-Containing Block Copolymers by Chain Transfer Emulsion
Polymerization and ATRP 155
Figure 33 TEM of partially sulfonated P(VDF-co-HFP)-b-PS block copolymer (IEC = 0.89 meq/g)
Wnek158-161 reported sulfonated styrene/ethylene-butylene /styrene (S-SEBS)
triblock copolymers (Figure 34) as a PEM for use in hydrogen fuel cells. These triblock
59
copolymers (Dais Analytic’s PEMs) are based on well-known commercial block
copolymers of the styrene-ethylene/butylene-styrene family (Kraton). To synthesize
these sulfonated PEMs, the unsulfonated polymer is dissolved in a
dichloroethane/cyclohexane solvent mixture. The sulfur trioxide/triethylphosphate
sulfonating complex in solution is then added and allowed to react at temperatures
between –5°C and 0°C. PEM can be solvent cast from lower alcohols such as n-propanol
to afford an elastomeric hydrogel with conductivities of 0.07 - 0.1 S/cm when fully
hydrated.161 For the process described above, the unsulfonated block copolymer could
have a number average molecular weight of about 50,000 g/mol with a styrene content of
20 to 35 weight % of the triblock copolymer. Transmission electron microscopy (TEM)
of cast films (50% mole of the styrene units sulfonated, stained with RuO4) reveals
distinct, phase-separated structures which indicate a lamellar structure with thickness of
approximately 25nm.162
Figure 34 Chemical Structure of Sulfonated SEBS Block Copolymer
The main drawback of employing hydrocarbon-based materials is their much
poorer oxidative stability compared to perfluorinated or partially perfluorinated
membranes due to the partially aliphatic character.163 For this reason, Dais membranes
are aimed at portable fuel cell power sources of 1 kW or less, for which operating
temperatures are less than 60°C.
60
1.4.4 Multiblock copolymers for PEM
1.4.4.1 Poly(arylene ether sulfone) Multiblock Copolymers
Ghassemi et al.77, 136, 164 reported new multiblock copolymers containing
perfluorinated poly(arylene ether) as the hydrophobic segment and highly sulfonated
poly(arylene ether sulfone) as hydrophilic segment, with the aim of providing polymeric
materials with a highly phase-separated morphology.
As shown in Figure 35, sulfonated–fluorinated poly(arylene ether) multiblocks
(MBs) 3 were synthesized by nucleophilic aromatic substitution of highly activated
fluorine terminated telechelics 2 made from decafluorobiphenyl with 4,4’-
(hexafluoroisopropylidene)diphenol and hydroxyl-terminated telechelics 1 made from
4,4’-biphenol and 3,3’-disulfonated-4,4’-dichlorodiphenylsulfone.
An increase sulfonated block size in the copolymer resulted in enhanced
membrane ion exchange capacity and proton conductivity. AFM images (Figure 36) of
these sulfonated–fluorinated poly(arylene ether) multiblocks revealed a very well defined
phase separation, which may explain their higher proton conductivities compared to the
random copolymers. In addition, multiblock copolymers exhibited higher proton
conductivities than Nafion at low relative humidity. This may be attributed to the
existence of nano-structure morphology forming sulfonated hydrophilic domains
surrounded by fluorinated hydrophobic segments as supported by AFM data.
61
Figure 35 Synthetic scheme of Sulfonated–fluorinated poly(arylene ether) multiblocks
62
Figure 36 AFM taping mode phase images of a Sulfonated–fluorinated poly(arylene ether)
multiblock
However, these sulfonated–fluorinated poly(arylene ether) multiblock copolymers
suffer from extensive water swelling.
Poly(arylene ether sulfone) block copolymers have been prepared based on the
polycondensation of α,ω-dihydroxy bisphenol A polysulfone precursors (Mn = 1800,
4900, 9500 g/mol) with α,ω-dibromo polyvinylidene fluoride (Mn = 1200 g/mol). 165
These polymers have been post-sulfonated to yield a series of block copolymers
containing sulfonated bisphenol A polysulfone and poly(vinylidene fluoride), SPSF-b-
PVDF, as shown in Figure 37. GPC (polystyrene base) showed the number average
molecular weights of these block copolymers were pretty low, in the range of 10,000 to
20,000 g/mol.
63
Figure 37 Synthesis of SPSF-b-PVDF
Membranes are compared with sulfonated bisphenol A polysulfone
homopolymers (SPSF) to examine the effect of the fluoropolymer blocks on membrane
64
morphology and proton conductivity.166 The difference in proton conductivity between
SPSF and SPSF-b-PVDF polymers is illustrated in Figure 38.
In the low IEC regime, SPSF-b-PVDF block copolymer exhibits proton
conductivity up to four times higher than the corresponding homopolymers. As IEC is
increased the conductivity of the homopolymer and block copolymer is increased, but the
difference in conductivity between the two series diminishes. At the highest IEC
examined, the effect of the fluoropolymer block is negligible. Water contents and λ
values ([H2O]/[SO3H]) are similar for both polymers at a given IEC. The enhancement in
conductivity of the block copolymers is therefore judged to be due to the presence of the
fluoropolymer block which promotes the formation of ionic aggregates and an ionic
network. For higher IEC membranes, where both the concentration of acidic sites and λ
values are much higher, and the network of ions more fully formed, the presence of the
relatively small fluoropolymer segments has little effect on conductivity.
Figure 38 Dependence of proton conductivity on IEC for SPSF (diamond) and SPSF-b-PVDF (square)
membranes. (30 oC, 95% relative humidity)
65
Figure 39 TEM micrographs of polymer membranes. (a): SPSF (IEC: 1.55); (b): SPSF-b-PVDF
(IEC:1.62); (c): SPFSF (IEC: 0.83); (d): SPSF-b-PVDF (IEC: 0.78)
Transmission electron microscopy (TEM) analysis of SPSF and SPSF-b-PVDF
membranes (Figure 39) show the presence of ionic aggregates. The size of the
aggregates is smaller in the block copolymer (~ 7 nm vs. ~ 11 nm) for the high IEC
copolymers. Ionic aggregates are also observed for low IEC copolymers (Figure 39 c and
Figure 39 d) but in addition to small aggregates the block copolymer possesses larger
regions of ionic aggregation, similar to those reported for polymer blends of sulfonated
polymers. It appears that these 50—200 nm size domains are the result of gross phase
separation of ionic and non-ionic regions. However, it should be noted that the TEMs are
66
measured under vacuum with the membrane in its dehydrated state, while conductivites
are measured in their hydrated state. Further work is need to determine why these
morphologies form and if they are the reason for the observed enhanced conductivity for
low IEC block copolymers. One thing that is need to point out is that, the molecular
weight of the PVDF block is relatively small, ~ 1200 g/mol, thus further refinement of
morphology and proton conductivity might be observed if longer blocks of fluoropolymer
were incorporated.
1.4.4.2 Polyimide Multiblock Copolymers
Multiblock copolymers have also been of interest in the design of tailored
sulfonated polyimides. Sequenced copolymers of phthalic polyimide have been prepared
via a two step condensation polymerization of aromatic diamines and dianhydrides. 126, 167
The average length of the ionic sequence is controlled by the molar ratios of monomers.
The morphology and the conducting properties of these phthalic polyimide membranes
are dependent on the distribution of ionic groups along the polymer chain. A small-angle
neutron scattering (SANS) study of the membrane was performed to determine if phase
separation between the ionic domains and the polymer matrix exists. SANS
measurements indicate the intensity of the ionomer peak is larger by a factor of 100 and
located at a lower q value for the sulfonated phthalic polyimide when compared to Nafion.
This suggests the ionic domains are larger in the polyimide. This result is reportedly due
to the block character of the sulfonated phthalic polyimide polymers. However, it is yet
to be determined how the size of ionic domains effects proton conductivity or other
properties related to fuel cell performance.
67
Sulfonated phthalic polyimides are sensitive to hydrolysis and possess a short
lifetime under typical fuel cell operating conditions. This has led to research on
sulfonated naphthalenic polyimides, which have a demonstrated lifetime of 3,000
hours.168, 169 Sequenced naphthalenic polyimides may be synthesized via a similar
process as for phthalic polyimides as shown in Figure 40. 123, 170-174
Figure 40 Synthesis of sequenced sulfonated naphthalenic polyimide block copolymers
Proton conductivity of naphthalenic polyimide block copolymers varies with ionic
sequence length as shown in Figure 41 . Proton conductivity does not linearly relate to
water uptake in the polymers, which is not in agreement with the general observation that
the ionic conductivity of ion exchange membranes increases with water content but,
rather, may be due to the concentration of protons in the hydrated membranes.
68
Figure 41 Ionic conductivity vs. ionic block length for BDSA/NTDA/ODA copolymers with X/Y =
30/70.
Of significance is that the observation that sequenced sulfonated naphthalenic
polyimides exhibit a higher conductivity than randomly sulfonated naphthalenic
polyimides. Both λ and conductivity values are systematically lower for random
polyimides. However, the difference in conductivity between sequenced and random
polymers decreases as IEC increases as observed in SPSF-b-PVDF block copolymers.123
In similar work, sequenced copolymers of naphthalene-1,4,5,8-tetracarboxylic
dianhydride (NTDA), 4,4-oxydianiline (ODA), and 9,9-bis(4-aminophenyl)fluorine-2,7-
disulfonic acid (BAPFDS), in which the ionic block length is 2, are reported to exhibit
higher proton conductivity than random copolymers. Increased phase separation is
suggested as an explanation. 172
69
1.5 Multiblock Copolymers Based on Substituted
Poly(phenylene)s
The goal of this research is to develop a series of hydrophilic-hydrophobic
nanophase-separated multiblock copolymers for proton exchange membranes. Recall
that two blocks have to be very incompatible (or Flory interaction parameter χ has to be
high) and block length of each block (number of monomers along the polymer chain) has
to be large enough to develop phase-separated lamellar structures. One must carefully
choose appropriate monomers and meet the requirement χN >>(χN)c ≈ 10.5.142
Keeping that in mind, telechelic substituted poly(phenylene) (PBP) oligomers
were chosen as hydrophobic blocks and telechelic sulfonated poly(arylene ether sulfone)
(PAES) oligomers as hydrophilic blocks. They are very incompatible since one is made
of hydrophobic hydrocarbon while the other is completely hydrophilic and therefore there
is a fairly high χ between these two monomers. In addition, PBP and PAES are quite
different in terms of chemical structures: PBP has very rigid polymer chain while the
PAES backbone is much more flexible because of the arylene-ether bonds. The N should
be very high when a high molecular weight copolymer is formed. Another requirement is
polymer chain composition f ≈ 0.5, which means that the ratio between two segments
should be close to 1:1. This should also be feasible to achieve.
Before going into the detail of multiblock synthesis, the method of preparing
substituted poly(p-phenylene) via Ni (0)-catalyzed coupling reaction is reviewed.
70
1.5.1 Ni (0) -Catalyzed Coupling Polymerization
Reactions
The coupling of aryl halides using the Ni (0)-catalyst system was first reported by
Colon and Kelsey175, 176 to quantitatively convert chlorobenzene to biphenyl. Using a
catalytic amount of nickel, triphenylphosphine, and excess zinc in a dry, dipolar aprotic
solvent at 60-80°C, chlorobenzene is coupled quantitatively in a few minutes. The
efficiency of this method lies in the fact that excess reducing metals (normally zinc) play
a crucial role, which allows biaryls to be formed rapidly in excellent yields from
chlorobenzene under mild conditions. In fact, rather than simply acting as an agent for
regenerating nickel(0), zinc is crucially involved in the formation of the active
intermediate, which is inaccessible from aryl chlorides without zinc.
Figure 42 Reaction Mechanism Proposed by Colon for the Ni(0)-Catalyzed Polymerization of
Arylene Chlorides
71
The catalytic cycle proposed by Colon and Kelsey contains three steps (Figure 42).
175, 177 These steps were oxidative addition of Ni (0) across the aryl chloride bond (step 1),
reduction resulting in an arylnickel (I) species (step 2), and oxidative addition of the
arylnickel (I) species to a second aryl chloride (step 3). The first two steps were shown to
be accelerated by the presence of an electron-withdrawing substituent on the aryl chloride.
In the third step, the presence of an electron-withdrawing group ortho to the Ni-C bond
would be expected to increase the stability of the electron-rich ArNi(I)L3 (where L= PPh3
or L2 = bipy) complex and thus slow the oxidative addition reaction of this complex with
a second aryl chloride. Colon and Kelsey proposed that early in the reaction (conversions
less than 80%) the reduction of the ArNi(II)ClL2 (step 2) was the rate-limiting step. As
the concentration of ArCl approached that of the nickel species, the oxidative addition of
ArNi-(I)L3 (step 3) became rate limiting. The degree of polymerization is related to the
conversion through [Xn] = 1/(1-ρ) ([Xn] = average degree of polymerization, ρ =
conversion). Oligomers of 10 repeat units equated to a 90% conversion of the aryl
chloride. At this concentration, the oxidative addition of ArCl to the ArNi(I)L3 would be
rate limiting.
Since Colon’s pioneering work, Sheares177-181 and Percec 182, 183 have extended
this reaction to polymerization, giving high-performance materials. In general, the Ni
(0)-catalyst system is fairly sensitive to atmosphere, solvent, and the functional group in
the starting material. For a successful Ni (0) coupling reaction, an inert atmosphere must
be used and the aprotic solvent should be dried thoroughly to suppress reduction of aryl
chlorides to arene. Besides, a small amount of 2, 2’-bipyridine can effectively suppress
some side reactions such as aryl transfer. For aryl chlorides, electron-withdrawing or
72
weakly electron-donating substituent gave high yields of biaryl. Functional groups such
as nitro groups and acidic substituents would either completely inhibit the reaction or
cause aryl chlorides to be reduced to arene. Aryl bromides and iodides need a longer
reaction time than those for aryl.175 A number of nickel salts can be used to generate the
catalyst, but nickel chloride and bromide were most effective. The hydrated nickel salts
led to substantial reduction. Although many metals have reduction potentials lower than
that of nickel, only zinc, magnesium, and manganese gave high yields of coupled
products from chlorobenzene. Triarylphosphines were the best ligands and other ligands
normally gave much slower reactions and lower yields.175
1.5.2 Poly(p-phenylene) (PPP) and Derivatives
Poly(p-phenylene) (PPP) and its derivatives are a promising class of high-
performance polymers because of their excellent thermal and mechanical properties.178,
184,185 However, high molecular weight PPPs derivatives are difficult to synthesize
mainly due to the poor solubility of the growing rigid-rod chains during polymerization.
This problem has been decreased by introducing pendant groups to the phenyl rings
which improve the solubility of PPP, while retaining many of the most useful
characteristics. Appropriately substituted PPPs are soluble and yet exhibit good thermal
and mechanical properties. Thus, poly(2,5-benzophenone) (PBP) is known for its high
degree of polymerization, good solubility in dipolar aprotic solvents, and excellent
thermal and mechanical properties.77, 183, 186
High molecular weight PBP shows a glass transition temperature of 206 oC and
5% weight loss temperatures in nitrogen and air of 526 and 520 oC, respectively. 177 In
73
addition, the tensile modulus of PBP was from two to four times greater than other high-
performance isotropic thermoplastics including polysulfone, poly(ether imide),
poly(ether ether ketone), poly(phenylene sulfide), and thermoplastic
polybenzimidazole.187 PBPs can also be processed by compression molding and
extrusion and are now commercially available under the trade name Parmax from Solvay
Advanced Polymer Inc.
Figure 43 Alkyl-substituted Poly(2,5-benzophenone)s
Sheares184 and Quirk185 independently successfully synthesized poly(2,5-
benzophenone)s and alkyl-substituted poly(2,5-benzophenone)s via Ni (0)-catalyzed
coupling reaction, shown in Figure 43. The resulting polymers are soluble and show no
evidence of crystallinity by DSC. Number average molecular weights are in the range of
9.2 ×103–11.7 ×103 g/mol by multiple angle laser light scattering (MALLS). These
polymers exhibit high thermal stability with glass transition temperatures ranging from
173 to 225°C and weight loss occurring above 450°C in nitrogen and 430°C in air.
74
The regiochemistry of the nickel-catalyzed polymerization of 2,5-
dichlorobenzophenone in the presence of excess zinc and triphenylphosphine with (PBP-
B) and without added coligand (PBP-A), 2,2’-bipyridine has been examined by Quirk188
and Wang and coworkers.189 Based on 13C NMR study of model dimers, Quirk
concluded that PBP-B contained fewer H–T units (71%) than PBP-A (88%).188 However,
PBP-B exhibits a longer UV-vis maximum wavelength, indicating of longer conjugation
lengths along the polymer backbone. Quirk argued what determines conjugation length
may not be the percentage of H–T units, but the absence of a significant number of
conjugation-interrupting head-to-head units. These H–H groups appear to be absent in
PBP-B, but to be present in PBP-A. This is the reason for the decreased conjugation
lengths in PBP-A, and for the increased conjugation lengths in PBP-B.
Wang and coworkers189 used a chemical method (controlled “intramolecular”
cyclization converting 2,2’-dibenzoylbiphenyl moieties into phenanthrene units)
combined with spectroscopic analysis (Raman spectroscopy) to study the regiochemistry
of PBP. The resulting polymers were extremely insoluble. They also concluded that
PBP-B prepared in the presence of 2,2’-bipyridyl had fewer H-T units than PBP-A.
More recently, Hagberg and Sheares177 studied the effects of solvent and
monomer structure on the Ni(0)-catalyzed polymerization with various monomers, as
monomer structure shown in Figure 44. They found solvent and the monomer structure
were shown to be key considerations in the Ni (0) polymerization of arylene dichloride
monomers containing electron-withdrawing substituents. Polymerization in DMAc
resulted in side reactions, such as reduction of the carbonyl groups, which limited the
materials utility. On the contrary, through the use of THF as solvent, high molecular
75
weight poly[3-(2’-thiophenecarbonyl)-2,5-thiophene] and poly(2,5-benzophenone) were
synthesized with no side reactions being observed.
Poly(2,5-benzophenone) of 58 ×103 g/mol ([η] = 1.15 dL/g) with a Tg of 180 °C
and a 10% weight loss temperature in nitrogen of 576 °C was synthesized. In addition to
higher molecular weights than previously reported179, 190, no reduction of the carbonyl
was observed, even without the addition of TPO. (Polymerization in amide solvents led
to reduction of nearly 15% of the carbonyl functionalities. This limitation was overcome
by simply oxidizing the material post-polymerization or by the addition of small
quantities of triphenylphosphine oxide (TPO) to the reaction mixture.) This result
eliminated the need for oxidation reactions post-polymerization. The polymer was
soluble in solvents such as chloroform, THF, and DMAc. Thin films of the polymer were
cast from chloroform. Although not highly creasable, the films were flexible and showed
much better overall filmforming properties than the polymers synthesized previously in
amide solvents.
Analysis of the results by NMR and consideration of the mechanism showed that
in Ni (0)-catalyzed polymerization there is a window of electron withdrawing ability for
which functionalities such as carbonyl-containing pendants meet the criteria and greatly
accelerate the reaction. Increasing the electron-withdrawing ability by substitution with a
sulfone slows the reaction due to the stabilization of a reactive intermediate.177
76
Figure 44 Structures of 3-benzoyl-2,5-dichlorothiophene (M1), 2,5-dichloro-3-(2’-
thiophenecarbonyl)thiophene (M2), 3-benzenesulfonyl-2,5-dichlorothiophene (M3), 2,5-
dichlorobenzophenone (M4), and 2-benzenesulfonyl-1,4-dichlorobenzene (M5).
1.5.3 Multiblock Copolymers Based on Substituted
Poly(p-phenylene)s
Multiblock copolymers have also been of interest in the design of tailored poly(p-
phenylene)s. Sheares et al. 180 synthesized rigid-rod poly(4’-methyl-2,5-benzophenone)
macromonomers (telechelic oligomers) by Ni(0) catalytic coupling of 2,5-dichloro-4’-
methylbenzophenone and end-capping agent 4-chloro-4’-fluorobenzophenone (Figure 45).
Since aryl fluorides do not participate in the Ni(0)-catalyzed coupling reaction, aryl
fluorides activated by an electron-withdrawing group can be used as reactive sites for
nucleophilic aromatic substitution in poly(p-phenylene)s. Because of the step-growth
nature of the Ni (0)-coupling polymerization and relatively high rates of reaction, an aryl
chloride monomer with a functionality of 1 can be introduced into the reaction mixture to
77
end cap poly(2,5-benzophenone)s. End capping of poly(2,5-benzophenone)s with 4-
chloro-4’-fluorobenzophenone will result in PPP chains that contain aromatic fluoride
end groups activated toward nucleophilic aromatic substitution. Furthermore, the
molecular weights of these polymers can be readily controlled by altering the amount of
4’-chloro-4-fluorobenzophenone added to the reaction mixture.
Substitution of the macromonomer end groups was determined to be nearly
quantitative by 1H NMR and gel permeation chromatography. The macromonomers
(telechelic oligomers) produced were proved to be labile to nucleophilic aromatic
substitution.
Figure 45 Synthesis of poly(4’-methyl-2,5-benzophenone) telechelics.
A similar approach was applied in the synthesis of hydrophilic-hydrophobic
multiblock copolymers of sulfonated poly (4’-phenyl-2,5-benzophenzone) and
poly(arylene ether sulfone) (as shown in Figure 46) by Ghassemi and McGrath164. Rigid-
rod poly(4’-phenyl-2,5-benzophenzone) telechelics were first synthesized by Ni (0)-
catalyzed coupling of 2,5-dichloro-4’-phenylbenzophenone and the end capping agent 4-
chloro-4’fluorobenzophenone. After sulfonating at 50°C with concentrated sulfuric acid,
78
the fluoroketone activated sulfonated poly(4’-phenyl-2,5-benzophenzone) oligomers
(Mn=3.05×103 g/mol) was copolymerized with hydroxyl terminated biphenol base
polyarylethersulfone (Mn=4.98×103 g/mol) to afford an alternating multiblock sulfonated
copolymer that formed flexible transparent films. These sulfonated multiblock
copolymers exhibited proton conductivity up to 0.036 S⋅cm -1 with IEC value varying
from 0.70 meq/g to 1.20 meq/g. While these values are lower than the control
commercial membrane Nafion® 1135 (IEC 0.91 meq/g and proton conductivity 0.12
S⋅cm -1), they are considered promising and further composition are being investigated.
DSC only showed one Tg value for sulfonated multiblocks, indicating the segments may
not be of sufficient length to develop two Tg values. Therefore, a study on longer
segments sulfonated multiblocks is needed.
79
ClCl
C O
Cl CO
FNMP, 80oC, 4h
C O OOFF+
NiCl2, Zn, PPh3, Bipy
m
NaCl
C O OOFF
SO3M
H2SO4, 50oC, 24h
SClO
OCl OHHO
SO
OOOHO OH
1
2
3
2 + 3NMAc/Toluene, K2CO3
+ NMP/Toluene, K2CO3
n
SO
OOOO O
C O OO
SO3M
A B
5
H2SO4 (1.5M), 25oC
SO
OOOO O
C O OO
SO3H
A B6
Figure 46 Synthesis of Hydrophilic-Hydrophobic Multiblock copolymers164
80
McGrath research group at Virginia Tech has been focusing on poly(aryl ether
sulfone) random copolymers for PEM for several years. The technique and procedure of
making hydrophilic blocks--synthesizing controlled molecular weight sulfonated
poly(aryl ether sulfone) is pretty mature in our lab. Therefore, it is feasible to control the
length of both hydrophobic and hydrophilic blocks during copolymerization and
synthesize multiblock copolymers via nucleophilic aromatic substitution reaction. By
varying the block length and chemical composition, one may expect some interesting
morphology phenomenon that controls the degree of connectivity between ionic domains
as well as water swelling and mechanical properties.
In summary, phase-separated hydrophilic-hydrophobic multiblock copolymers are
promising PEM materials due to their ability to provide good conductivity at low water
content. Other advantages include less water swelling and better mechanical and thermal
properties. So, more detailed research is needed on this topic.
81
Chapter 2
2 EXPERIMENTAL
2.1 Solvent Purification
2.1.1 N,N-Dimethylacetamide (DMAc)
H3C C N
CH3
CH3O
Source: Aldrich Chemical
Boiling Point: 164.5-166 oC
Density: 0.937 g/mL
Molecular Weight: 87.12 g/mol
Purification: DMAc was dried over calcium hydride (CaH2) or
phosphorus pentoxide (P2O5) for at least 12 hours. DMAc
was distilled under reduced pressure (10 mmHg / 60 oC)
and stored over molecular sieves under nitrogen.
82
2.1.2 N-Methyl-2-Pyrrolidone or 1-Methyl-2-Pyrrolidone (NMP)
N CH3
O
Source: Aldrich Chemical
Boiling Point: 202 oC
Density: 1.028 g/mL
Molecular Weight: 99.13 g/mol
Purification: NMP was dried over calcium hydride (CaH2) for at least 12
hours then distilled under reduced pressure (10 mmHg /80
oC) and stored over molecular sieves under nitrogen.
2.1.3 Fuming Sulfuric Acid
H2SO4· xSO3
Source: Aldrich Chemical
Molecular Weight: 98.08 g/mol
Density: 1.925g/mL
Purification: Sulfuric acid containing oleum (27-30% SO3) was used as
received. Film was used to wrap and seal the bottle after
each use.
83
2.1.4 Toluene
Source: Aldrich Chemical
Boiling Point: 110-111 oC
Density: 0.865 g/mL
Molecular Weight: 92.14 g/mol
Purification: Toluene was transferred into 250 mL round-bottom flasks
and stored over molecular sieves under nitrogen.
2.1.5 Ethanol or Ethyl Alcohol
CH3CH2OH
Source: Aldrich Chemical
Boiling Point: 78 oC
Density: 0.789 g/mL
Molecular Weight: 46.07 g/mol
Purification: Absolute (99%) ethanol was used as a coagulating and / or
recrystallization solvent and used without any purification.
CH3
84
2.1.6 Methanol or Methyl Alcohol
CH3OH
Source: Fisher
Boiling Point: 64.7 oC
Density: 0.791 g/mL
Molecular Weight: 32.04 g/mol
Purification: Methanol was used as a coagulating and / or
recrystallization solvent and was used without any
purification.
2.1.7 Isopropyl Alcohol or 2-Propanol or Isopropanol
(CH3)2CHOH
Source: Fisher
Boiling Point: 82 oC
Density: 0.785 g/mL
Molecular Weight: 60.01 g/mol
Purification: Isopropanol was used as a coagulating and / or as a
recrystallization solvent and used without any purification.
85
2.1.8 Tetrahydrofuran (THF)
C4H8O
Source: Aldrich Chemical
Boiling Point: 65-67 oC
Density: 0.889 g/mL
Molecular Weight: 72.11 g/mol
Purification: Tetrahydrofuran (THF) was fresh distilled from sodium and
benzophenone before using.
2.1.9 Hydrochloride Acid (HCl)
Source: Aldrich Chemical
Boiling Point: > 100 oC
Density: 1.2 g/mL
Molecular Weight: 36.46 g/mol
Purification: A.C.S. reagent grade hydrochloride acid (HCl) was used as
received.
2.1.10 Benzene
C6H6
Source: Aldrich Chemical
Boiling Point: 80 oC
Density: 0.874 g/mL
Molecular Weight: 78.11 g/mol
86
Purification: Anhydrous 99.8% benzene was used and stored under
nitrogen and used without any purification
2.1.11 Cyclohexane
C6H12
Source: Aldrich Chemical
Boiling Point: 80.7 oC
Density: 0.779 g/mL
Molecular Weight: 36.46 g/mol
Purification: A.C.S. reagent grade cyclohexane was used as
recrystallization solvent and used without any purification.
2.1.12 Acetic Anhydride
(CH3CO)2O
Source: Aldrich Chemical
Boiling Point: 138-140 oC
Density: 1.08 g/mL
Molecular Weight: 102.09 g/mol
Purification: A.C.S. reagent grade acetic anhydride was used as
recrystallization solvent and used without any purification.
87
2.2 Reagents and Purification of Monomers
2.2.1 2,2’-bis(4-hydroxyphenol)propane (Bisphenol A) or (Bis A)
Source: Dow Chemical
Molecular Weight: 228.29 g/mol
Melting Point: 152-153 oC
Purification: Bisphenol A was recrystallized from a 25% (w/v) solution
of toluene. After refluxing for several hours, the solution
was allowed to cool to room temperature. The milky white
crystals were collected by filtration and dried in a vacuum
for over 12 hours at 60 oC, then at least 12 hours at 90 oC.
2.2.2 4,4’-Biphenol (BP)
Source: BP-Amoco
Molecular Weight: 186.21 g/mol
HO C
CH3
CH3
OH
OHHO
88
Melting Point: 108-110 oC
Purification: Monomer grade biphenol was used as received. Prior to
use, it was dried in a vacuum oven for at least 12 hours at
100 oC. Biphenol can be recrystallized from deoxygenated
acetone or toluene.
2.2.3 4,4'-(hexafluoroisopropylidene) diphenol (6F-Bisphenol A)
Source: DuPont Chemical
Molecular Weight: 336.33 g/mol
Melting Point: 160-162 oC
Purification: 6F bisphenol A (sometimes called bisphenol AF) was
received as a slightly colored (pink) powder. It was first
dissolved in warm acetic acid and precipitated with
deionized water. The precipitated powder was then filtered
and recrystallized from refluxing toluene yielding off-white
crystals. The collected crystals were dried in a vacuum
oven for at least 12 hours at 120 oC. 6F can also be
purified by sublimation.
HO C
CF3
CF3
OH
89
2.2.4 4,4’-Dichlorodiphenyl sulfone (DCDPS)
Source: Solvay Advanced Polymer
Molecular Weight: 287.13 g/mol
Melting Point: 145-147 oC
Purification: Monomer grade dichlorodiphenyl sulfone was used as
received. Prior to use, it was dried in a vacuum oven for at
least 12 hours at 100 oC. DCDPS can be recrystallized
from toluene and yields large crystals that are pulverized
prior to drying.
2.2.5 Potassium Carbonate (Anhydrous)
K2CO3
Source: Aldrich
Molecular Weight: 138.21 g/mol
Melting Point: 891 oC
Purification: Potassium carbonate was dried under vacuum at 140 oC for
at least 12 hours prior to use.
S
O
O
Cl Cl
90
2.2.6 Chlorosulfonic acid
ClSO3H
Source: Aldrich
Molecular Weight: 116.52 g/mol
Density: 1.753 g/mL
Purification: Chlorosulfonic acid was used as received. Film was used
to wrap and seal moisture from the contents. The container
was stored in a desiccator once opened.
2.2.7 Chlorotrimethylsilane
Source: Aldrich
Boling Point: 57-59 oC
Density: 0.85 g/mL
Molecular Weight: 108.64 g/mol
Purification: Chlorotrimethylsilane (98%) was used as received. A
positive pressure of nitrogen was used when syringing the
reagent from the sealed container. Chlorotrimethylsilane
was stored in a desiccator once opened.
Cl Si
CH3
CH3
CH3
91
2.2.8 2,2’-Bipyridyl (bipy)
N N
C10H8N2
Source: Aldrich
Molecular Weight: 156.18 g/mol
Melting Point: 70-73 oC
Purification: 2,2’-Bipyridyl (bipy) (> 99%) was purified by
recrystallization from ethanol and dried at 50 oC
under vacuum for 12h.
2.2.9 Triphenylphosphine (PPh3)
P
(C6H5)3P
Source: Aldrich
Molecular Weight: 262.29 g/mol
Melting Point: 79-81 oC
Purification: Triphenylphosphine (PPh3) (97%)was purified by
recrystallization from cyclohexane and dried at 60
oC under vacuum for 12h.
92
2.2.10 Dichlorobis(triphenylphosphine)nickel-(II)
Ni ClCl
Ph3P
Ph3P
[(C6H5)3P]2NiCl2
Source: Aldrich
Molecular Weight: 654.2 g/mol
Purification: Dichlorobis(triphenylphosphine)nickel-(II) (> 99.5%)was
used as received without further purification and stored
under nitrogen.
2.2.11 Nickel(II) Chloride (NiCl2)
Source: Aldrich
Molecular Weight: 129.6 g/mol
Purification: Anhydrous nickel(II) chloride was dried under vacuum at
120 oC for 24h and kept in the oven to avoid moisture.
2.2.12 Zn Powder
Source: Aldrich
Molecular Weight: 65.39 g/mol
Purification: Zn powder was washed with acetic anhydride, filtered,
washed with dry Et2O, and dried under vacuum at 150 °C
for at least 48h.
93
2.2.13 2,5-dichlorobenzoic acid
COOH
Cl Cl
Source: Aldrich
Molecular Weight: 191.01 g/mol
Melting Point: 151-153 oC
Purification: 2,5-dichlorobenzoic acid (97%) was used to
prepare dichloro monomers for Ni (0)-catalyzed
coupling reaction and used without further
purification.
2.2.14 4-Fluorobenzoyl chloride
CO
FCl
Source: Aldrich
Molecular Weight: 158.56 g/mol
Melting Point: 10-12 oC
Density: 1.342 g/mL
Purification: 4-Fluorobenzoyl chloride (98%) was stored in desiccator and used
without further purification
94
2.2.15 4-chlorophenly-4’-fluorophenyl sulfone
Cl SO
FO
Source: Aldrich
Molecular Weight: 270.71 g/mol
Melting Point: 60-62 oC
Density: 1.302 g/mL
Purification: 4-Chlorophenyl-4’-fluorophenyl sulfone (99%) was used without
further purification
2.2.16 Thionyl chloride (SOCl2)
Source: Aldrich
Molecular Weight: 118.97 g/mol
Boiling Point: 79 oC
Melting Point: -105 oC
Density: 1.631 g/mL
Purification: Thionyl chloride (> 99%) was used without
further purification.
95
2.2.17 Chlorobenzene
Cl
Source: Aldrich
Molecular Weight: 112.56 g/mol
Boiling Point: 132 oC
Density: 1.106 g/mL
Purification: Chlorobenzene (> 99.5%) was used without further
purification
2.2.18 Fluorobenzene
F
Source: Aldrich
Molecular Weight: 96.10 g/mol
Boiling Point: 85 oC
Density: 1.024 g/mL
Purification: Fluorobenzene (> 99%) was used without further
purification
96
2.2.19 Biphenyl
Source: Aldrich
Molecular Weight: 154.21 g/mol
Boiling Point: 255 oC
Melting Point: 68-70 oC
Purification: Biphenyl (99.5%) was used without further purification
2.2.20 Diphenyl ether
O
Source: Aldrich
Molecular Weight: 170.21 g/mol
Boiling Point: 259 oC
Melting Point: 25-27 oC
Density: 1.073 g/mL
Purification: Biphenyl ether (> 99%) was used without further
purification
97
2.2.21 Aluminum Chloride (AlCl3)
Source: Aldrich
Molecular Weight: 133.34 g/mol
Melting Point: 190 oC
Purification: Anhydrous aluminum chloride was handled under
hood and was used without further purification.
98
2.3 Monomer Synthesis
2.3.1 2,5-Dichlorobenzophenone
C O
Cl Cl
Source: Synthesized in house
Molecular Weight: 251.11 g/mol
Melting Point: 88-89 oC.
Procedure: Thionyl chloride (21.5 mL, 0.2945 mol) and 2,5-dichlorobenzoic
acid (45.0 g, 0.2356 mol) were added together in 500 mL 3-neck
flask and stirred at 63 oC for 12 h. Excessive thionyl chloride was
removed under reduced pressure by gradually increasing the
temperature; light yellow 2,5-dichlorobenzoyl chloride (90-95%)
was obtained by distillation at 76-79 oC (5-6 mmHg).
2,5-Dichlorobenzoyl chloride (34 g, 0.16 mol) was added dropwise
via an addition funnel to a suspension of AlCl3 (24 g, 0.18 mol) in
80 mL of dry benzene (0.89 mol) in a 250-mL flask fitted with a
reflux condenser followed by heating at 63 oC for 12 h. The
reaction mixture was quenched by pouring onto acidified, ground
ice and then extracted with toluene. The organic layer was washed
99
with water, aqueous NaHCO3, and water and then dried with
anhydrous Na2S04. After removal of the toluene, the resulting
yellow-red residue was first recrystallized from ethanol/water
(90/10, v/v) and then from hexane/toluene (50-80 vol % hexane) to
give white crystals. (yield = 89%)
ClCl
COOH
SOCl2
ClCl
CO
ClR
AlCl3
ClCl
CO
R
ClCl
CO
Cl
O
R
H
+63oC
12h
Distilled under vacum
+
(dropwise)
63oC 12h
Quenced with ice
extracted with toluene
Recrystallization
ethnol/water=90/10
Recrystallization
hexane/toluene = 50/50White Crystals (90%)
Light yellow liquid (99%)
(slightly excess)
(slightly excess)
Friedel-Crafts Acylation
(in excess)
1
2
3
F
4
Figure 47 Synthesis scheme of substituted 2,5-dichlorobenzophenone monomers
100
2.3.2 2,5-Dichloro-4’-fluorobenzophenone
C O
Cl Cl
F
Source: Synthesized in house
Molecular Weight: 269.10 g/mol
Melting Point: 87-88 oC.
Procedure: Thionyl chloride (21.5 mL, 0.2945 mol) and 2,5-dichlorobenzoic
acid (45.0 g, 0.2356 mol) were added together in 500 mL 3-neck
flask and stirred at 63 oC for 12 h. Excessive thionyl chloride was
removed under reduced pressure by gradually increasing the
temperature; light yellow 2,5-dichlorobenzoyl chloride (90-95%)
was obtained by distillation at 76-79 oC (5-6 mmHg).
2,5-Dichlorobenzoyl chloride (34 g, 0.16 mol) was added dropwise
via an addition funnel to a suspension of AlCl3 (24 g, 0.18 mol) in
84 mL of fluorobenzene (0.89 mol) in a 250-mL flask fitted with a
reflux condenser followed by heating at 63 oC for 12 h. The
reaction mixture was quenched by pouring onto acidified, ground
ice and then extracted with toluene. The organic layer was washed
with water, aqueous NaHCO3, and water and then dried with
anhydrous Na2S04. After removal of the toluene, the resulting
101
yellow-red residue was first recrystallized from ethanol/water
(90/10, v/v) and then from hexane/toluene (50-80 vol % hexane) to
give white crystals. (yield = 85%)
2.3.3 2,5-Dichloro-4’-phenylbenzophenone
C O
Cl Cl
Source: Synthesized in house
Molecular Weight: 327.20 g/mol
Melting Point: 126-127 oC.
Procedure: Thionyl chloride (21.5 mL, 0.2945 mol) and 2,5-dichlorobenzoic
acid (45.0 g, 0.2356 mol) were added together in 500 mL 3-neck
flask and stirred at 63 oC for 12 h. Excessive thionyl chloride was
removed under reduced pressure by gradually increasing the
temperature; light yellow 2,5-dichlorobenzoyl chloride (90-95%)
was obtained by distillation at 76-79 oC (5-6 mmHg).
2,5-Dichlorobenzoyl chloride (34 g, 0.16 mol) was added dropwise
via an addition funnel to a suspension of AlCl3 (24 g, 0.18 mol)
and biphenyl (28 g, 0.18 mol) in 100 mL of cyclohexane in a 250-
102
mL flask fitted with a reflux condenser followed by heating at 80
oC for 12 h. The reaction mixture was quenched by pouring onto
acidified, ground ice and then extracted with toluene. The organic
layer was washed with water, aqueous NaHCO3, and water and
then dried with anhydrous Na2S04. After removal of the toluene,
the resulting yellow-red residue was first recrystallized from
ethanol/water (90/10, v/v) and then from hexane/toluene (50-80
vol % hexane) to give white crystals. (yield = 80%)
2.3.4 2,5-Dichloro-4’-oxyphenylbenzophenone
C O
Cl Cl
O
Source: Synthesized in house
Molecular Weight: 343.20 g/mol
Melting Point: 97 oC.
Procedure: Thionyl chloride (21.5 mL, 0.2945 mol) and 2,5-dichlorobenzoic
acid (45.0 g, 0.2356 mol) were added together in 500 mL 3-neck
flask and stirred at 63 oC for 12 h. Excessive thionyl chloride was
removed under reduced pressure by gradually increasing the
103
temperature; light yellow 2,5-dichlorobenzoyl chloride (90-95%)
was obtained by distillation at 76-79 oC (5-6 mmHg).
2,5-Dichlorobenzoyl chloride (34 g, 0.16 mol) was added dropwise
via an addition funnel to a suspension of AlCl3 (24 g, 0.18 mol)
and biphenyl ether (30.6 g, 0.18 mol) in 100 mL of cyclohexane in
a 250-mL flask fitted with a reflux condenser followed by heating
at 80 oC for 12 h. The reaction mixture was quenched by pouring
onto acidified, ground ice and then extracted with toluene. The
organic layer was washed with water, aqueous NaHCO3, and water
and then dried with anhydrous Na2S04. After removal of the
toluene, the resulting yellow-red residue was first recrystallized
from ethanol/water (90/10, v/v) and then from hexane/toluene (50-
80 vol % hexane) to give white crystals. (yield = 82%)
2.3.5 4-Chloro-4'-fluorobenzophenone
CO
FCl
Source: Synthesized in house
Molecular Weight: 234.65 g/mol
Melting Point: 97 oC.
Procedure: A 1L three-necked, round-bottomed flash was equipped with a
mechanical stirrer and p-chlorobenzoyl chloride (86.19 g, 0.496
104
mol) and fluorobenzene (400 mL). Iron (III) chloride (104.33 g,
0.640 mol) was then added to the reaction mixture through a solid
additional funnel. The reaction was stirred at 5-10 oC for 1.5 h,
then at 50-60 oC for an additional 3 h. The orange-yellow reaction
mixture was quenched with water, and the product was extracted
with chloroform. The chloroform extracts were dried over sodium
sulfate. The solvent was evaporated in vacuum and the residue
was distilled under vacuum to give the desired product as solid.
The final product was then recrystallized from ethanol to give a
yield of 79%.
Cl C ClO
+ FAlCl3 Cl C
OF
Figure 48 Synthesis scheme for 4-Chloro-4'-fluorobenzophenone
105
2.3.6 Sodium Salt of 3,3’-Disulfonated-4,4’-dichlorodiphenyl sulfone
(SDCDPS)
SO
O
SO3Na
NaO3S
ClCl
Source: Synthesized in house
Molecular Weight: 491.25 g/mol
Melting Point: > 350 oC.
Procedure: The procedure utilized was slightly modified from previously
reported procedures. To a 100-mL, three necked flask equipped
with a mechanical stirrer and a nitrogen inlet/outlet was added 28.7
g of DCDPS dissolved in 60mL 30% fuming sulfuric acid. The
solution was heated to 110 oC for 6 hours. The reaction mixture
was allowed to cool to room temperature and added into 400ml of
ice-water. Next, 180 g of NaCl was added and, subsequently, the
disodium salt of disulfonated dichlorodiphenyl sulfone precipitated
as a white powder. The latter was filtered and re-dissolved in 400
mL of cold deionized water, treated by 2N NaOH aqueous solution
to a pH of 6~7, and finally an excess amount of NaCl was added
again to salt out the sodium form of the sulfonated monomer. The
crude product was filtered and recrystallized from a mixture of
106
alcohol (methanol or isopropanol) and deionized water (9/1, v/v),
producing white needle-like crystals. Yield: 87%.
Figure 49 Synthesis scheme for 3,3’-Disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS) and its
salt form.
107
2.4 Synthesis of Homopolymers from Substituted
Poly(phenylene)s via Ni(0)-catalyzed Coupling
Reaction
2.4.1 Poly(2,5-benzophenone)
A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.0470 g, 1.6
mmol), PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g,
1.6 mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated
under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2
followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe
through the rubber septum to initiate the reaction, which became deep red after 10
minutes. Then, 2,5-dichlorobenzophenone (4.0178 g, 16 mmol) was charged under N2
flow. The mixture was stirred and heated at 65 oC for 24 h. After cooling to room
temperature, the reaction mixture was added into concentrated aqueous HCl/methanol
(40/60 v/v) solution and stirred overnight. The resulting precipitate was collected by
filtration and washed thoroughly with 10% sodium bicarbonate solution and deionized
water. After drying in a vacuum oven at 100 oC overnight, the light yellow polymer was
isolated with a yield of 95%.
2.4.2 Poly(4’-fluoro-2,5-benzophenone)
A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),
PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6
108
mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated
under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2
followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe
through the rubber septum to initiate the reaction, which became deep red after 10
minutes. Then, 2,5-dichloro-4’-fluorobenzophenone (4.3056 g, 16 mmol) was charged
under N2 flow. The mixture was stirred and heated at 65 oC for 24 h. After cooling to
room temperature, the reaction mixture was added into concentrated aqueous
HCl/methanol (40/60 v/v) solution and stirred overnight. The resulting precipitate was
collected by filtration and washed thoroughly with 10% sodium bicarbonate solution and
deionized water. After drying in a vacuum oven at 100 oC overnight, the light yellow
polymer was isolated with a yield of 99%.
2.4.3 Poly(4’-phenyl-2,5-benzophenone)
A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),
PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6
mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated
under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2
followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe
through the rubber septum to initiate the reaction, which became deep red after 10
minutes. Then, 2,5-dichloro-4’-phenylbenzophenone (5.2352 g, 16 mmol) was charged
under N2 flow. The mixture was stirred and heated at 65 oC for 24 h. After cooling to
room temperature, the reaction mixture was added into concentrated aqueous
HCl/methanol (40/60 v/v) solution and stirred overnight. The resulting precipitate was
109
collected by filtration and washed thoroughly with 10% sodium bicarbonate solution and
deionized water. After drying in a vacuum oven at 100 oC overnight, the light yellow
polymer was isolated with a yield of 92%.
2.4.4 Poly(4’-oxyphenyl-2,5-benzophenone)
A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),
PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6
mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated
under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2
followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe
through the rubber septum to initiate the reaction, which became deep red after 10
minutes. Then, 2,5-dichloro-4’-oxyphenylbenzophenone (5.4912 g, 16 mmol) was
charged under N2 flow. The mixture was stirred and heated at 65 oC for 24 h. After
cooling to room temperature, the reaction mixture was added into concentrated aqueous
HCl/methanol (40/60 v/v) solution and stirred overnight. The resulting precipitate was
collected by filtration and washed thoroughly with 10% sodium bicarbonate solution and
deionized water. After drying in a vacuum oven at 100 oC overnight, the light yellow
polymer was isolated with a yield of 98%.
110
2.5 Telechelic Oligomer Synthesis
2.5.1 Synthesis of Telechelic Poly(2,5-benzophenone) (PBP)
Oligomers
A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),
PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6
mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated
under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2
followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe
through the rubber septum to initiate the reaction, which became deep red after 10
minutes. Then, 2,5-dichlorobenzophenone (3.7667 g, 15 mmol) and the end-capping
agent 4-chloro-4’-fluorobenzophenone (0.2346 g, 1 mmol) were charged under N2 flow.
The mixture was stirred and heated at 65 oC for 24 h. After cooling to room temperature,
the reaction mixture was added into concentrated aqueous HCl/methanol (40/60 v/v)
solution and stirred overnight. The resulting precipitate was collected by filtration and
washed thoroughly with 10% sodium bicarbonate solution and deionized water. After
drying in a vacuum oven at 100 oC overnight, the light yellow functional oligomer (or
polymer) was isolated with a yield of 95%.
2.5.2 Synthesis of Telechelic Poly(4’-oxyphenyl-2,5-benzophenone)
(PPBP) Oligomers
A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),
PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6
111
mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated
under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2
followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe
through the rubber septum to initiate the reaction, which became deep red after 10
minutes. Then, 2,5-dichloro-4’-oxyphenylbenzophenone (5.1480 g, 15 mmol) and the
end-capping agent 4-chloro-4’-fluorobenzophenone (0.2346 g, 1 mmol) were charged
under N2 flow. The mixture was stirred and heated at 65 oC for 24 h. After cooling to
room temperature, the reaction mixture was added into concentrated aqueous
HCl/methanol (40/60 v/v) solution and stirred overnight. The resulting precipitate was
collected by filtration and washed thoroughly with 10% sodium bicarbonate solution and
deionized water. After drying in a vacuum oven at 100 oC overnight, the light yellow
functional oligomer was isolated with a yield of 98%.
2.5.3 Synthesis of Telechelic Disulfonated Poly(arylene ether sulfone)
(PAES) Oligomers
The aromatic nucleophilic reaction was conducted in a 3-neck flask equipped with
a mechanical stirrer, nitrogen inlet and a Dean-Stark trap. In a typical polymerization to
prepare 5,000 g/mol oligomer, biphenol (0.3742 g, 2 mmol), SDCDPS (0.8725 g, 1.776
mmol) and potassium carbonate (0.3179 g, 2.3 mmol) were added to the flask. Dry NMP
(10 mL) and toluene(5 mL) were added as the solvents. The reaction mixture was heated
under reflux at 150 oC for 4 h, which stripped off most of the toluene to dehydrate the
system. The temperature was then raised slowly to 190 oC for 36 h. The viscous
polymer solution was cooled to room temperature, diluted with NMP and filtered to
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remove most of the salts. The oligomer was isolated by precipitation in isopropanol.
After drying in a vacuum oven at 100 oC overnight, the white functional oligomer was
isolated with a yield of 99%.
2.6 Synthesis of Multiblock Copolymers
2.6.1 Synthesis of PAES-PBP Multiblock Copolymers
The viscous solution of poly(arylene ether sulfone) (PAES) oligomers prepared
above was first cooled to room temperature. Poly(2,5-benzophenone) telechelic oligomer
(1.1171 g, 0.223 mmol), potassium carbonate (0.069 g, 0.5 mmol), dry NMP (5 mL) and
toluene (10 mL) were added into the same reaction flask under nitrogen. The reaction
mixture was heated to 150 oC and refluxed for 4 h. The toluene was removed to
dehydrate the system and the temperature was raised to 190 oC for another 48 h. The
viscous polymer solution was cooled to room temperature, diluted with NMP and filtered
to remove most of the salts. The block copolymer was isolated by precipitation in
isopropanol. It was washed extensively with deionized water and chloroform and dried at
120 oC under vacuum for 24 h.
2.6.2 Synthesis of PPAES-PBP Multiblock Copolymers
The viscous solution of poly(arylene ether sulfone) (PAES) oligomers prepared
above was first cooled to room temperature. Poly(4’-oxyphenyl-2,5-benzophenone)
telechelic oligomer (1.1171 g, 0.223 mmol), potassium carbonate (0.069 g, 0.5 mmol),
dry NMP (5 mL) and toluene (10 mL) were added into the same reaction flask under
113
nitrogen. The reaction mixture was heated to 150 oC and refluxed for 4 h. The toluene
was removed to dehydrate the system and the temperature was raised to 190 oC for
another 48 h. The viscous polymer solution was cooled to room temperature, diluted
with NMP and filtered to remove most of the salts. The block copolymer was isolated by
precipitation in isopropanol. It was washed extensively with deionized water and
chloroform and dried at 120 oC under vacuum for 24 h.
2.7 Characterization
2.7.1 Nuclear Magnetic Resonance (NMR) Spectroscopy
Proton (1H), Carbon (13C) and Fluorine (19 F) Nuclear Magnetic Resonance were
used to confirm the chemical composition of the monomers and polymers in this research.
Samples were dissolved in appropriate deuterated solvents (DMSO-d6, CDCl3, or D2O),
at a typical concentration of 5% (0.05g / 1 mL). NMR spectra were obtained on a Varian
Unity Spectrometer operating at 400 MHz. Quantitive Carbon (13C) NMR was
conducted with a concentration of at least 10% (0.3 g/3 mL) and longer T1 (> 6s)
relaxation time.
2.7.2 Fourier Transform Infrared (FTIR) Spectroscopy
FTIR was utilized to confirm the functional groups of synthesized homo-and
copolymers. Measurements were conducted on a Nicolet Impact 400 FTIR Spectrometer
using thin solution-cast polymer films. The typical thickness of the films is around 10
µm.
114
2.7.3 Gel Permeation Chromatography (GPC)
GPC measurements were used to determine molecular weight and molecular
weight distributions of synthesized polymers and oligomers. GPC experiments were
performed on a liquid chromatograph equipped with a Waters 1515 isocratic HPLC pump,
Waters Autosampler, Waters HR5-HR4-HR3 column, Waters 2414 refractive index
detector and Viscotek 270 RALLS/viscometric dual detector. NMP (containing 0.05 M
LiBr) was used as the mobile phase. The addition of LiBr is well known to shield the
polyions from intramolecular expansion for ion-containing multiblock polymers. The
column temperature was maintained at 60 oC because of the viscous nature of NMP.
Both the mobile phase solvent and sample solution were filtered before introduction to
the GPC system.
2.7.4 Intrinsic Viscosity Determinations ([η])
Intrinsic viscosity measurements were carried out with a Cannon Ubbelholde
viscometer in NMP (containing 0.05 M LiBr) as solvent at 25 oC. The addition of LiBr is
well known to shield the polyions from intramolecular expansion for ion-containing
multiblock polymers. The measurements of intrinsic viscosities of ion-containing
multiblock copolymers as well as oligomers provide valuable information on the size of a
polymer molecule in solution and shield some light on the molecular weight. Intrinsic
viscosities are very important in this dissertation since GPC can not be effectively used
for ion-containing multiblock copolymers. This is because ion-containing multiblock
115
copolymers typically form micelles-like structures in solvents such as NMP and DMAc,
leading to non-true solution. Therefore, gel permeation chromatography (GPC) can not
provide reasonable values of molecular weights for this series ion-containing multiblock
copolymers.
2.7.5 Thermogravimetric Analysis (TGA)
Thermogravimetric analyses (TGA) were performed on a TA Instrument TGA Q-
500 thermogravimetric analyzer in either air or nitrogen to assess the thermal and thermo-
oxidative stability of multiblock copolymers as well as various oligomers. All the
samples were first vacuum dried and kept in the TGA furnace at 150 oC in a nitrogen
atmosphere for 30 min to remove water before TGA characterization. The typical
heating rate was 10 oC/min in nitrogen or air.
2.7.6 Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry (DSC) was conducted using TA Instrument
DSC Q-1000 at a heating rate of 10 oC/min, under a stream of nitrogen to determine the
glass transition temperatures (Tg) of multiblock copolymers as well as various oligomers
and melting points (Tm) of various monomers. Second heat Tg values were reported as
the midpoints of the change in the slopes of the baselines.
116
2.7.7 Elemental Analysis
Galbraith Laboratories (TN) performed elemental analysis of all synthesized
monomers. The atomic compositional percentages of carbon, hydrogen, oxygen, sulfur
and sodium were obtained.
2.7.8 Non-Aqueous Potentiometric Titration
Non-aqueous potentiometric titrations were employed to determine the milli-
equivalent (meq) weight of sulfonic acid groups in sulfonated poly(arylene ether
sulfone)s. This value was then used to experimentally determine the Ion-Exchange
Capacity (IEC) of the sulfonated copolymers containing pendant sulfonic acid groups.
The ion-exchange capacity is defined as the milli-equivalents of reactive –SO3H sites per
gram of polymer and have units, therefore, of meq/g. The measured IEC values are
compared to the theoretical or calculated IEC value based on the moles of sulfonated
dihalide monomer charged to the reaction flask. Potentiometric titrations were performed
using a MCI GTOS Automatic Titrator equipped with a standard calomel electrode and a
reference electrode.
A typical titration is conducted as follows: Tetramethyl ammonium hydroxide
(TMAH) was utilized as titrant (~ 0.02 N) for the titration of sulfonic acid groups in the
polymer. The titrant was standardized against dry potassium hydrogen phthalate (KHP)
immediately prior to titrating. Sulfonated polymers were dried at ~120 oC before being
weighed and dissolving in dimethyl acetamide (DMAc). The end-point was detected as
the maximum of the first derivative for the potential versus the used volume of titrant.
The end point was then used to calculate the IEC (meq/g) of the sulfonated membrane.
117
The reported experimentally calculated IECs were those of the average of at least three
titrated samples.
2.7.9 Film Preparation
Films were prepared by casting solutions of sulfonated copolymers dissolved in
DMAc on clean glass substrates. The concentration of the polymer solution was varied
from 5 – 15% (w/v) which allowed some control over the thickness of the films. Polymer
solutions were filtered through a disposable syringe and disc filters (0.45 µm) prior to
casting to remove particulates or gels. Evaporation of DMAc was accomplished via an
infrared lamp in an inert (N2) environment with gradual increasing lamp intensity over 24
hours. The films were then placed in a vacuum oven at 120 oC for approximately 24
hours and 150 oC for four hours.
2.7.10 Water Uptake
The membranes were vacuum-dried at 100 oC for 24 h, weighed and immersed in
deionized water at room temperature for 24 h to measure the water uptake. The wet
membranes were wiped dry and quickly weighed again. The water uptake of the
membranes is reported in weight percent as follows:
100uptakewater ×−
=dry
drywet
WWW
Equation 5
where Wwet and Wdry are the weights of the wet and dry membranes, respectively.
118
2.7.11 Conductivity Measurements
Proton conductivity at 30 °C at full hydration (in liquid water) was determined in
a window cell geometry using a Solartron 1252+1287 Impedance/Gain-Phase Analyzer
over the frequency range of 10 Hz - 1 MHz. The cell geometry was chosen to ensure that
the membrane resistance dominated the response of the system. The resistance of the
film was taken at the frequency which produced the minimum imaginary response. The
conductivity of the membrane can be calculated from the measured resistance and the
geometry of the cell according to:
AZl'
=σ Equation 6
where σ is the proton conductivity, l is the length between the electrodes, A is the
cross sectional area available for proton transport, and Z’ is the real impedance response.
For determining proton conductivity in liquid water, membranes were equilibrated at 30
°C in deionized water for 24 h prior to the testing. For determining proton conductivity
under partially hydrated conditions, membranes were equilibrated in a relative humidity
chamber (ESPEC SH-240) at the specified relative humidity (RH) and 80 °C for 24 h
before measurement.
119
Figure 50 Schematic of a four-point membrane proton conductivity cell.
2.7.12 Ion Exchange Capacity (IEC)
Ion Exchange Capacity (IEC) was determined by aqueous potentiometric
titrations using an MCI Automatic Titrator Model GT-05. The membranes were soaked
in deionized water containing sodium sulfate (1 M solution) for 24 h at room temperature
to form the salt form polymer and an acid form, water soluble compound (i.e., sodium
hydrogen sulfate). The solutions were titrated by standard sodium hydroxide solution
(0.01 M) to quantitatively determine sulfonic acid concentration in the sulfonated
polymers in terms of ion exchange capacity (IEC, mequiv/g).
2.7.13 UV-visible Absorption Spectroscopy (UV-vis)
Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry
(UV/ VIS) involves the spectroscopy of photons and spectrophotometry. It uses light in
120
the visible and adjacent near ultraviolet (UV) and near infrared (NIR) ranges. In this
region of energy space molecules undergo electronic transitions. UV/Vis spectroscopy is
routinely used in the quantitative determination of solutions of transition metal ions and
highly conjugated organic compounds.
The method is most often used in a quantitative way to determine concentrations
of an absorbing species in solution, using the Beer-Lambert law:
− Equation 7
where A is the measured absorbance, I0 is the intensity of the incident light at a
given wavelength, I is the transmitted intensity, L the pathlength through the sample, and
c the concentration of the absorbing species. For each species and wavelength, ε is a
constant known as the molar absorptivity or extinction coefficient. This constant is a
fundamental molecular property in a given solvent, at a particular temperature and
pressure, and has units of 1 / M * cm or often AU / M * cm. The absorbance and
extinction ε are sometimes defined in terms of the natural logarithm instead of the base-
10 logarithm.
The Beer-Lambert law states that the absorbance of a solution is due to the
solution's concentration. Thus UV/VIS spectroscopy can be used to determine the
concentration of a solution. It is necessary to know how quickly the absorbance changes
with concentration. This can be taken from references (tables of molar extinction
coefficients), or more accurately, determined from a calibration curve.
121
An ultraviolet-visible spectrum is essentially a graph of light absorbance versus
wavelength in a range of ultraviolet or visible regions. Such a spectrum can often be
produced by a more sophisticated spectrophotometer. Wavelength is often represented by
the symbol λ. Similarly, for a given substance, a standard graph of extinction coefficient ε
vs. wavelength λ may be made or used if one is already available. Such a standard graph
would be effectively "concentration-corrected" and thus independent of concentration.
For the given substance, the wavelength at which maximum absorption in the spectrum
occurs is called λmax.
UV-visible Absorption Spectroscopy (UV-vis) were conducted using a Shimadzu
Model UV-1601 UV–visible spectrometer. The samples were dissolved in methanol or
THF to maintain several ppm solutions and absorbance data were generated.
2.7.14 Atomic Force Microscopy (AFM)
Atomic force microscopy (AFM) images were obtained using a Digital
Instruments MultiMode scanning probe microscope with a NanoScope IVa controller
(Veeco) in tapping mode. A silicon probe (Veeco) with an end radius of <10 nm and a
force constant of 5 N/m was used to image samples. The ratio of amplitudes used in the
feedback control was adjusted to a constant value for most samples (the ratio of
amplitudes is 0.83 of the free air amplitude for both the PAES-6k-PBP-6k and PAES-
10k-PBP-10k samples). Samples were dried under vacuum at 60 °C for 3 h and then
equilibrated at 30% relative humidity for at least 12 h before being imaged immediately
at room temperature in a relative humidity of approximately 15-20%.
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Chapter 3
3 Results and Discussion
3.1 Introduction
We are currently interested in the synthesis of potentially economical, and highly
thermooxidatively stable polymers as candidates for proton-exchange membranes (PEMs)
in fuel cells. PEMFCs have potential as energy conversion devices for both
transportation, mobile and stationary applications.191 The primary demands on the
hydrated PEM are high proton conductivity ( ≥ 0.1 S cm-1), low fuel and O2 permeability,
limited swelling in water and high chemical, thermal and mechanical stability.192
Conventional PEMFCs typically operate with Nafion® membranes29, which offer quite
good performance below 90 oC. However, today there is a strong need for PEMs capable
of sustained operation up to 120 oC to offer many benefits including increased resistance
to fuel impurities, most notably carbon monoxide (CO), fast electrode kinetics, and
simplified water/thermal management. Unfortunately, the proton conductivity of Nafion
suffers greatly at temperatures above 90 oC due to the depressed hydrated α-relaxation or
too restrained transport.18, 29 Also, the barrier properties of this membrane are poor when
methanol is used as fuel. These factors, in addition to the high cost of Nafion, have
encouraged extensive research on alternative PEM materials. 9, 91, 136
Our group at Virginia Tech has been focusing on synthesis, characterization and
performance of disulfonated poly (arylene ether sulfone) copolymers from the sodium
form of 3,3’-disulfonated-4,4’-dichlorophenyl sulfone, 4,4’-dichlorophenyl sulfone and
123
4,4’-biphenol via direct step-growth polycondensation. There wholly aromatic biphenol-
bases copolymers are oxidatively stable, which is critical for long term performance
under harsh conditions within fuel cells. The incorporation of different functional groups
generates structurally distinct sulfonated copolymers and allows qualitative and
quantitive structure-properties relationships.193-195 Additionally, the incorporation of polar
groups, such as phenyl phosphine oxides and phenyl nitriles, may aid in preparation and
improve stability of filled composite (e.g., inorganic heter-polyacids).196-198
Figure 51 Chemical structure of disulfonated poly(arylene ether sulfone) copolymers where the letter
x in abbreviation refers to the sulfonation percentage of a disulfonated monomer.
124
Structure–property–performance relationships of directly copolymerized
poly(arylene ether sufone) were investigated by Kim et al. 195 The polysulfone membrane
incorporated with hexafluoro bisphenol A (6F) showed decreased water uptake compared
to non-fluorinated polysulfones (BP) while conductivity increased at a similar IEC,
attributed to a greater degree of phase separation. The polysulfone membranes
incorporated with polar groups such as benzonitrile and PPO, on the other hand, showed
decreased water uptake, conductivity and methanol permeability. Increased conductivity
of the fluorine incorporated system improved fuel cell performance by reducing cell
resistance, while polar group incorporation adversely impacted the fuel cell performance
by lowering conductivity.
As mentioned before in Chapter 1, the ideal PEM materials would show good
proton conductivity at high temperature and low relative humidities, controlled swelling
and deswelling, and exhibit good oxidative stability and mechanical properties. Although
we have obtain very good results by using disulfonated poly (arylene ether sulfone)
random copolymers (BPSH series) in DMFC fuel cells (mainly because of very low
methanol permeability), the performance in H2/air fuel cells is poor compared with
Nafion, especially under low relative humidities. For the random copolymers, we found
the proton conductivity decrease drastically with a decrease in hydration level.98
Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are
promising PEM materials, particularly due to their ability to form unique morphologies.
It is well known that the membrane morphology is important for the PEM fuel cell
performance, and depends strongly on the water content, and on the type concentration
and distribution of the acidic moieties.27 The unique morphologies of multiblock
125
copolymers may play an important role in providing good proton transport at low water
contents and high temperatures. 135
In order to synthesize nanophase separated hydrophobic-hydrophilic multiblock
copolymers, we decided to choose telechelic substituted poly(phenylene) (PBP)
oligomers as hydrophobic blocks and telechelic sulfonated poly(arylene ether sulfone)
(PAES) oligomers as hydrophilic blocks. This is mainly because PBP and PAES are very
incompatible since one is made of hydrophobic hydro carbon while the other is
completely hydrophilic and therefore fairly high χ between these two monomers. In
addition, PBP and PAES are quite different in terms of chemical structures: PBP has very
rigid polymer chain while PAES backbone is much more flexible because of the arylene-
ether bonds. As a result, PAES-PBP multiblock copolymer would develop a continuous,
nanophase separated, hydrophobic-hydrophilic morphology, which would enhance the
proton transport and maybe even control the extensive swelling problem.
126
3.2 Synthesis of Substituted Poly(p-phenylene)s
Homopolymers
The coupling of bis(aryl halide)s is one of the most recent approaches for the
development of high performance materials. The synthetic procedure was first reported
by Colon et al.175 for synthesis of biphenyl from chlorobenzene. They reported that a
Ni(0)-catalyzed reaction would quantitatively convert chlorobenzene to biphenyl at mild
temperatures and short reaction times. Colon et al.176 have demonstrated a method for
synthesizing poly (arylene ether sulfone)s by a catalytic process involving the coupling of
bis(aryl chlorides) with a Ni(0) complex. Ueda et al. have also investigated the Ni(0)-
catalyzed coupling reaction to make poly(arylene ether sulfone)s199 as well as several
poly(arylene ether ketone)s.200
Our interests in high-performance polymers via nickel-catalyzed coupling has led
us to various poly(p-phenylene) (PPP) derivatives. These polymers are currently
receiving considerable interest because of their high thermal stability and potential in
numerous thermally robust materials including composites, lubricant additives, and
thermoset precursors. However, because the material is quite rigid, it is difficult to
synthesize and to form into useful products. Several steps have been taken to improve
the solubility of PPP while retaining the useful characteristics.201, 202 Appropriately
substituted materials are soluble and yet exhibit thermal and mechanical properties
comparable to the unsubstituted polymer. Percec et al. 183, 203 have synthesized a variety
of soluble polyphenylenes by the Ni(0)-catalyzed coupling of bis(aryl mesylates). The
resulting polymers are high molecular weight, soluble, and thermally stable.
127
There have been several reports on the synthesis of substituted polyphenylenes
utilizing techniques other than the Ni(0)-catalyzed reaction. Schluter et al. 204-207 have
described the palladium-catalyzed polymerization of alkyl-substituted poly(p-phenylene)
derivatives. Tour et al.208, 209 have also investigated the functionalization of brominated
poly(p-phenylene) with several alkynes with the goal of producing thermoset precursors.
Novak has reported on the synthesis of water-soluble poly(p-phenylenes) via the
palladium-mediated, Suzuki cross-coupling of aryl halides and arylboronic acids.210-212
Our efforts are focused on the Ni(0)-catalyzed coupling of various substituted 2,5-
dichlorobenzophenones. DeSimone et al. have previously reported on the synthesis of
poly(2,5-benzophenone) from 2,5-dichlorobenzophenone utilizing this catalyst sytem.178
Maxdem Inc. (now Solvay Advanced Polymer Inc.) has also reported the synthesis of
poly(2,5-benzophenone) in a similar manner but with different glass transition and
thermal decomposition temperatures. poly(2,5-benzophenone)s can also be processed by
compression molding and extrusion and are now commercially available under the trade
name Parmax from Solvay Advanced Polymer Inc.
Wang and Quirk185 have reported that higher glass transition temperatures, which
result from a more stereoregular head-to-tail geometry in the polymer backbone, are
obtained when 2,2’dipyridyl is used as a coligand in the reaction. The versatility and
ease of the monomer synthesis led us to study poly (2,5-benzophenone) materials for
potential applications such as conducting polymers and gas separation membranes.
128
3.2.1 Monomer Synthesis
2,5-Dichlorobenzophenone and its derivatives were synthesized by aluminum
chloride-catalyzed, Friedel-Crafts acylation of benzene and other aromatic compounds
with 2,5-dichlorobenzoyl chloride as shown in Figure 52. In the case of acylation of
biphenyl and diphenylether, addition of cyclohexane as solvent was necessary in order to
dissolve the aromatic compound and form a suspension with aluminum chloride.
Excessive thionyl chloride was removed under reduced pressure by gradually
increasing the temperature; light yellow 2,5-dichlorobenzoyl chloride (90-95%) was
obtained by distillation at 76-79 oC (5-6 mmHg). The dark-colored acid chloride could
also be directly used from the first reaction for the second step, by simply degassing
under vacuum to remove excess thionyl chloride. One thing needs to be addressed here is
that 2,5-dichlorobenzoyl chloride should be added dropwise (slowly) to the suspension of
AlCl3 and dry benzene or other aromatic compounds since Friedel-Crafts acylation is a
extremely exothermic reaction. The color of the suspension changed quickly to deep
green and then to black color at the end.
After heating at 63 oC for 12h, the reaction mixture was quenched by pouring
onto acidified, ground ice to neutralize all residual AlCl3. Then the organic/water
mixture was extracted with toluene for at least three times. The organic layer was
washed with water, aqueous NaHCO3, and water and then dried with anhydrous Na2S04
to remove all residual water. After removal of the toluene, the resulting yellow-red
residue was first recrystallized from ethanol/water (90/10, v/v) and then from
hexane/toluene (50-80 vol % hexane). Extra care should be taken in recrystallization step
to minimize the loss of final product. After recrystallization, white needle crystals were
129
obtained. The yield of the final product was about 84% based on the acid when acid
chloride was used without purification. If purified (distillation) acid chloride (light
yellow colored liquid) was used, the overall yields were 90-95%.
Figure 52 Synthesis of substituted 2,5-Dichlorobenzophenones by Friedel-Crafts Reactions
The recrystallized monomers were analyzed by proton NMR (Figure 53, Figure
54 and Figure 55) with d-chloroform as solvent to confirm the structure and purity.
130
Figure 53 Proton NMR spectrum of 2,5-dichlorobenzophenone
Figure 54 Proton NMR spectrum of 2,5-dichloro-4’-phenyl-benzophenone
1.00 1.02 1.00
3.02
8.0 7.8 7.6 7.4 7.2 PPM
2.10 2.10
1.02
2.09 1.90
1.00
7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 PPM
131
Figure 55 Proton NMR spectrum of 2,5-dichloro-4’-oxyphenyl-benzophenone
3.2.2 Polymer Synthesis
Colon and Kelsey175 have reported detailed general procedures for the efficient
synthesis of biaryls from aryl chlorides utilizing a coupling reagent composed of a
mixture of a catalytic amount of an anhydrous nickel salt and triphenylphosphine in the
presence of an excess of a reducing metal (Zn, Mg, or Mn). Variables investigated
included the effects of the reducing metal, added coligands, halide salts, and substituents
on the aromatic ring. From the results of Kaeriyama and co-workers213 and the
requirements for high molecular weight, step-growth (condensation) polymerization, it
was apparent that the use of high-purity, dry reagents were required for the preparation of
high molecular weight, substituted poly- (p-phenylene)s.
Colon and Kelsey175, 176 also observed that the nickel-catalyzed coupling reaction
of aryl chlorides in the presence of zinc is accelerated dramatically by addition of 1 equiv
2.04
4.07
1.00 0.96 1.92 2.01
8.0 7.8 7.6 7.4 7.2 7.0 6.8PPM
132
of 2,2'-bipyridyl (BPY). It was also reported that 2,2'-bipyridyl effectively suppresses the
principal side reaction of phenyl transfer from triphenylphosphine which would limit the
polymer molecular weight. The effect of 2,2'-bipyridyl on the polymerization of 2,5-
dichlorobenzophenone was investigated by Quirk185 using the same conditions as
described previously, i.e., NiCl2, Zn, and 4-7 equiv of triphenylphosphine, with the
addition of 1 mol equiv of 2,2'-bipyridyl relative to nickel. The red-brown color of the
catalyst formed within 5 min after addition of solvent and at a temperature of 40 oC.
Another difference observed for polymerizations in the presence of bipyridyl was the fact
that gel formation was observed after approximately 40 min at 70 oC ([η] = 0.94 dL/g).
The gel disappeared upon increasing the temperature to 130 oC; however, the molecular
weight did not increase significantly upon raising the temperature and continued heating
([η] = 0.94 dL/g).
More recently, Hagberg and Sheares177 studied the effects of solvent and
monomer structure on the Ni(0)-catalyzed polymerization with various monomers. They
found solvent and the monomer structure were shown to be key considerations in the Ni
(0) polymerization of arylene dichloride monomers containing electron-withdrawing
substituents. Polymerization in DMAc or DMF resulted in side reactions, such as
degradation of reduction of the carbonyl groups, which limited the materials utility. On
the contrary, through the use of THF as solvent, high molecular weight poly(2,5-
benzophenone) were synthesized with no side reactions being observed.
Poly(2,5-benzophenone) of 58 ×103 g/mol ([η] = 1.15 dL/g) with a Tg of 180 °C
and a 10% weight loss temperature in nitrogen of 576 °C was synthesized. In addition to
higher molecular weights than previously reported179, 190, no reduction of the carbonyl
133
was observed, even without the addition of TPO. (Polymerization in amide solvents led
to reduction of nearly 15% of the carbonyl functionalities. This limitation was overcome
by simply oxidizing the material post-polymerization or by the addition of small
quantities of triphenylphosphine oxide (TPO) to the reaction mixture.) This result
eliminated the need for oxidation reactions post-polymerization. The polymer was
soluble in solvents such as chloroform, THF, and DMAc. Thin films of the polymer were
cast from chloroform. Although not highly creasable, the films were flexible and showed
much better overall filmforming properties than the polymers synthesized previously in
amide solvents.
Analysis of the results by NMR and consideration of the mechanism showed that
in Ni (0)-catalyzed polymerization there is a window of electron withdrawing ability for
which functionalities such as carbonyl-containing pendants meet the criteria and greatly
accelerate the reaction. Increasing the electron-withdrawing ability by substitution with a
sulfone slows the reaction due to the stabilization of a reactive intermediate.177
Therefore, we chosen THF as solvent in Ni (0)-catalyzed polymerization with the
catalyst mixture as: NiCl2(PPh3)2 (0.1 equiv), PPh3 (0.2 equiv) , Zn (3.1 equiv), 2,2’-
bipyridyl (0.1 equiv) and dichloride monomer ( 1.0 equiv), as shown in Figure 56.214 The
Ni (0)-catalyzed polymerization was carried out under oxygen and water free atmosphere.
After charging the catalyst mixture and dry THF, the solution first became brown and
deep green and at last deep-red. The red color is a good indication of formation of low
valent arylnickel (I) complexes, which are responsible for rapid coupling of aryl
chlorides.175
134
ClCl
C O C O
m
R R
R= OH, F,,
1 mole
NiCl2(PPh3)2(0.1), Zn (3.1)
PPh3(0.2), 2,2'-Dipyridine(0.1), THF
Figure 56 Synthesis of substituted poly(2,5-benzophenone)s by Ni (0)-catalyzed coupling reaction
Then, the dichloride monomers were charged under N2 flow. The mixture was
stirred and heated at 65 oC for 4h to 12 h. The polymerization proceeded until the
viscosity increased to a point which the reaction could no longer be stirred efficiently.
After cooling to room temperature, the reaction mixture was added into concentrated
aqueous HCl/methanol (40/60 v/v) solution and stirred overnight to remove all residual
Zinc powder. The resulting precipitate was collected by filtration and washed thoroughly
with 10% sodium bicarbonate solution and deionized water. After drying in a vacuum
oven at 100 oC overnight, the light yellow polymer was isolated with a yield of 95%.
The polymers are insoluble in organic solvents such as hexanes, toluene, and
acetone. However, the polymers are soluble in solvents such as chloroform,
tetrahydrofuran, DMAc and NMP.
135
3.2.3 Polymer Characterization
1H and 13C NMR and FTIR were employed to provide structural and
compositional characterizations of the poly(2,5-benzophone) and its derivatives
synthesized from Ni(0)-catalyzed coupling polymerization. UV-visible spectrum was
carried out to shed more light on the level of conjugation in this series of substituted
poly(p-phenylene)s. Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were used to explore the thermal behavior of these rigid rod poly(p-
phenylene)s.
Figure 57 Proton NMR of Poly (2,5-benzophenone) in CDCl3
8.5 8.0 7.5 7.0 6.5 6.0 PPM
136
Figure 58 13C NMR of Poly (2,5-benzophenone) in CDCl3
3.2.3.1 1H and 13C NMR Spectroscopy
Proton NMR spectroscopy has been repeatedly employed to provide structural
confirmation of the polymers.215 Compared proton NMR of monomer 2,5-
dichlorobenzophenone, the biggest difference in the polymer (Poly (2,5-benzophenone))
spectrum is the chemical shift of three protons on the dichloride benzene rings from 7.3-
7.5 ppm to high field 6.5-7.2 ppm due to the coupling of aryl chlorides. Before
polymerization, these three protons are adjunct to chlorides, which is electron-
withdrawing group. However, after coupling polymerization, the three protons are
nearby other benzene rings (electron-donating group) since all chlorides are reacted and
the benzene rings are connected to each other to form a rigid rod polymer chain. As a
137
consequence, the chemical shift of these three protons changes from low field (7.3-7.5
ppm) to high field (6.5-7.2 ppm).
Because the 2,5-dichlorobenzophenone monomers are not symmetric, there is a
possibility of several region-isomers in the polymer backbone. 13C-NMR was utilized to
elucidate the isomerism of the synthesized polymers. The 13C-NMR of poly (2,5-
benzophenone) is shown in Figure 58. This polymer shows only two peaks in the
carbonyl region of the spectrum (196-198 ppm) with the main peak at 197.6 ppm, while
polymer prepared in the presence of 2,2'-bipyridyl shows an additional distinct peak at
196.3 ppm.185 Therefore, this polymer seems to generate carbonyl groups which are in a
more regular environment, i.e., in fewer different regiochemical environments, than the
polymer prepared in the absence of 2,2'-bipyridyl.
Figure 59 shows the different regiochemical regularities in the placement of the
lateral benzoyl groups along the polymer backbone. This polymer seems to have a more
regular structure, i.e., more head-to-tail structures (1) and fewer head-to-head structures
(2) and tail-to tail structures (3).
Figure 59 Poly (2,5-benzophenone)s Structures
138
3.2.3.2 Fourier Transform Infrared Spectroscopy (FTIR)
Fourier transform infrared spectroscopy is a powerful tool used to characterize the
functional groups in a material.216 It basically shows at which wavelengths the sample
absorbs the IR, and allows an interpretation of which bonds are present. This technique
works almost exclusively on covalent bonds, and as such is of most use in organic
chemistry.217 Figure 60 shows the FTIR spectrum of poly (2,5-benzophenone) and the
big peak at 1670 cm-1 is assigned to the C=O stretching frequency from the lateral
carbonyl substitution along the poly(p-phenylene)s chain.
Figure 60 FTIR spectrum of poly (2,5-benzophenone)s
1670
139
3.2.3.3 Intrinsic Viscosity [η] (IV)
The intrinsic viscosity measured in a specific solvent is related to the molecular
weight, M, by the Mark-Houwink equation.218
αη vKM=][ Equation 8
where K and α are Mark-Houwink constants that depend upon the type of polymer,
solvent, and the temperature of the viscosity determinations. Exponent α is a function of
polymer geometry, and varies from 0.5 to 2.0. α contains information about the shape of
the molecules219:
• a = 1/2 ⇒ “bad” solvent, theta condition, a tighter configuration of polymer
chain, intrinsic viscosity is low
• 0.5 < a < 0.8 ⇒ “good” solvent, fairly loosely extended polymer chain, intrinsic
viscosity is high.
• a > 0.8 ⇒ stiff chain
If K and α are known, molecular weight can be derived from intrinsic viscosity.
However, for a newly synthesized polymer, Mark–Houwink constants may not be
available in the literature. In this case, these constants can be determined experimentally
by measuring the intrinsic viscosities of several polymer samples for which the molecular
weight has been determined by an independent method (i.e. osmotic pressure or light
scattering) provided that samples tested have sharp molecular weight distribution.220, 221
A plot of the log[η] vs logM (Mark-Houwink plot) usually gives a straight line. The
slope of this line is the α value and the intercept is equal to the log of the K value.
140
As shown in Table 1, the intrinsic viscosities of this series of substituted poly(2,5-
benzophenone)s are in the range of 0.85 to 0.96 dl/g, indicating fairly high molecular
weight polymers have been prepared by Ni (0) catalyzed coupling reactions.214 For this
series of poly(p-phenylene)s, however, the α value should be at least close to 0.8 because
of rod-like stiff chain in solvents.222, 223 In other words, the intrinsic viscosity of poly(p-
phenylene)s would appear to be higher than normal coil-like polymers even if they may
have the same molecular weight.
Table 1 Properties of substituted poly(2,5-benzophenone)s
Polymer Mn
a
(×103 g/mol)
Mw a
(×103 g/mol) PDI a
Intrinsic
Viscosity b
(dL/g)
Tg c
(oC)
TGA
(5% weight loss)
N2 (oC) d
P1 36.5 66.8 1.83 0.96 190 535
P2 25.2 52.1 2.07 0.87 188 520
P3 26.8 69.8 2.60 0.91 197 528
P4 23.2 53.4 2.30 0.85 202 506
a Determined by GPC with THF as solvent at 30 oC (polystyrene standard)
b Determined in THF at 30 oC
c Determined by DSC during second run with heating rate of 10 oC/min
d Obtained by TGA at heating rate of 10 oC/min in N2
141
C O
n
C O
n
F
C O
n
C O
n
O
P1 P2 P3 P4
Figure 61 Substituted poly (2,5-benzophenone)s P1-P4 structures
3.2.3.4 Gel Permeation Chromatography (GPC)
Unlike other modes of chromatography, gel permeation or size exclusion
chromatography (GPC or SEC) is an entropically controlled separation technique in
which molecules are separated on the basis of hydrodynamic volume or size. With
proper column calibration or by the use of molecular-weight-sensitive detectors, such as
light scattering, viscometry, or mass spectrometry, the molecular weight distribution
(MWD) and the statistical molecular weight averages can be obtained readily.224-226
In order to utilize GPC method to determine the molecular weight of a polymer,
one must make a good choice of solvent, temperature, sample concentration, column
selection, detector selection and molecular weight calibration method. Among these
factors, solvent is probably the most critical factor that influences the accuracy of GPC
measurement since partially soluble sample can introduce many problems, which might
be multiplied because of high column pressure. It is also important to dissolve the
142
sample at appropriate temperature and for sufficiently long before injecting in order to
allow the macromolecular coils to fully swell in the solvent or to break down any
aggregates.224 In some cases, the addition of electrolytes can be required to achieve
disaggregation.227 This issue will be addressed in details later when we have to deal with
ion-containing copolymers.
THF was used as solvent for GPC characterization and the results are summarized
in Table 1. This series of poly (2,5-benzophenone)s exhibit number-average molecular
weight varying from 23.2 × 103 g/mol to 36.5 × 103 g/mol. However, in the case of
poly(p-phenylene)s, it is worthy to point out that GPC can only give a rough estimation
rather than exact molecular weight due to the nature of rigid-rod polymer chain. The
deviation from the expected value of 2.0 is consistent with a broad range of
polydispersities previously reported for similar materials made via Ni(0)-catalyzed
coupling polymerization.184, 185, 196 The origin of varying polydispersities is not clear yet.
3.2.3.5 Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry (DSC) is a technique for measuring the energy
necessary to establish a nearly zero temperature difference between a substance and an
inert reference material, as the two specimens are subjected to identical temperature
regimes in an environment heated or cooled at a controlled rate.228
DSC is one important characterization technique utilized for determining the glass
transition temperature Tg and the crystalline melting transition temperature for polymers.
The crystalline-melting transition temperature (Tm) a polymer is the melting temperature
of the semi-crystalline regions or domains within the polymer.229 The glass transition
143
temperature (Tg) of a polymer is the temperature at which backbone segments in the
amorphous domains of the polymer attain sufficient thermal energy to move in a
coordinated manner. In the region of the glass transition temperature, the polymer will
change from a glassy material to a rubbery material. 230, 231
There are many factors that affect the glass transition of a polymer. Some of
these factors include polymer structure, molecular symmetry, molecular weight,
structural rigidity, and the presence of secondary forces.232 Differential scanning
calorimetry (2nd Heat) analysis was used to characterize the thermal transition for this
series of poly(p-phenylene)s synthesized in this research. A heating rate of 10 oC/min
was used on polymer films of typical weights of 5-10 mg.
These polymers exhibit no evidence of crystallinity by DSC, probably due to
existence of region-isomers as discussed previously, resulting in soluble and processible
polymers. These polymers exhibit fairly high Tgs owing to their wholly aromatic
backbones and pendant groups. Another interesting phenomenon is that as one increase
molecular bulkiness from P1 to P4, Tg constantly goes up from 188 to 202 oC because of
the hindered internal rotation by the larger pendant groups.
3.2.3.6 Thermogravimetric Analysis (TGA)
Thermal Gravimetric Analysis (TGA) is a simple analytical technique that
measures the weight loss (or weight gain) of a material as a function of temperature.233,
234 As materials are heated, they can loose weight from a simple process such as drying,
or from chemical reactions that liberate gasses. Some materials can gain weight by
reacting with the atmosphere in the testing environment.
144
The thermal and thermo-oxidative stability as a function of weight loss of this
series of poly(p-phenylene)s were investigated by thermogravimetric analysis. All the
samples were pre-heated at 150oC for 30 minutes in TGA furnace to remove trace solvent
and moisture. Then, the dynamic TGA experiments were run from 50 to 700 oC, at a
heating rate of 10oC/min under nitrogen.
These polymers exhibit high thermal stability as is expected for rigid rod poly(p-
phenylene)s with 5% weight loss temperature varies from 520 to 538 oC in nitrogen. The
high thermal stabilities of these polymers were no doubt derived from their fully aromatic
structure extending through the backbone and pendent groups. P1 exhibits the highest
thermal stability with a 5% weight loss temperature of 538 oC in nitrogen with 60% of the
polymer remaining at 600 oC. Polymer P4 had lower 5% weight loss temperature
compared to others, which may have been due to the presence of relatively less thermally
stable phenyl ether pendent group.
145
Figure 62 TGA thermogram for poly(2,5-benzophenenone) in N2
3.2.3.7 Ultraviolet-visible Spectroscopy (UV/ VIS)
Ultraviolet-visible spectroscopy (UV/ VIS) uses light in the visible and adjacent
near ultraviolet (UV) and near infrared (NIR) ranges. In this region of energy space
molecules undergo electronic transitions.235
UV/Vis spectroscopy is routinely used in the quantitative determination of
solutions of transition metal ions and highly conjugated organic compounds. Organic
compounds, especially those with a high degree of conjugation, absorb light in the UV or
visible regions of the electromagnetic spectrum.236 The solvents for these determinations
are often water for water soluble compounds, or ethanol for organic-soluble compounds.
146
Note organic solvents may have significant UV absorption and not all solvents are
suitable for use in UV spectroscopy. Ethanol absorbs very weakly at most wavelengths.
Samples for UV/Vis spectrophotometry are most often liquids are typically placed
in a transparent cell, known as a cuvette. Cuvettes are typically rectangular in shape,
made of high quality quartz, commonly with an internal width of 1 cm. (This width
becomes the path length, L, in the Beer-Lambert law.) An ultraviolet-visible spectrum is
essentially a graph of light absorbance versus wavelength in a range of ultraviolet or
visible regions. Wavelength is often represented by the symbol λ. For the given
substance, the wavelength at which maximum absorption in the spectrum occurs is called
λmax. 236
It is well known that poly(p-phenylene)s absorb light in the UV or visible regions
due to their highly conjugated phenyl rings. Figure 63 shows UV-vis spectrum of
poly(2,5-benzophene) in CHCl3 and the absorption maximum λmax is at 352 nm, which is
good agreement with values for high molecular weight poly(2,5-benzophene) in
literature.178, 185 In addition, it has been well-established that the wavelength of
maximum absorbance in the p-phenylene series increases (bathochromic shifts) as the
number of conjugated phenyl rings increases. However, this does not necessarily mean
that the higher the λmax, the higher the molecular weight because larger conjugated
systems does not ensure longer polymer chain. This is mainly due to the fact that region-
isomers exist in this polymer backbone, which break up some conjugated phenyl rings.
This is also the reason why these poly(p-phenylene)s are not crystalline.
147
Figure 63 UV-vis spectrum of poly(2,5-benzophenone) in CHCl3
Poly(2,5-benzophenone)
-0.4
0
0.4
0.8
1.2
1.6
2
200 250 300 350 400 450 500
Wavelength (nm)
Abs
orba
ncee
U)
352 nm
256 nm
148
3.3 Disulfonated Poly (arylene ether sulfone) Copolymers
(BPSH series)
Poly(arylene ether sulfone)s are a promising class of proton exchange membranes
due to their excellent thermal and chemical stability and high glass transition
temperatures.237 High proton conductivity can be achieved through post-sulfonation of
poly(arylene ether) materials, but this most often results in very high water sorption or
even water solubility. In addition, post-sulfonation lacks good control of ionic groups’
distribution along the polymer chain.
Previously, our group has reported the synthesis and properties of several directly
polymerized disulfonated poly (arylene ether sulfone) random copolymers, which exhibit
high proton conductivity and excellent stability, and also reduced methanol permeability
and a higher upper limit use temperature compared to Nafion.10, 16, 18, 89, 102
3.3.1 Synthesis of Disulfonated Poly (arylene ether sulfone)
Copolymers (BPSH)
Figure 64 shows the synthesis of poly (arylene ether sulfone) random copolymers
via using biphenol with various molar stoichiometric amounts of dichlorodiphenyl
sulfone (DCDPS) and sulfonated dichlorodiphenyl sulfone (SDCDPS). The respective
molar amount of the sodium salt of 3,3’-disulfonated 4,4’-dichlorodiphenyl sulfone
(SDCDPS) was increased in increments of ten mole % while, in order to maintain
stoichiometric balance, the dichlorodiphenyl sulfone (DCDPS) molar amount was
correspondingly decreased. Series of sulfonated copolymerizations were attempted using
149
0 to 60 mol% sulfonated comonomer. This series of polymers are called BPSH-xx,
where BP stands for biphenol, S is for sulfonated, and H denotes the proton form of the
acid where xx represents the degree of disulfonation.
A typical reaction, using similar polymerization conditions as the control
polymers, is given for biphenol containing 40 mol% sulfonated dichlorodiphenyl sulfone
and 60 mol% unsulfonated sulfone monomer. First, 1.86 g (10 mmol) biphenol, 1.1487 g
(4 mmol) 4,4’-dichlorodiphenyl sulfone (DCDPS), 2.9476 g (6 mmol) disodium-3,3’-
Figure 64 Synthesis of “BPSH” series random copolymers via direct copolymerization
disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS) and 1.15 equivalents (1.6 g) of
potassium carbonate were added to a 3-neck flask fitted with a mechanical stirrer, a
nitrogen inlet and a Dean Stark trap. Dry N,N-dimethylacetamide (DMAc) (~30 mL)
was introduced to afford a 20% solids concentration, and toluene (12 mL) was used as an
azeotroping agent. The reaction mixture was refluxed at 150 ºC for 4 hours to remove
150
most of the toluene and dehydrate the system. Next, the reaction temperature was raised
slowly to 175 ºC for 48 hours. The viscous solution obtained was cooled and diluted
with DMAc to allow easier filtering. The reaction product was filtered to remove most
of the salts, and isolated by addition to stirred deionized water. The disulfonated
copolymers were dried under vacuum at 120 ºC for 24 hours after being washed several
times with deionized water.
The sulfonated copolymers were isolated from deionized water as swollen fibers.
The amount of swelling in the copolymers was a direct function of the degree of
sulfonation, based on the amount of sulfonated dichlorodiphenyl sulfone utilized. For
each sulfonated copolymer, films were cast on glass substrates by re-dissolving
copolymers as described in the experimental section.
In order to be used as proton exchange membranes in fuel cells, these BPS series
copolymers must be in their free-acid forms--BPSH. To convert the films into their acid-
form, they were typically boiled in dilute 0.5 M H2SO4 for 2h, followed by boiling in
deionized water for another 2h. We have published papers about the methods of
acidification for the copolymer.101 Boiling in dilute 0.5 M H2SO4 for 2h is sufficient to
fully acidify the copolymer, but does not increase the level of sulfonation and that was
proved by NMR before and after acidification.
3.3.2 Characterization of BPSH copolymers
Proton NMR is a powerful tool to identify and characterize BPSH series of
sulfonated copolymers. Integration and appropriate analysis of known reference protons
151
of the copolymer allowed for determination of the relative composition of the BPSH
series copolymers. Figure 65 shows a representative 1H NMR spectrum of the
copolymer BPSH-35. As shown in Figure 65, the protons adjacent to the sulfonated
group derived from the disulfonated dihalide monomer in the copolymer were well
separated from the other aromatic protons (~8.25 ppm). The degree of sulfonation is
determined by comparing integrals of the aromatic protons of the sulfonated sulfone
(SDCDPS) moiety a relative to those of unsulfonated sulfone (DCDPS) g. The
calculation for degree of sulfonation is provided for BPSH-35 copolymer. The
calculation concluded that 34.2 mol% of 3,3’-disulfonated-4,4’-dichlorodiphenyl sulfone
(SDCDPS) was incorporated into the random copolymer, which agrees well with target
value 35%. Similar calculations done for other BPSH series sulfonated copolymer are
tabulated on Table 2.
Figure 65 Proton NMR spectrum of BPSH 35 in d6-DMSO
7.92
2.00
8.5 8.0 7.5 7.0 6.5 PPM
c
g
b
e,i d,f,h
a
152
%2.34%100
42
2n sulfonatio of percentage Mole =×+
=ga
a
Equation 9
Intrinsic viscosities of BPSH series sulfonated copolymers were measured in
NMP with 0.05 M LiBr and listed in Table 2. All BPSH sulfonated copolymers show
high viscosities, varying from 0.7 to 0.9 dL/g. Recall that the intrinsic viscosity
measured in a specific solvent is related to the molecular weight, M, by the Mark-
Houwink equation.
αη vKM=][ Equation 8
Table 2 Degree of sulfonations and intrinsic viscosities of BPSH series of disulfonated copolymers
BPSH
series
DCDPS/SDCDPS
(mole ratio)
Degree of
sulfonation
(100%) a
Degree of
sulfonation
(100%) b
Intrinsic
viscosity [η] c
(dL/g)
BPSH-0 10/0 0 0 0.69
BPSH-10 9/1 10 9.6 0.77
BPSH-20 8/2 20 19.5 0.80
BPSH-35 6.5/3.5 35 34.2 0.92
BPSH-40 6/4 40 38.9 0.88
a Target value
b Experimental values determined by proton NMR
c NMP with 0.05M LiBr as solvent at 25 oC
153
Table 3 Influence of degree of disulfonation on several properties of BPSH copolymers
BPSH
series
Degree of sulfonation
(100%) a
Ion Exchange
Capacity b
(mequiv./g)
Water uptake
(wt. %)
Proton
Conductivity c
(S/cm)
BPSH-10 9.6 0.41 5 0.01
BPSH-20 19.5 0.88 12 0.03
BPSH-35 34.2 1.40 40 0.08
BPSH-40 38.9 1.50 45 0.09
a Experimental values determined by proton NMR
b Determined by back-titration of acid groups
c Measured in liquid water at 30 oC
However, common characterization of molecular weight in ion containing
systems, such as BPSH sulfonated copolymers, is complicated by the presence of ionic
groups (sulfonated groups) attached to the polymer backbone where ion-ion interactions
affect the characteristic size of the macromolecule in a solvent. This ion effect on chain
size is often termed as the polyelectrolyte effect. The addition of a strong electrolyte
(such as LiBr) to the solvent is helpful in suppressing the polyelectrolyte effect to allow
characterization of the ion containing materials. Therefore, intrinsic viscosity measured
in solvent of NMP with 0.05 M LiBr can be treated as reasonable data, and can thus be
compared with normal polymers without any ion groups.
154
This series of BPSH sulfonated random copolymers exhibit good conductivity
(0.08-0.09 S/cm) when the sulfonation degree is higher than 35%. As shown in Table 3,
the conductivity and water uptake of this series of copolymers increase with disulfonation
(or with increasing IEC). However, as we report previously, a semi continuous
hydrophilic phase was observed and the membranes swelled dramatically--once the
degree of disulfonation reached 60 mole %, resulting in a hydrogel that can not be used
as a proton exchange membrane. Therefore, to obtain a decent proton exchange
membrane, we here limit the ionic exchange capacity (IEC) to around 1.5 mequiv/g, or in
other words, to limit the sulfonation degree to around 35%, to balance the proton
conductivity with other physical properties.
155
3.4 Multiblock Copolymers
Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are
promising PEM materials, particularly due to their ability to form unique morphologies.
It is well known that the membrane morphology is important for the PEM fuel cell
performance, and depends strongly on the water content, and on the type concentration
and distribution of the acidic moieties.27 The unique morphologies of multiblock
copolymers may play an important role in providing good proton transport at low water
contents and high temperatures. 135
This chapter reports the end-capping of 2,5-dichlorobenzophenone with 4-chloro-
4’-fluorobenzophenone via Ni (0)-catalyzed coupling reaction to produce difunctional
poly (2,5-benzophenone) (PBP) oligomers with aryl fluoride endgroups, which are labile
to subsequent nucleophilic aromatic substitution. These functionalized PBP
(hydrophobic) telechelic oligomers were then combined with phenoxide terminated
disulfonated wholly aromatic biphenyl poly(arylene ether sulfone) oligomers (hydrophilic)
to produce high molecular weight multiblock copolymers capable of forming flexible
films. Randomization by ether–ether interchange reaction was obviously avoided.
Morphology studies on these multiblock copolymers as well as its relationship to proton
conductivity and water uptake have been initiated and are discussed.
156
3.4.1 Synthesis of poly (2,5-benzophenone) (PBP) telechelic
oligomers
As reviewed in Chapter 3.1 and 3.2, the 2,5-dichlorobenzophenone can react
readily via Ni(0)-catalyzed coupling to form isomeric high molecular weight poly(2,5-
benzophenone) (PBP) with excellent thermal properties.
Molecular weight and end-group functionality of step-growth polymers, such as
poly (aryl ether sulfone)s, can be controlled by the addition of a monofunctional end-
capping agent. Because of the step-growth nature of the Ni(0)-coupling polymerization
and relatively high rates of reaction, an aryl chloride monomer with a functionality of 1
can be introduced into the reaction mixture to end cap poly(2,5-benzophenone)s.175 Since
aryl fluorides do not participate in the Ni(0)-catalyzed coupling reaction, end capping of
poly(2,5-benzophenone)s with 4-chloro-4’-fluorobenzophenone will result in PBP chains
that contain aromatic fluoride end groups.180 These aryl fluorides activated by electron-
withdrawing group (carbonyl group) can be used as reactive sites for nucleophilic
aromatic substitution block copolymerization. Furthermore, the molecular weights of
these oligomers are controlled by altering the amount of 4-chloro-4’-fluorobenzophenone.
The end-capping agent 4-chloro-4’-fluorobenzophenone was synthesized by an
iron (III) chloride-catalyzed Friedel–Crafts acylation of chlorobenzene with 4-
fluorobenzoyl chloride. Sheares et al.180 and Ghassemi et al.164 had previously
demonstrated the use of this end-capping agent in the synthesis of substituted poly(2,5-
benzophenone) oligomers.
The synthesis of fluorine-terminated PBP telechelic oligomers, shown in Figure
66, was conducted using Ni (0)-catalyzed coupling method from 2,5-
157
dichlorobenzophenone and a controlled amount of the end-capping agent 4-chloro-4’-
fluorobenzophenone. The monomer and end-capping agent were added quickly under
nitrogen flow to the polymerization flask containing a mixture of nickel chloride, 2, 2’-
bypyridyl and triphenylphosphine in THF.
ClCl
C O
Cl CO
F
THF, 60oC, 12h
C OOO
FF
+NiCl2(PPh3)2, Zn, PPh3, Bpy
n
Figure 66 Synthetic scheme of fluorine terminated poly(2,5-benzophenone) oligomers
The molecular weights of PBP oligomers were controlled by varying the end-
capping agent stoichiometry. Note here although proton NMR has a much higher
sensitivity than 13C NMR (because the major isotope of carbon, the 12C isotope, has a
spin quantum number of zero and is not magnetically active) 238, there are so many
aromatic protons overlapping together, making it impossible to identify the end units and
determine the number average molecular weight Mn. Therefore, quantitive 13C NMR
spectra were used to determine the degree of polymerization of the PBP telechelics.
158
The peaks at 194.6 ppm and in the range of 198.3–196.2 ppm are assigned to
carbonyl moieties in the end-capping agent c and in the main chain d, respectively
(Figure 67). The integrals of peaks 194.6 ppm and in the range of 198.3–196.2 ppm are
compared to calculate the degrees of polymerization of the PBP telechelics. As
summarized in Table 4, the experimental values from 13C NMR agree well with the
target values. Other prominent features in the 13C NMR spectra are the signals from the
ipso carbon a attached to the fluoride and its adjacent carbon b from the end-capping
agent. The ipso carbon a is assigned as a doublet at 165.0 ppm with a coupling constant
of 255.1 Hz. The adjacent carbon b signal is clearly visible as a doublet at 115.3 ppm
with a coupling constant of 23.1 Hz.
Figure 67 13C NMR spectrum of a fluorine terminated poly(2,5-benzophenone) oligomer in CDCl3
d c a
b
C O
CO
CO
FF
n
ab
c
d
159
Figure 68 19 F NMR of a fluorine terminated poly(2,5-benzophenone) oligomer in CDCl3
19 F NMR was conducted to confirm the functionality of fluorine at each end of
poly(2,5-benzophenone) oligomers. As shown in Figure 68, only one peak shows up at
106.2-106.4ppm, which agrees well with literature value of 106.7-106.9ppm.164
As expected, intrinsic viscosities η of this series PBP oligomers increase with
molecular weight (Table 4). Intrinsic viscosity η can be related to molecular weight by
the Mark-Houwink-Sakurada equation
αη vKM=][ Equation 8
where Mv is viscosity average molecular weight; K and α are the Mark-Houwink
constant and the Mark-Houwink-Sakurada exponent. The plot of log [η] with log Mn ()
gave an approximately straight line and the Mark-Houwink-Sakurada exponent α of
these rigid PBP oligomers was found to be 0.83. The data confirm that the molecular
weight control of this series of PBP oligomers was successful.
160
The thermal properties of these low molecular weight PBP telechelics were found
to be very robust. A steady increase in the glass transition temperature Tg was observed
from 151 to 187 °C, as molecular weight increased from 2200 to 9400 g/mol (Table 4).
The highest Tg at 187 °C is close to that found for the non-end-capped homopolymer,
namely 190 °C.
Figure 69 Double logarithmic plot of [η] against Mn of fluorine terminated poly(2,5-benzophenone)
oligomers
y = 0.8253x - 3.5863
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1
Log Mn
Log [η]
161
Table 4 Characterization of fluorine terminated poly(2,5-benzophenone) oligomers
Polymer
Molar Fraction
of End-capping
Agent
Mn a
(g/mol)
Mn b
(g/mol)
Intrinsic
Viscosity c
[η] (dl/g)
Tg d
(oC)
1 0.20 2200 1980 0.16 151
2 0.10 4000 3800 0.21 163
3 0.07 5600 5500 0.32 167
4 0.05 7600 7000 0.46 176
5 0.04 9400 -- 0.48 187
6 0.00 -- -- 0.96 190
a Target molecular weight
b Determined from 13C NMR
c NMP as solvent at 25 oC
d Determined by DSC
162
Figure 70 UV-vis spectra of a series of poly(2,5-benzophenone) oligomers
Figure 70 shows the UV-vis spectra of a series of poly(2,5-benzophenone)
oligomers with different molecular weight, varying from 2,200 to 9,400 g/mol. Recall
that when an atom or molecule absorbs energy, electrons are promoted from their ground
state to an excited state. As one might expect, they all exhibit similar absorptions at two
wavelength regions: one is around 256 nm, which is assigned to the jump of π→π*; the
other is in the neighborhood of 350 nm, which is corresponded to the promotion of n→π.
It has been well-established that the wavelength of maximum absorbance in the p-
phenylene series increases (bathochromic shifts) as the number of conjugated phenyl
rings increases.
-0.5
0
0.5
1
1.5
2
200 250 300 350 400
7.6k
control
2.2k
9.4k
5.6k
4.0k
Wavelength (nm)
Absorbanc
163
However, it seems that the wavelength of maximum absorbance λmax does not
correlate very well with molecular weight for this series of poly(2,5-benzophenone)
oligomers. (Table 5) This is probably because the regiochemical irregularity (Head to
Head and Tail to Tail) disrupts the conjugation along the polymer backbone. Thus, there
is no clear trend in the wavelength of maximum absorbance as molecular weight
increases. Nevertheless, for the sample PBP-control (uncapped polymer), the absorption
maximum λmax is at 352 nm, which is match well with the values for high molecular
weight poly(2,5-benzophene) in literature.178, 185
Table 5 Summary of UV-vis λmax of a series of poly(2,5-benzophenone) oligomers
Samples λ1 (nm) λmax2 (nm)
PBP-2.2k 256 346
PBP-4.0k 256 344
PBP-5.6k 256 342
PBP-7.6k 256 346
PBP-9.4k 256 350
PBP-control 254 352
164
Figure 71 Synthetic scheme of fluorophenyl sulfone terminated PBP oligomers
Another endcapping agent can be used in synthesizing fluorine terminated PBP
oligomers is 4-chlorophenyl-4’-fluorophenyl sulfone, as shown in Figure 71. Because
sulfone is a stronger electron-withdrawing group than ketone, it would promote
nucleophilic substitution reaction in the next coupling reaction. Thus, 4-chloroph enly-4’-
fluorophenyl sulfone is a better endcapping agent than 4-chloro-4’-fluorobenzophenone.
PBP telechelic oligomers endcapped with fluorophenyl sulfone group were also prepared
in the same way as described above. The molecular weights of PBP oligomers were
controlled by varying the end-capping agent stoichiometry.
Figure 72 shows the 19F NMR spectra of a fluorophenyl sulfone-terminated and
fluorophenyl ketone-terminated PBP telechelic oligomers. This again confirms the
functionality of fluorine at each end of PBP oligomers. Additionally, the fluorine peak
ClCl
C O
Cl SO
F
THF, 60oC, 12h
C O
SO
SO
FF
+NiCl2(PPh3)2, Zn, PPh3, Bpy
n
a bO
OO
Mn = 5k to 10k
2,5-dichlorobenzophenone 4-chlorophenly-4’-fluorophenyl sulfone
ClCl
C O
Cl SO
F
THF, 60oC, 12h
C O
SO
SO
FF
+NiCl2(PPh3)2, Zn, PPh3, Bpy
n
a bO
OO
Mn = 5k to 10k
2,5-dichlorobenzophenone 4-chlorophenly-4’-fluorophenyl sulfone
165
shifts from -106 ppm in ketone-terminated PBP to -104 ppm in sulfone-terminated PBP,
which is presumably due to the stronger electron-withdrawing nature of sulfone.
Figure 72 19 F NMR of a fluorophenyl sulfone-terminated and fluorophenyl ketone-terminated PBP
telechelic oligomers in CDCl3
Proton NMR was used to determine the number average molecular weight of
these fluorophenyl sulfone-terminated PBP oligomers. Compared with 13C NMR, proton
NMR has a much higher sensitivity because the major isotope of carbon, the 12C isotope,
has a spin quantum number of zero and is not magnetically active 238. Thus, proton NMR
offers a much better way than 13C NMR to characterize number average molecular
weight. Figure 73 shows proton NMR of a fluorophenyl sulfone-terminated PBP
oligomer with target Mn of 6,000 g/mol. Number-average molecular weight Mn was
determined by comparing integrals of the aromatic protons of the phenyl sulfone moiety
in the terminal group a,b relative to the rest protons in repeat units.
100 -102 -104 -106 -108 PPM
F19 OBS ERVEST ANDARD PARAM ET ERS
F19 OBS ERVEST ANDARD PARAM ET ERS
Fluorophenyl sulfone-terminated
Fluorophenyl ketone-terminated -106 ppm
-104 ppm
100 -102 -104 -106 -108 PPM
F19 OBS ERVEST ANDARD PARAM ET ERS
F19 OBS ERVEST ANDARD PARAM ET ERS
Fluorophenyl sulfone-terminated
Fluorophenyl ketone-terminated -106 ppm
-104 ppm
166
For example:
g/mol 5700 272280192
unit end ofweight Molecular 2unit)repeat ofweight Molecular (n) unitsrepeat ofNumber (
g/mol 272unit end ofweight Molecular g/mol 801unitrepeat ofweight Molecular
29n 240 48n protonsrest of intergel 8, Hb and Ha of Intergel
=×+×=
×+×=
==
=⇒=+==
nM
Figure 73 Proton NMR of a fluorophenyl sulfone-terminated PBP oligomer with target Mn of 6,000
g/mol
3.4.2 Synthesis of disulfonated poly(arylene ether sulfone) (PAES)
telechelic oligomers
Hydroxy (phenoxide) terminated sulfonated poly(arylene ether sulfone) (PAES)
telechelics were synthesized (Figure 74) as hydrophilic blocks in multiblock copolymers.
8.00 4.43
240.31
8.0 7.5 7.0 6.5 6.0 PPM
a,b
C O
SO
SO
FF
n OO
a b c d
c
d 8.00 4.43
240.31
8.0 7.5 7.0 6.5 6.0 PPM
a,b
C O
SO
SO
FF
n OO
a b c d
c
d
a,b
C O
SO
SO
FF
n OO
a b c dC O
SO
SO
FF
n OO
a b c d
c
d
167
The 4,4’-biphenol monomer was used in excess base on the total amount of disulfonated
4,4’-dichlorodiphenylsulfone (SDCDPS) to produce telechelics with the target number
average molecular weight of 3000, 6000 and 10,000 g/mol.
SClO
OCl
+NaO3S
SO3Na+
OHHO
SO
OMO3S
SO3M
OOH OH
BPS100
(in excess)
K2CO3
NMP/Toluene Reflux 150oC for 4hrs190oC for 36 hrs under N2
n
+
O
endgroups were acidified byacetic acid
Figure 74 Synthetic scheme for hydroxy terminated disulfonated poly(arylene ether sulfone)
oligomers
In order to use proton NMR to determine the number average molecular weight
Mn of this series of telechelic oligomers, Two-dimensional COSY (COrrelation
SpectroscopY) experiment was first performed to confirm the protons assignments in
proton NMR, as shown in Figure 75. It is well known that 2-D COSY NMR determines
168
the connectivity of a molecule by determining which protons are spin-spin coupled.239, 240
Usually, COSY experiment gives information regarding three-bond couplings (from a
proton to its carbon, to the adjacent carbon, then to that carbon's proton). Off-diagonal
peaks denote splitting between protons on adjacent carbons.240, 241 Thus, the protons are
assigned as in Figure 76.
Figure 75 2D-COSY spectrum (COrrelation SpectroscopY) of a phenol terminated disulfonated
poly(arylene ether sulfone) oligomer
85 80 75 70 65
8.5
8.0
7.5
7.0
PP
M In
dire
ct D
imen
sion
1
g
fa
i d
b
h c
g
f
ai
d
b
e
c
85 80 75 70 65
8.5
8.0
7.5
7.0
PP
M In
dire
ct D
imen
sion
1
g
fa
i d
b
h c
85 80 75 70 65
8.5
8.0
7.5
7.0
PP
M In
dire
ct D
imen
sion
1
g
fa
i d
b
h c
g
f
ai
d
b
e
c
169
Figure 76 Proton NMR spectrum of a phenol terminated disulfonated poly(arylene ether sulfone)
oligomer
Number-average molecular weight Mn was determined by comparing integrals of
the aromatic protons of the biphenol moiety in the terminal group c relative to those in
repeat units a. Peaks at 7.71 ppm and 6.84 ppm are assigned to c and a, respectively.
For example:
g/mol 5900 1806379.0
unit end ofweight Molecular unit)repeat ofweight Molecular (n) unitsrepeat ofNumber (g/mol 180unit end ofweight Molecular
g/mol 637unitrepeat ofweight Molecular
0.9aproton of Intergralcproton of Intergraln unitsrepeat ofNumber
=+×=
+×==
=
===
Mn
HaHc
OO S
O
O
OHH
n
SO3K
KO3S
b a a b e f
g
g
f e
h i d cj
170
Figure 77 13 C NMR spectrum of a phenol terminated disulfonated poly(arylene ether sulfone)
oligomer
13 C NMR (Figure 77) was also conducted to confirm the structural of this series
of phenol terminated disulfonated poly(arylene ether sulfone) oligomers. By comparing
the carbon peak in repeat units (for example C-1 at 157 ppm) with carbon peak in end
units (for example C-11 at 156 ppm), one can calculate the degree of polymerization and
thus the number-average molecular weight Mn. However, carbon NMR is much less
sensitive than proton NMR since the major isotope of carbon, the 12C isotope, has a spin
quantum number of zero and is not magnetically active. Only the less common 13C
isotope present naturally at 1.1% abundance is magnetically active with a spin quantum
number of 1/2 much like a proton. Therefore, only the few carbon-13 nuclei present
resonate in the magnetic field, resulting in reduced sensitivity. 238 For this reason, we
SO
O
SO3M
OOM OM
MO3S
O1
2 34
n
56 7
8
910
1112
C-4C-10
C-7
C-9
C-6 C-3
C-2
C-5 C-1 C-8
C-11 C-12
171
used much more sensitive proton NMR to determine the number-average molecular
weight Mn.
As shown in Table 6, for this series of PAES oligomers, Mn determined by proton
NMR agree well with the target values. Besides, the intrinsic viscosity [η] increases from
0.15 to 0.35 dL/g as number-average molecular weight Mn increases from 3,000 to 10,000
g/mol. These results indicate that the synthesis of sulfonated poly(arylene ether sulfone)
(PAES) telechelic oligomers was successful and number-average molecular weight Mn
can be readily controlled by upsetting the stoichiometry of polycondensation.
Table 6 Properties of hydroxy terminated poly(arylene ether sulfone) oligomers
Polymer
Molar Excess of
Biphenol
Monomer (%)
Mn a
(g/mol)
Mn b
(g/mol)
Intrinsic
Viscosity c
[η] (dl/g)
1 22.6 3000 3100 0.15
2 10.9 6000 5900 0.24
3 6.40 10,000 9900 0.35
a Target molecular weight
b Determined from proton NMR
c NMP with 0.05M LiBr as solvent at 25 oC
172
3.4.3 Synthesis of PAES-PBP Multiblock Copolymers
In general, one of the drawbacks of poly(2,5-benzophenone) (PBP) is their
inability to form flexible films. On the other hand, PAES forms excellent films but lacks
the high modulus and strength of PBP. To develop more flexible, film-forming PBP-base
materials, hydroxy terminated sulfonated PAES telechelics (hydrophilic) prepared above
were copolymerized with fluorine terminated PBP telechelics (hydrophobic) via
nucleophilic aromatic substitution (Figure 78).89
K2CO3
OOFF
C O
PBP
SO
OO
MO3S
SO3M
OO OKK
BPS100
SO
OO
MO3S
SO3M
OO OOO
C O
m
NMP/Toluene
150oC, 4h190oC, 48h
n+
x n
Hydrophilic Hydrophobic
+ KFy
Figure 78 Synthetic scheme of a PAES-PBP hydrophobic-hydrophilic multiblock copolymer
The stoichiometry of the copolymerization was based on a 1:1 molar ratio
between functional end groups of hydrophilic (PAES)/hydrophobic (PBP) oligomers
prepared above. Randomization, which is to be expected in the high temperature
poly(arylene ether) multiblock copolymers forming step, was avoided since there is no
ether–ether interchange possible in the PBP blocks. By varying the block length of
173
hydrophilic (PAES) /hydrophobic (PBP) telechelics from 3,000 to 10,000 g/mol, three
multiblock copolymers: PAES-3k-PBP-3k, PAES-6k-PBP-6k and PAES-10k-PBP-10k
were synthesized.
3.4.4 Characterization of Multiblock Copolymers
1H and 13C NMR and FTIR were employed to provide structural and
compositional characterizations of the PAES-PBP multiblock copolymers synthesized
from the coupling reaction. Thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) were used to explore the thermal behavior of these novel
multiblock copolymers. Gel permeation chromatograph (GPC) was utilized to shed more
light on molecular weight as well as molecular distribution of these multiblock
copolymers. Atomic force microscopy (AFM) was carried out to explore the surface
morphology of the hydrophilic-hydrophobic multiblock copolymers. X-ray
Photoelectron Spectroscopy (XPS) experiment was performed to give qualitative and
quantitative elemental analysis for the surface of multiblock copolymer films. Proton
conductivity, water uptake and ion change capacity (IEC) of these multiblock copolymers
were also measured to examine their potentials to use as proton exchange membrane in
fuel cell.
3.4.4.1 1H and 13C NMR Spectroscopy
Proton NMR spectroscopy has been repeatedly employed to provide structural
confirmation of the synthesized polymers.215 Figure 79 shows the proton NMR spectrum
of PAES-6k-PBP-6k multiblock copolymer with block lengths of approximately 6,000
174
g/mol. Compared with proton NMR spectra of PBP and PEAS oligomers (Figure 76), it
is very clear that this spectrum contains the characteristic proton peaks from both PBP
and PEAS oligomers, indicating good incorporation of both oligomers. For instance, the
two broad peaks at 7.5 ppm and 7.3 ppm belong to the aromatic protons in PBP’s
phenylene backbone; while the unique peak at 8.3 ppm comes from the aromatic protons
in the repeat unit of polysulfone (PAES).
Figure 79 Proton NMR of PAES-PBP hydrophobic-hydrophilic multiblock copolymer with block
lengths of approximately 6000 g/mol
In addition, the proton peak at 6.8 ppm, which is assigned to aromatic protons of
the biphenol moiety in the terminal group in PAES oligomers (refer to Figure 76),
completely disappears in the multiblock copolymer spectrum. This peak is supposed to
reflect the connecting units between two blocks and the disappearance of this peak
strongly suggest that high molecular weight multiblock copolymers were successfully
175
synthesized. In other words, the proton peaks in connecting units are so small compared
with protons in each blocks that one can not observe them any more.
Figure 80 13C NMR spectra of a PAES-PBP multiblock copolymer (a), a PBP oligomer (b) and a
PAES oligomer (c).
Figure 80 exhibit the comparison of 13C NMR of a PAES-PBP multiblock
copolymer (a), a PBP oligomer (b) and a PAES oligomer (c). As expected, the
multiblock copolymer spectrum shares characteristic peaks of both PBP and PAES
oligomers. For example, the peaks at 195-197 ppm, a unique peak for carbonyl groups
in the PBP lateral substituent, were also observed in the spectra of multiblock copolymers;
176
on the other hand, the characteristic peaks of PAES at 159 ppm and 158 ppm also can be
found in the spectra of multiblock copolymers. This again confirms that both PBP and
PAES oligomers have been incorporated into multiblock copolymers.
3.4.4.2 Fourier Transform Infrared Spectroscopy (FT-IR)
Fourier transform infrared spectroscopy is a powerful tool used to characterize the
functional groups in a material. Usually, the mid- infrared (approx. 4000-400 cm-1) is
more useful in studying the fundamental vibrations and associated rotational-vibational
structure. FT-IR instrument measure all wavelengths at once and a transmittance or
absorbance spectrum may be plotted. It basically shows at which wavelengths the sample
absorbs the IR, and allows an interpretation of which bonds are present. This technique
works almost exclusively on covalent bonds, and as such is of most use in organic
chemistry.
177
Figure 81 FT-IR spectrum of a PAES-PBP multiblock copolymer
FT-IR experiment was conducted to confirm the chemical structure of PAES-PBP
multiblock copolymer (Figure 81), especially shed more light on the functional groups.
As one can observed from the spectrum, a strong peak at 1666 cm-1 are corresponded to
lateral carbonyl groups in PBP blocks; while two peaks at 1027 and 1096 cm-1 (1030 and
1098 in literature89) clearly come from a vibrational symmetric and asymmetric
stretching of the sulfonic acid groups in PAES segments. This result confirms that
PAES-PBP multiblock copolymers contain both PBP and PAES blocks and multiblock
copolymers were successfully synthesized.
1006
10271096
1666
1164
1006
10271096
1666
1164
1006
10271096
1666
1164
178
3.4.4.3 Gel Permeation Chromatography (GPC) and Intrinsic Viscosity
[η] (IV)
Gel Permeation Chromatography (GPC) is a powerful, fast and economic way to
determine MW of polymers in which molecules are separated on the basis of
hydrodynamic volume or size.242 With proper column calibration or by the use of
molecular-weight-sensitive detectors, such as light scattering, viscometry, or mass
spectrometry, the molecular weight distribution (MWD) and the statistical molecular
weight averages can be obtained readily. 224-226
However, characterization of MW in ion containing systems (in this case
hydrophilic-hydrophobic multiblock copolymers) is complicated by the presence of ionic
groups attached to the polymer backbone. Ion-containing polymers are usually termed as
“polyelectrolytes”,227 which generally describe any macromolecules which can dissociate
into highly charged polymeric molecules upon being placed in water or other ionizing
solvent irrespective of their ion content.243, 244 As shown in Figure 82, it is well known
that a polyelectrolyte strongly extend with dilution due to the mutual repulsion of the
charges on the polymer backbone.227 Therefore, it is especially important to allow the
macromolecular coils to fully swell in the solvent or to break down any aggregates in
GPC measurements.
179
Figure 82 Comparison of neutral and ion-containing polymers with different ion concentration.
When the ionic strength of the solution is increased, a polyelectrolyte tends to
become more coiled due to the screening effects of polymer charges by the excessive
presence of smaller salt counterions in solution as shown in Figure 83. 245 This
characteristic behavior of polyelectrolyte was often explained by counterion condensation
and chain expansion due to intramolecular repulsion.245 Therefore, the conformation of
polyelectrolyte chain is strongly influenced by the strength of electrostatic interaction,
which may be moderated by the addition of small-molar-mass salt. It has been
established experimentally that the larger the amount of the added salt, the closer the
macromolecular behavior of the polyelectrolyte solutions approaches that of solutions of
conventional, uncharged macromolecules.246-249 However, high concentrations of salt
may also lead to clog of the columns and irreproducible results.250 Plus, limited solubility
of the salt in solvent is another big problem.
c < c* c = c* c* < c < c** c = c**
Neutral macromolecules keep the coil conformation, even below the overlap concentration.
Charged macromolecules strongly extend with dilution due to the mutual repulsion of the charges on the polymer backbone.
c < c* c = c* c* < c < c** c = c**
Neutral macromolecules keep the coil conformation, even below the overlap concentration.
Charged macromolecules strongly extend with dilution due to the mutual repulsion of the charges on the polymer backbone.
c < c* c = c* c* < c < c** c = c**
Neutral macromolecules keep the coil conformation, even below the overlap concentration.
Charged macromolecules strongly extend with dilution due to the mutual repulsion of the charges on the polymer backbone.
180
Figure 83 Effect of ion strength on the shape of a polyelectrolyte molecular in solution
In view of the above discussion, we chose NMP with 0.05 M LiBr as a solvent
both in GPC and Intrinsic viscosity measurements for this series of hydrophilic-
hydrophobic multiblock copolymers. LiBr is good salt with very good solubility in NMP
and 0.05 M concentration is proved to be good enough to suppress the “polyelectrolyte
effect” but not easily cause the clog of the columns in GPC instrument. 251
These multiblock copolymers, as shown in Table 7, exhibit Mn and Mw in the
range of 36,000 to 66,000 g/mol. Additionally, they all showed reasonable high intrinsic
viscosity [η] from 0.68 to 0.82 dL/g in NMP (0.05 M LiBr). Furthermore, they can all be
solvent-cast into transparent and creasable films, which can then be used in proton
conductivity and water uptake measurements. Therefore, both GPC and IV data again
confirm that high molecular weight multiblock copolymers were successfully synthesized.
181
Table 7 Molecular Weight Data of PAES-PBP Multiblock Copolymers
No.
Hydrophilic
Block
Length
Hydrophobic
Block
Length
Mn
(× 103 g/mol)
Mw
(× 103 g/mol)
Intrinsic
Viscosity
(dL/g)
PDI
1 3,000 3,000 36.5 62.8 0.68 1.72
2 6,000 6,000 38.5 61.4 0.72 1.60
3 10,000 10,000 56.9 65.9 0.82 1.16
3.4.4.4 Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry (DSC) is a technique for measuring the energy
necessary to establish a nearly zero temperature difference between a substance and an
inert reference material, as the two specimens are subjected to identical temperature
regimes in an environment heated or cooled at a controlled rate.
DSC is one important characterization technique utilized for determining the glass
transition temperature Tg and the crystalline melting transition temperature Tm for
polymers.220
As is well known, multiblock copolymers consist of chemically distinct blocks
covalently linked to form a single molecule. Owing to mutual repulsion between
dissimilar blocks, they tend to segregate into separate nanophases. However, because
each block is chemically joined, macroscopic phase separation is prevented. Instead,
nanophase separation takes place. 138 As a result, two separate glass transition
temperatures (Tgs) may be observed by methods such as DSC.
182
Figure 84 DSC thermogram of PAES-PBP hydrophobic-hydrophilic multiblock copolymer with
block lengths of approximately 6000 g/mol showing two Tgs
Figure 84 shows the DSC thermogram of a PAES-6k-PBP-6k multiblock
copolymer with block lengths of approximately 6000 g/mol. Two glass transition
temperatures (Tgs) were observed: one is 174 oC, which assigned to the Tg of hydrophobic
PBP segment; the other is around 224oC, which corresponded to the Tg of hydrophilic
disulfonated PAES block. This suggests nanophase separation happened due to the
immiscibility of the PBP and PAES blocks. This is expected since PBP and PAES are
quite different in terms of chemical structures: PBP has very rigid polymer chain while
PAES backbone is much more flexible because of the arylene-ether bonds. This two Tgs
phenomenon was also observed in a PAES-10k-PBP-10k sample (Tgs: 180oC and 225oC)
Temperature (oC)
Heat Flow
(W/g)
183
but only one Tg (225oC) was found in a PAES-3k-PBP-3k multiblock copolymer (Table
8). This is expected since longer blocks should increase the immiscibility between
dissimilar blocks, favoring the nanophase separation in the multiblock systems. 138
Recall that the theory of nano-phase separation has been developed by Leibler et
al. 142 in the 1980s and the essential conclusion is that, at equilibrium, the state of the
system is determined by only two relevant parameters: the polymer chain composition f
and the product χN. The composition of the A-type segments f =NA/N and N=NA+NB,
where NA and NB are degrees of polymerization of A-type and B-type blocks. χ is the
Flory-Huggins interaction parameter and N is the number of monomers along the
copolymer backbone. For a symmetric (50/50 vol%) copolymer, f = 0.5, the transition
from a disordered state to one possessing long-range order occurs when χN = (χN)c ≈ 10.5
in the absences of critical fluctuation. 144 However, Flory-Huggins interaction parameter
χ is a fairly complicated parameter and the χ value of poly(arylene ether sulfone) is still
unknown.
Nevertheless, two separate Tgs were observed via DSC for multiblock copolymers
when the block length was longer than 6,000 g/mol. This strongly suggests that nano-
phase separation take place due to the immiscibility of two blocks.
3.4.4.5 Thermogravimetric Analysis (TGA)
Thermal Gravimetric Analysis (TGA) is a simple analytical technique that
measures the weight loss (or weight gain) of a material as a function of temperature. As
materials are heated, they can loose weight from a simple process such as drying, or from
184
chemical reactions that liberate gasses. Some materials can gain weight by reacting with
the atmosphere in the testing environment. 233, 234
Table 8 DSC and TGA results for PAES-PBP multiblock copolymers
No.
Hydrophilic
Block
Length
Hydrophobic
Block
Length
Tgsa
(oC)
5% weight
loss (air)b
(oC)
5% weight
loss (N2)b
(oC)
1 3,000 3,000 225 413 440
2 6,000 6,000 174, 224 445 465
3 10,000 10,000 180, 225 459 486
a Determined by DSC during second run with heating rate of 10 oC/min
b Obtained by TGA at heating rate of 10 oC/min
The thermal and thermo-oxidative stability as a function of weight loss of this
series of PAES-PBP multiblock copolymers (salt form) were investigated by
thermogravimetric analysis (TGA). All the samples were pre-heated at 150oC for 30
minutes in TGA furnace to remove trace solvent and moisture. Then, the dynamic TGA
experiments were run from 50 to 700 oC, at a heating rate of 10oC/min under nitrogen or
air.
These multiblock copolymers (salt form) exhibit high thermal stability with 5%
weight loss temperature varies from 440 to 486 oC in nitrogen and 413 to 459 oC. This is
probably due to the excellent thermostability of both the wholly aromatic PBP blocks and
PAES segments.
185
Figure 85 TGA thermogram of a PAES-PBP multiblock copolymer, a BPS 40 random copolymer and
a PBP homopolymer
Figure 85 shows the comparison of TGA thermogram of a PAES-PBP multiblock
copolymer, a BPS 40 random copolymer and a PBP homopolymer. Usually, block
copolymers offer no advantages over comparable homopolymers or random copolymers
in terms of thermal and thermal-oxidative stability. This is because a macromolecule is
only as stable as its most sensitive bond. It can be clearly seen that these multiblock
copolymers (salt form) show comparable thermal stability with BPS random copolymers
and PBP homopolymers. Thus, to some extent, this indicates that high molecular weight
Multiblock
BPS random
PBP homopolymer
186
PAES-PBP multiblock copolymers were successfully synthesized and the linkage units
are pretty thermal stable.
3.4.4.6 Atomic force microscopy (AFM)
The atomic force microscope (AFM) system has evolved into a useful tool for
direct measurements of micro-structural parameters and unraveling the intermolecular
forces at nanoscale level with atomic-resolution characterization. 252, 253
Typically, these micro-cantilever systems are operated in three open-loop modes;
non-contact mode, contact mode, and tapping mode.253 In order to probe electric,
magnetic, and/or atomic forces of a selected sample, the non-contact mode is utilized by
moving the cantilever slightly away from the sample surface and oscillating the cantilever
at or near its natural resonance frequency. Alternatively, the contact mode acquires
sample attributes by monitoring interaction forces while the cantilever tip remains in
contact with the target sample.
The tapping mode of operation combines qualities of both the contact and non-
contact modes by gleaning sample data and oscillating the cantilever tip at or near its
natural resonance frequency while allowing the cantilever tip to impact the target sample
for a minimal amount of time. As a matter of fact, the technique of tapping mode AFM
(T-AFM) is a key advance in AFM technology. This technique allows high resolution
imaging of soft samples that are difficult to examine using the contact AFM technique. It
overcomes problems such as friction and adhesion that are usually associated with
conventional AFM imaging systems.253
187
Compared with electron microscopes such as TEM and SEM, T-AFM provides a
true three-dimensional surface profile. Additionally, samples viewed by AFM do not
require any special treatments (such as metal/carbon coatings) that would irreversibly
change or damage the sample. While an electron microscope needs an expensive vacuum
environment for proper operation, most AFM modes can work perfectly well in ambient
air or even a liquid environment.254
Tapping mode AFM images were obtained under ambient conditions on a 500 nm
× 500 nm size scale to investigate nanophase separation for multiblock copolymers of
PAES-6k-PBP-6k and PAES-10k-PBP-10k (Figure 86). The dark regions of the phase
images are assigned to the softer blocks, which represent the hydrophilic ionic groups
(sulfonic acid) containing small amounts of water. The light regions are related to hard
segments, derived from the hydrophobic hydrocarbon units. It can be clearly seen that
for the PAES-6k-PBP-6k multiblock sample, the dark regions (hydrophilic domains) are
connected to each other and form channels and networks that extend through the whole
sample surface; while light regions (hydrophobic domains) are divided into small
domains, which have diameters of 20 to 40 nm. This nanophase separation phenomenon
develops further in the PAES-10k-PBP-10k sample, where even the light regions
(hydrophobic domains) start to aggregate too, forming bigger and longer hydrophobic
continuous phases. This again confirms that nanophase separation took place in these
hydrophilic-hydrophobic multiblock copolymers and is considered significant in terms of
the transport properties.135
188
Figure 86 Tapping mode AFM phase images of (a) a PAES-6K-PBP-6K multiblock copolymer, and
(b) a PAES-10K-PBP-10K multiblock copolymer with block lengths of approximately 6000 and
10,000 g/mol, respectively.(Scan sizes are 500 nm; Z ranges for phase images are 10 o)
3.4.4.7 X-ray Photoelectron Spectroscopy (XPS)
X-ray Photoelectron Spectroscopy (XPS) is a surface chemical analysis technique
that can be used to analyze the chemistry of the surface of a material. In fact, XPS is
arguably the most useful surface analysis technique because of the excellent qualitative
and quantitative elemental analysis it provides. In 1967, it was established that energy
analyzed electrons, photo-emitted during the irradiation of a solid sample by
monochromatic x-rays, exhibited sharp peaks corresponding to the binding energy of
core-level electrons. These binding energies can be used to identify the chemical
constituents of materials to a depth of 50 Å, and can distinguish between oxidation states
of the elements analyzed.
189
Since these hydrophilic-hydrophobic multiblock copolymers were cast on clean
glass to form films, one may wonder if there is any element enrichment on either side of
the films. Thus, XPS was utilized to examine both the air side and glass side of the films.
Table 9 summarizes the XPS data of a PAES-10k-PBP-10k multiblock copolymer.
Basically, atomic % of sulfur only reflects the PAES hydrophilic blocks enrichment at the
surface because of its only source is from the sulfones and sulfuric acids. However,
atomic % values at each side are quite close to each other, meaning no significant
element difference between air and glass sides of the films. This result also suggests that
since there is no real difference between each side of the membrane, it does not matter
which side to put in when these films are made into MEA to test in real fuel cell
instrument.
Table 9 XPS data of a PAES-PBP multiblock copolymer
C (1s)
(Atomic %)
O (1s)
(Atomic %)
S (2p)
(Atomic %)
Air-side 86.5 12.7 0.9
Glass-side 88.1 11.1 0.8
3.4.4.8 Proton conductivity and water uptake
The hydrophilic-hydrophobic nanophase separation morphology is considered to
be particularly good for PEM materials because it may increase the water self diffusion
coefficient, which may facilitate proton transportation at even low humidities.
190
Recently, Roy and McGrath 135 investigated of the effect of chemical composition,
morphology, and ion exchange capacity (IEC) on the transport properties of proton
conducting membranes. The self diffusion coefficient of water was measured by pulsed-
field gradient spin echo nuclear magnetic resonance (PGSE NMR) technique.255-261 They
found that both proton conductivity and self diffusion coefficient of water increased with
an increase in block length and irrespective of the IEC values. This clearly emphasizes
the importance of connectivity between the hydrophilic domains on the transport
properties.
Since protons are able to migrate through these continuous hydrophilic channels
through the membrane with less hindrance by hydrophobic domains, theoretically,
smaller amounts of water (lower relative humidities) would be required to assist the
proton transport. Moreover, extensive swelling, which is very normal for most ion
containing hydrocarbon random copolymers, can be minimized.
This series of multiblock copolymers showed surprising low water uptake values
of only 7%~10% (Figure 87). These values are not only lower than most ion-containing
random copolymers such as poly(arylene ether sulfone)s (BPSH-series, where BP stands
for biphenol, S is for sulfonated, and H denotes the proton form of the acid)89, but even
lower than highly hydrophobic perfluorinated copolymers like Nafion®. The reason
behind this interesting phenomenon is not yet clear, but one possible explanation is the
rigid hydrophobic phase may be important.
191
38
21
7 7
10
0
5
10
15
20
25
30
35
40
Wat
er u
ptak
e %
BPSH 35 Nafion 112 Multiblock(PAES-3k-PBP-3k)
Multiblock(PAES-6k-PBP-6k)
Multiblock(PAES-10k-PBP-10k)
BPSH 35Nafion 112 Multiblock (PAES-3k-PBP-3k) Multiblock (PAES-6k-PBP-6k) Multiblock (PAES-10k-PBP-10k)
Figure 87 Comparison of water uptake of BPSH 35, Nafion, and PAES-PBP multiblock copolymers
The proton conductivities of these multiblock copolymer membrane samples were
measured under full hydrated condition in water at room temperature. The acid form
membranes showed conductivity of 0.03, 0.04 and 0.06 S/cm, for PAES-3k-PBP-3k (IEC
0.95 meq/g), PAES-6k-PBP-6k (IEC 1.05 meq/g), PAES-10k-PBP-10k (IEC 1.10 meq/g),
respectively (Table 10). These values are considered to be good considering low IEC and
very low uptake. Thus, these multiblock copolymers are promising PEM materials for
fuel cells. However, for all three multiblock copolymers the experimental IEC values
were lower than the theoretical values (1.57 meq/g) calculated for an ideal alternating
192
multiblock, indicating that there was somewhat less than quantitative incorporation of
sulfonated telechelics.
Table 10 Characterization of PPP-PAES hydrophobic-hydrophilic multiblock copolymers
Multiblock
copolymers
Hydrophilic
block
length
Hydrophobic
block
length
Intrinsic
viscosity
(dl/g)a
Ion
exchange
capacity
(meq/g)b
Ion
exchange
capacity
(meq/g)c
Water
uptake
Proton
conductivity
(mS/cm)d
1 3000 3000 0.68 1.57 0.95 7% 30
2 6000 6000 0.72 1.57 1.05 7% 40
3 10,000 10,000 0.82 1.57 1.10 10% 60
a NMP with 0.05M LiBr as solvent at 25 oC
b. Target value
c Determined by back-titration of acid groups
d Measured in liquid water at 30 oC
Proton exchange membrane materials that enable higher temperature operation
would be preferred in vehicle applications because it demands smaller radiator, higher
CO tolerance and faster cathode reaction kinetics. For a higher temperature system to be
feasible, the membrane must have improved proton conductivity at low relative humidity
(RH) vs. current materials. For determining proton conductivity under partially hydrated
conditions, membranes were equilibrated in a relative humidity chamber (ESPEC SH-240)
at the specified relative humidity (RH) and 80 °C for 24 h before measurement.
193
0.0001
0.001
0.01
0.1
1
30 40 50 70 90 100
Relative Humidity (%)
Prot
on C
ondu
ctiv
ity (S
/cm
)
Nafion 117BPSH 35Multiblock
Figure 88 Influence of relative humidity on proton conductivity for Nafion 117, BPSH 35 and
multiblock copolymer at 80 oC.
Figure 88 exhibits the plot of proton conductivities vs relative humidity (RH) for
three different PEM materials: Nafion 117, BPSH 35 and a PAES-6k-PBP-6k multiblock
copolymer. Three materials all show similar proton conductivity at high RH regions.
However, below 50% RH, the proton conductivity decreases significantly for the random
sulfonated aromatic copolymers—BPSH 35. This is mainly due to the discontinuous
morphological structure that limits the proton transport under partially hydrated
conditions.
For Nafion, the extent of decrease in conductivity at low RHs is not as severe as
that measured for random copolymers. This is related to the unique chemical structure of
the Nafion, which consists of highly flexible side chain hydrophilic sulfonic acid groups
194
and a hydrophobic fluorinated flexible backbone. These chemical features apparently
promote strong nano-phase separation between distinctly hydrophilic and hydrophobic
domains even in random ionomers and allow proton transport between the interconnected
hydrophilic domains even at low hydration levels.
For PAES-6k-PBP-6k multiblock copolymer, proton conductivity shows much
less RH dependence than random sulfonated aromatic copolymers BPSH 35, almost same
or less RH dependence as Nafion with decreasing RHs. This phenomenon lies in the fact
that this multiblock copolymer possesses a co-continuous nanophase separated
morphology, as confirmed previously by AFM and DSC data. Since this unique co-
continuous morphology (interconnected channels and networks) dramatically facilitate
the proton transport (increase the diffusion coefficient of water), improved proton
conductivity under partially hydrated conditions becomes feasible.
However, the proton conductivities of these multiblock copolymers are still not as
good as Nafion, probably because the experimental IEC values are still pretty low, just
around 1.0 meq/g. Since the water uptake of this series of multiblock copolymers are
very low, we can push the IEC to a higher value to further increase the proton
conductivity. Therefore, future studies in multiblock copolymer systems with higher IEC
systems (1.5-2.0 meq/g), longer block length (15k-20k) and higher molecular weight are
needed.
195
Chapter 4
Taken from Journal of Polymer Science, Part A: Polymer Chemistry, Accepted, Oct 2006
4 Multiblock Copolymers of Poly (2,5-benzophenone)
and Disulfonated Poly (arylene ether sulfone) for
Proton Exchange Membranes. I. Synthesis and
Characterization
Hang Wang, Anand S. Badami, Abhishek Roy, James E. McGrath∗
Macromolecules and Interfaces Institute, Virginia Polytechnic Institute and State
University, Blacksburg, VA 24061
∗ Corresponding author. Tel.: 540-2315976; fax: 540-2318517.
E-mail address: [email protected] (J.E. McGrath).
Attribution: Mr. Anand S. Badami did a great job on Atom Force Microscopy
experiment and Mr. Ahbishek Roy kindly helped me on proton conductivity
measurements. Professor James E. McGrath is my advisor and he gave me tremendous
help and guidance on this work.
196
ABSTRACT: Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are
promising PEM materials due to their ability to form various morphological structures
which enhance transport. A series of poly(2,5-benzophenone) activated aryl fluoride
telechelic oligomers with different block molecular weights were successfully
synthesized by Ni (0)- catalyzed coupling of 2,5-dichloro-benzophenone and the end-
capping agent 4-chloro-4'-fluorobenzophenone. These telechelic oligomers (hydrophobic)
were then copolymerized with phenoxide terminated disulfonated poly (arylene ether
sulfone)s (hydrophilic) by nucleophilic aromatic substitution to form hydrophilic-
hydrophobic multiblock copolymers. High molecular weight multiblock copolymers
with number average block lengths ranging from 3,000 to 10,000 g/mol were successfully
synthesized. Two separate Tgs were observed via DSC in the transparent multiblock
copolymer films when each block length was longer than 6,000 g/mol (6k). Tapping
mode atomic force microscopy (AFM) also showed clear nanophase separation between
the hydrophilic and hydrophobic domains and the influence of block length, as one
increased from 6k to 10k. Transparent and creasable films were solvent-cast and
exhibited moderate proton conductivity and low water uptake. These copolymers are
promising candidates for high temperature proton exchange membranes in fuel cells,
which will be reported separately in part II of this series.
Keywords: proton exchange membrane; fuel cell; multiblock copolymers; phase
separation; morphology; poly(arylene ether sulfone); poly (p-phenylene) derivatives
197
INTRODUCTION
Poly(p-phenylene)s (PPPs) and especially their derivatives continue to receive
much attention due to their excellent thermal, mechanical, and electrical properties.178,
184,185 However, high molecular weight PPPs derivatives are difficult to synthesize
mainly due to the poor solubility of the growing rigid-rod chains during polymerization.
This problem has been decreased by introducing pendent groups to the phenyl rings
which improve the solubility of PPP, while retaining many of the most useful
characteristics. Appropriately substituted PPPs are soluble and yet exhibit thermal and
mechanical properties approaching the unsubstituted polymer. Thus, poly(2,5-
benzophenone) (PBP) is known for its high degree of polymerization, good solubility in
dipolar aprotic solvents, and excellent thermal and mechanical properties.77, 183, 186 High
molecular weight PBP shows a glass transition temperature of 206 oC and 5% weight loss
temperatures in nitrogen and air of 526 and 520 oC, respectively. 5 In addition, the tensile
modulus of PBP was shown to be from two to four times greater than other high-
performance isotropic thermoplastics including polysulfone, poly(ether imide),
poly(ether ether ketone), poly(phenylene sulfide), and thermoplastic
polybenzimidazole.187 PBPs can also be processed by compression molding and
extrusion and are now commercially available under the trade name Parmax from Solvay
Advance Polymer Inc.
High molecular weight PBPs can be synthesized by the nickel-catalyzed
polymerization of 2,5-dichlorobenzophenone using the coupling method of Colon and
Kelsey.175 However, the inability to form good flexible films is a still major drawback of
most PPPs. Even amorphous, high molecular weight poly(2,5-benzophenone) suffers
198
from this problem, possibly because of its stiff chain. The inability to form flexible,
creasable films from these stiff chains remains a major barrier in the testing and
development of PPP-based proton-exchange and gas separation membranes and in other
applications.
One alternative is to use a rigid-rod PBP difunctional telechelic oligomer,
combined with a flexible, random coil, thermally stable telechelic oligomer, to produce
thermally stable multiblock copolymers. This methodology was reported earlier by
Ghassemi 164 and Sheares 180 and flexible and transparent films have been formed from
these PBP based multiblock copolymers.
We are currently interested in the synthesis of potentially economical, and highly
thermooxidatively stable polymers as candidates for proton-exchange membranes (PEMs)
in fuel cells. PEMFCs have potential as energy conversion devices for both
transportation, mobile and stationary applications.191 The primary demands on the
hydrated PEM are high proton conductivity (≥0.1 S cm-1), low fuel and O2 permeability,
limited swelling in water and high chemical, thermal and mechanical stability.192
Conventional PEMFCs typically operate with Nafion® membranes29, which offer quite
good performance below 90 oC. However, today there is a strong need for PEMs capable
of sustained operation up to 120 oC to offer many benefits including increased resistance
to fuel impurities, most notably carbon monoxide (CO), fast electrode kinetics, and
simplified water/thermal management. Unfortunately, the proton conductivity of Nafion
suffers greatly at temperatures above 90 oC due to the depressed hydrated α-relaxation or
too restrained transport.18, 29 Also, the barrier properties of this membrane are poor when
199
methanol is used as fuel. These factors, in addition to the high cost of Nafion, have
encouraged extensive research on alternative PEM materials. 9, 91, 136, 262, 263
Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are
promising PEM materials, particularly due to their ability to form unique morphologies.
It is well known that the membrane morphology is important for the PEM fuel cell
performance, and depends strongly on the water content, and on the type concentration
and distribution of the acidic moieties.27 The unique morphologies of multiblock
copolymers may play an important role in providing good proton transport at low water
contents and high temperatures. 135
This paper reports the end-capping of 2,5-dichlorobenzophenone with 4-chloro-
4’-fluorobenzophenone via Ni (0)-catalyzed coupling reaction to produce difunctional
PBP oligomers with aryl fluoride endgroups, which are labile to subsequent nucleophilic
aromatic substitution. These functionalized PBP (hydrophobic) telechelic oligomers
were then combined with phenoxide terminated disulfonated wholly aromatic biphenyl
poly(arylene ether sulfone) oligomers (hydrophilic) to produce high molecular weight
multiblock copolymers capable of forming flexible films. Randomization by ether–ether
interchange reaction was obviously avoided. Morphology studies on these multiblock
copolymers as well as its relationship to proton conductivity and water uptake have been
initiated and are discussed.
200
EXPERIMENTAL
Materials
All reagents were purchased from Aldrich and used without further purification
unless otherwise noted. N,N-Dimethylacetamide (DMAc) and N-methyl-2-pyrrolidione
(NMP) were dried over calcium hydride, distilled under vacuum and stored under
nitrogen before use. Tetrahydrofuran (THF) was distilled from sodium and benzophenone.
Then 2,2’-bipyridyl (bipy) and triphenylphosphine were purified by recrystallization from
ethanol and cyclohexane, respectively. Dichlorobis(triphenylphosphine)nickel-(II) was
stored under N2. Zn powder was washed with acetic anhydride, filtered, washed with dry
Et2O, and dried under vacuum at 150 °C. 4,4’-Dichlorodiphenylsulfone (DCDPS) and
4,4’-biphenol were obtained from Solvay Advanced Polymers and Eastman Chemical,
respectively. The 3,3’-disulfonate-4,4’-dichlorodiphenylsulfone (SDCDPS) was
synthesized from DCDPS according to a modified literature method,264 which has been
further developed in our laboratory.88 The 2,5-dichlorobenzophenone was prepared
according to a procedure reported in the literature.77
Characterization
1H and 13C NMR spectra were recorded on a Varian 400 spectrometer, using
CDCl3 and DMSO-d6 as solvents. Glass transition temperatures (Tg) were determined by
differential scanning calorimetry, using a TA Instrument DSC Q-1000 at a heating rate of
10 oC/min, under a stream of nitrogen. Second heat Tg values were reported as the
midpoints of the change in the slopes of the baselines. Thermogravimetric analyses (TGA)
were performed on a TA Instrument TGA Q-500 thermogravimetric analyzer. All the
201
samples were first vacuum dried and kept in the TGA furnace at 150 oC in a nitrogen
atmosphere for 30 min to remove water before TGA characterization. The typical
heating rate was 10 oC/min in nitrogen. Atomic force microscopy (AFM) images were
obtained using a Digital Instruments MultiMode scanning probe microscope with a
NanoScope IVa controller (Veeco) in tapping mode. A silicon probe (Veeco) with an
end radius of <10 nm and a force constant of 5 N/m was used to image samples. The
ratio of amplitudes used in the feedback control was adjusted to 0.83 of the free air
amplitude for both the PAES-6k-PBP-6k and PAES-10k-PBP-10k samples. Samples
were dried under vacuum at 60 °C for 3 h and then equilibrated at 30% relative humidity
for at least 12 h before being imaged immediately at room temperature in a relative
humidity of approximately 15-20%.
Intrinsic viscosity measurements were carried out with a Cannon Ubbelholde
viscometer in NMP (containing 0.05 M LiBr) as solvent at 25 oC. The addition of LiBr is
well known to shield the polyions from intramolecular expansion. GPC experiments
were performed on a liquid chromatograph equipped with a Waters 1515 isocratic HPLC
pump, Waters Autosampler, Waters HR5-HR4-HR3 column, Waters 2414 refractive
index detector and Viscotek 270 right angle laser light scattering (RALLS)/viscometric
dual detector. NMP (containing 0.05 M LiBr) was used as the mobile phase. The column
temperature was maintained at 60 oC because of the viscous nature of NMP. Both the
mobile phase solvent and sample solution were filtered before introduction to the GPC
system.
202
Water Uptake, Ion Exchange Capacity and Proton Conductivity
The membranes were vacuum-dried at 100 oC for 24 h, weighed and immersed in
deionized water at room temperature for 24 h to measure the water uptake. The wet
membranes were wiped dry and quickly weighed again. The water uptake of the
membranes is reported in weight percent as follows:
100uptakewater ×−
=dry
drywet
WWW
1 where Wwet and Wdry are the weights of the wet and dry membranes, respectively.
Ion Exchange Capacity (IEC) was determined by aqueous potentiometric
titrations using an MCI Automatic Titrator Model GT-05. The membranes were soaked
in deionized water containing sodium sulfate (1 M solution) for 24 h at room temperature
to form the salt form polymer and an acid form, water soluble compound (i.e., sodium
hydrogen sulfate). The solutions were titrated by standard sodium hydroxide solution
(0.01 M) to quantitatively determine sulfonic acid concentration in the sulfonated
polymers in terms of ion exchange capacity (IEC, mequiv/g).
Proton conductivity at 30 °C at full hydration (in liquid water) was determined in
a window cell geometry12 using a Solartron 1252+1287 Impedance/Gain-Phase Analyzer
over the frequency range of 10 Hz - 1 MHz. The cell geometry was chosen to ensure that
the membrane resistance dominated the response of the system. The resistance of the
film was taken at the frequency which produced the minimum imaginary response.265
The conductivity of the membrane can be calculated from the measured resistance and
the geometry of the cell according to:
AZl'
=σ 2
203
where σ is the proton conductivity, l is the length between the electrodes, A is the cross
sectional area available for proton transport, and Z’ is the real impedance response.
For determining proton conductivity in liquid water, membranes were equilibrated at 30
°C in deionized water for 24 h prior to the testing.
Monomer synthesis
The 2,5-dichlorobenzophenone was prepared by Friedel–Crafts acylation of
benzene with 2,5-dichlorobenzoyl chloride and the yield was 80%. The monomer was
purified by recrystallization twice from ethanol/water (90/10, v/v) and dried under
vacuum at 50 oC for 24 h. mp 88–89 oC 1H NMR δ 7.80 (m, 2H), 7.62 (ddd, 1H), 7.48
(m, 2H), 7.42 (m, 2H), 7.36 (m, 1H); 13C NMR δ 193.6 (C=O), 139.9 (C1), 135,8 (C1’),
134.1 (C4), 132.9 (C2), 131.3 and 131.1 (C3, C6), 130.1 (C2’), 129.6 (C5), 128.9 (C4’),
128.8 (C3’).
Synthesis of telechelic poly(2,5-benzophenone) (PBP) oligomers
A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),
PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6
mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated
under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2
followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe
through the rubber septum to initiate the reaction, which became deep red after 10
minutes. Then, 2,5-dichlorobenzophenone (3.7667 g, 15 mmol) and the end-capping
agent 4-chloro-4’-fluorobenzophenone (0.2346 g, 1 mmol) were charged under N2 flow.
204
The mixture was stirred and heated at 65 oC for 24 h. After cooling to room temperature,
the reaction mixture was added into concentrated aqueous HCl/methanol (40/60 v/v)
solution and stirred overnight. The resulting precipitate was collected by filtration and
washed thoroughly with 10% sodium bicarbonate solution and deionized water. After
drying in a vacuum oven at 100 oC overnight, the light yellow functional oligomer (or
polymer) was isolated with a yield of 95%.
Synthesis of PAES telechelic oligomers and PAES-PBP multiblock copolymers
The aromatic nucleophilic reaction was conducted in a 3-neck flask equipped with
a mechanical stirrer, nitrogen inlet and a Dean-Stark trap. In a typical polymerization to
prepare 5,000 g/mol oligomer, biphenol (0.3742 g, 2 mmol), SDCDPS (0.8725 g, 1.776
mmol) and potassium carbonate (0.3179 g, 2.3 mmol) were added to the flask. Dry
NMP(10 mL) and toluene(5 mL) were added as the solvents. The reaction mixture was
heated under reflux at 150 oC for 4 h, which stripped off most of the toluene to dehydrate
the system. The temperature was then raised slowly to 190 oC for 36 h. The viscous
solution obtained was cooled to room temperature. Poly(2,5-benzophenone) telechelic
oligomer (1.1171 g, 0.223 mmol), potassium carbonate (0.069 g, 0.5 mmol), dry NMP (5
mL) and toluene (10 mL) were added into the same reaction flask under nitrogen. The
reaction mixture was heated to 150 oC and refluxed for 4 h. The toluene was removed to
dehydrate the system and the temperature was raised to 190 oC for another 48 h. The
viscous polymer solution was cooled to room temperature, diluted with NMP and filtered
to remove most of the salts. The block copolymer was isolated by precipitation in
205
isopropanol. It was washed extensively with deionized water and chloroform and dried at
120 oC under vacuum for 24 h.
Membrane preparation
Membranes were prepared by dissolving the salt form copolymers in DMAc (10%
w/v). The solution was then filtered through a 0.45 µm TEFLON syringe filter and cast
onto a clean glass substrate. The membranes were slowly dried with a heat IR lamp for
24-48 h and then under vacuum at 100 °C for 24 h. The copolymer membranes in their
salt form were then converted to a corresponding acid form by boiling in 0.5 M sulfuric
acid solution for 2 h followed by immersion in boiling deionized water for 2 h.
206
RESULTS AND DISCUSSION
Synthesis of Monomers and Oligomers
The monomer 2,5-dichlorobenzophenone was synthesized by an aluminum
chloride catalyzed Friedel–Crafts acylation of benzene with 2,5-dichlorobenzoyl
chloride.180 2,5-Dichlorobenzoyl chloride was prepared from 2,5-dichlorobenzoic acid
and thionyl chloride and was purified by fractional vacuum distillation. The monomer
2,5-dichlorobenzophenone structure was confirmed by 1H and 13C NMR.
The 2,5-dichlorobenzophenone can react readily via Ni(0)-catalyzed coupling to
form isomeric high molecular weight poly(2,5-benzophenone) (PBP) with excellent
thermal properties. Since aryl fluorides do not participate in the Ni(0)-catalyzed coupling
reaction, end capping of poly(2,5-benzophenone)s with 4-chloro-4’-fluorobenzophenone
will result in PBP chains that contain aromatic fluoride end groups.180 These aryl
fluorides activated by electron-withdrawing group (carbonyl group) can be used as
reactive sites for nucleophilic aromatic substitution block copolymerization. Furthermore,
the molecular weights of these oligomers are controlled by altering the amount of 4-
chloro-4’-fluorobenzophenone.
The end-capping agent 4-chloro-4’-fluorobenzophenone was synthesized by an
iron (III) chloride-catalyzed Friedel–Crafts acylation of chlorobenzene with 4-
fluorobenzoyl chloride. Sheares et al.180 and Ghassemi et al.164 had previously
demonstrated the use of this end-capping agent in the synthesis of substituted poly(2,5-
benzophenone) oligomers.
207
Scheme 1 Synthesis of fluorine terminated poly(2,5-benzophenone) oligomers
ClCl
C O
Cl CO
F
THF, 60oC, 12h
C OOO
FF
+NiCl2(PPh3)2, Zn, PPh3, Bpy
n
The synthesis of fluorine-terminated PBP telechelic oligomers, shown in Scheme
1, was conducted using Ni (0)-catalyzed coupling method from 2,5-
dichlorobenzophenone and a controlled amount of the end-capping agent 4-chloro-4’-
fluorobenzophenone. The monomer and end-capping agent were added quickly under
nitrogen flow to the polymerization flask containing a mixture of nickel chloride, 2, 2’-
bypyridyl and triphenylphosphine in THF. Previously, Sheares et al. 177 found the use of
THF rather than amide solvents (DMF and NMP) yielded higher molecular weight
polymers with better film-forming properties and no reduction of the carbonyl groups
during the Ni(0)-catalyzed coupling reaction.
208
Figure 89 13C NMR spectrum of a fluorine terminated poly(2,5-benzophenone) oligomer in CDCl3
The molecular weights of PBP oligomers were controlled by varying the end-
capping agent stoichiometry. 13C NMR spectra were used to determine the degree of
polymerization of the PBP telechelics. The peaks at 194.6 ppm and in the range of
198.3–196.2 ppm are assigned to carbonyl moieties in the end-capping agent c and in the
main chain d, respectively (Figure 89). The integrals of peaks 194.6 ppm and in the
range of 198.3–196.2 ppm are compared to calculate the degrees of polymerization of the
PBP telechelics. As summarized in Table 11, the experimental values from 13C NMR
agree well with the target values. Other prominent features in the 13C NMR spectra are
the signals from the ipso carbon a attached to the fluoride and its adjacent carbon b from
the end-capping agent. The ipso carbon a is assigned as a doublet at 165.0 ppm with a
d c a
b
C O
CO
CO
FF
n
ab
c
d
209
coupling constant of 255.1 Hz. The adjacent carbon b signal is clearly visible as a doublet
at 115.3 ppm with a coupling constant of 23.1 Hz.
As expected, intrinsic viscosities η of this series PBP oligomers increase with
molecular weight (Table 11). Intrinsic viscosity η can be related to molecular weight by
the Mark-Houwink-Sakurada equation
[η] = KMva
where Mv is viscosity average molecular weight; K and α are the Mark-Houwink
constant and the Mark-Houwink-Sakurada exponent. The plot of log [η] with log Mn
(Figure 90) gave an approximately straight line and the Mark-Houwink-Sakurada
exponent α of these rigid PBP oligomers was found to be 0.83. The data confirm that
the molecular weight control of this series of PBP oligomers was successful.
The thermal properties of these low molecular weight PBP telechelics were found
to be very robust. A steady increase in the glass transition temperature Tg was observed
from 151 to 187 °C, as molecular weight increased from 2200 to 9400 g/mol (Table 11).
The highest Tg at 187 °C is close to that found for the non-end-capped homopolymer,
namely 190 °C.
210
y = 0.8253x - 3.5863
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1
Log Mn
Log [η]
Figure 90 Double logarithmic plot of [η] against Mn of fluorine terminated poly(2,5-benzophenone)
oligomers
Hydroxy (phenoxide) terminated sulfonated poly(arylene ether sulfone) (PAES)
telechelics were synthesized (Scheme 2) as hydrophilic blocks in multiblock copolymers.
The 4,4’-biphenol monomer was used in excess base on the total amount of disulfonated
4,4’-dichlorodiphenylsulfone (SDCDPS) to produce telechelics with the target number
average molecular weight of 3000, 6000 and 10,000 g/mol.
211
Table 11 Characterization of fluorine terminated poly(2,5-benzophenone) oligomers
Polymer
Molar Fraction
of End-capping
Agent
Mn a
(g/mol)
Mn b
(g/mol)
Intrinsic
Viscosity c
[η] (dl/g)
Tg d
(oC)
1 0.20 2200 1980 0.16 151
2 0.10 4000 3800 0.21 163
3 0.07 5600 5500 0.32 167
4 0.05 7600 7000 0.46 176
5 0.04 9400 -- 0.48 187
6 0.00 -- -- 0.96 190
a Target molecular weight
b Determined from 13C NMR
c NMP as solvent at 25 oC
d Determined by DSC
212
SClO
OCl
+NaO3S
SO3Na+
OHHO
SO
OMO3S
SO3M
OOH OH
BPS100
(in excess)
K2CO3
NMP/Toluene Reflux 150oC for 4hrs190oC for 36 hrs under N2
n
+
O
endgroups were acidified byacetic acid
Scheme 2 Synthesis of hydroxy terminated disulfonated poly(arylene ether sulfone) oligomers
1H NMR (Figure 91) was employed to determine number-average molecular
weight Mn by comparing integrals of the aromatic protons of the biphenol moiety in the
terminal group c relative to those in repeat units a. Peaks at 7.71 ppm and 6.84 ppm are
assigned to c and a, respectively. As shown in Table 12, for this series of PAES
oligomers, Mn determined by proton NMR agree well with the target values.
213
OO S
O
O
OHH
n
SO3K
KO3S
b a a b e f
g
g
f e
h i d cj
Figure 91 Proton NMR spectrum of a phenol terminated disulfonated poly(arylene ether sulfone)
oligomer
Table 12 Properties of hydroxy terminated poly(arylene ether sulfone) oligomers
Polymer
Molar Excess of
Biphenol
Monomer (%)
Mn a
(g/mol)
Mn b
(g/mol)
Intrinsic
Viscosity c
[η] (dl/g)
1 22.6 3000 3100 0.15
2 10.9 6000 5900 0.24
3 6.40 10,000 9900 0.35
a Target molecular weight
b Determined from proton NMR
c NMP with 0.05M LiBr as solvent at 25 oC
214
Synthesis of PAES-PBP Multiblock Copolymers
In general, one of the drawbacks of poly(2,5-benzophenone) (PBP) is their
inability to form flexible films. On the other hand, PAES forms excellent films but lacks
the high modulus and strength of PBP. To develop more flexible, film-forming PBP-base
materials, hydroxy terminated sulfonated PAES telechelics (hydrophilic) prepared above
were copolymerized with fluorine terminated PBP telechelics (hydrophobic) via
nucleophilic aromatic substitution (Scheme 3).89 The stoichiometry of the
copolymerization was based on a 1:1 molar ratio between functional end groups of
hydrophilic (PAES)/hydrophobic (PBP) oligomers prepared above. Randomization,
which is to be expected in the high temperature poly(arylene ether) multiblock
copolymers forming step, was avoided since there is no ether–ether interchange possible
in the PBP blocks. By varying the block length of hydrophilic (PAES) /hydrophobic
(PBP) telechelics from 3,000 to 10,000 g/mol, three multiblock copolymers: PAES-3k-
PBP-3k, PAES-6k-PBP-6k and PAES-10k-PBP-10k were synthesized. They all showed
reasonable high intrinsic viscosity η from 0.68 to 0.82 dl/g in NMP (0.05 M LiBr) and
formed transparent, creasable films. All these results strongly indicate high molecular
weight PAES-PPP multiblock copolymers were successfully synthesized.
215
K2CO3
OOFF
C O
PBP
SO
OO
MO3S
SO3M
OO OKK
BPS100
SO
OO
MO3S
SO3M
OO OOO
C O
m
NMP/Toluene
150oC, 4h190oC, 48h
n+
x n
Hydrophilic Hydrophobic
+ KFy
Scheme 3 Synthesis of a PBP-PAES hydrophobic-hydrophilic multiblock copolymer
Table 13 Characterization of PPP-PAES hydrophobic-hydrophilic multiblock copolymer
Multiblock
copolymers
Hydrophilic
block
length
Hydrophobic
block
length
Intrinsic
viscosity
(dl/g)a
Ion
exchange
capacity
(meq/g)b
Ion
exchange
capacity
(meq/g)c
Water
uptake
Proton
conductivity
(mS/cm)d
1 3000 3000 0.68 1.57 0.95 7% 30
2 6000 6000 0.72 1.57 1.05 7% 40
3 10,000 10,000 0.82 1.57 1.10 10% 60
a NMP with 0.05M LiBr as solvent at 25 oC
b. Target value
c Determined by back-titration of acid groups
d Measured in liquid water at 30 oC
216
Morphology
As is well known, multiblock copolymers consist of chemically distinct blocks
covalently linked to form a single molecule. Owing to mutual repulsion between
dissimilar blocks, they tend to segregate into separate nanophases. However, because
each block is chemically joined, macroscopic phase separation is prevented. Instead,
nano or nanophase separation takes place. 138 As a result, two separate glass transition
temperatures (Tgs) may be observed by methods such as DSC. Figure 92 shows the DSC
thermogram of a PAES-6k-PBP-6k multiblock copolymer with block lengths of
approximately 6000 g/mol. Two glass transition temperatures (Tgs) were observed: one
is 174 oC, which assigned to the Tg of hydrophobic PBP segment; the other is around
224oC, which corresponded to the Tg of hydrophilic disulfonated PAES block. This
suggests nanophase separation happened due to the immiscibility of the PBP and PAES
blocks. This is expected since PBP and PAES are quite different in terms of chemical
structures: PBP has very rigid polymer chain while PAES backbone is much more
flexible because of the arylene-ether bonds. This two Tgs phenomenon was also observed
in a PAES-10k-PBP-10k sample (Tgs: 180oC and 225oC) but only one Tg (225oC) was
found in a PAES-3k-PBP-3k multiblock copolymer. This is expected since longer blocks
should increase the immiscibility between dissimilar blocks, favoring the nanophase
separation in the multiblock systems. 138
217
Temperature (oC)
Heat Flow
(W/g)
Figure 92 DSC thermogram of PPP-PAES hydrophobic-hydrophilic multiblock copolymer with
block lengths of approximately 6000 g/mol showing two Tgs
Tapping mode AFM images were obtained under ambient conditions on a 500 nm
× 500 nm size scale to investigate nanophase separation for multiblock copolymers of
PAES-6k-PBP-6k and PAES-10k-PBP-10k (Figure 93). The dark regions of the phase
images are assigned to the softer blocks, which represent the hydrophilic ionic groups
(sulfonic acid) containing small amounts of water. The light regions are related to hard
segments, derived from the hydrophobic hydrocarbon units. It can be clearly seen that
for the PAES-6k-PBP-6k multiblock sample, the dark regions (hydrophilic domains) are
connected to each other and form channels and networks that extend through the whole
sample surface; while light regions (hydrophobic domains) are divided into small
domains, which have diameters of 20 to 40 nm. This nanophase separation phenomenon
develops further in the PAES-10k-PBP-10k sample, where even the light regions
218
(hydrophobic domains) start to aggregate too, forming bigger and longer hydrophobic
continuous phases. This again confirms that nanophase separation took place in these
hydrophilic-hydrophobic multiblock copolymers and is considered significant in terms of
the transport properties.135
Figure 93 Tapping mode AFM phase images of (a) a PAES-6K-PBP-6K multiblock copolymer, and
(b) a PAES-10K-PBP-10K multiblock copolymer with block lengths of approximately 6000 and
10,000 g/mol, respectively.(Scan sizes are 500 nm; Z ranges for phase images are 10 o)
Ion exchange capacity, Water uptake and Proton conductivity
The hydrophilic-hydrophobic nanophase separation morphology is considered to
be particularly good for PEM materials because it may increase the water self diffusion
coefficient, which may facilitate proton transportation at even low humidities.135 Since
protons are able to migrate through these continuous hydrophilic channels through the
membrane with less hindrance by hydrophobic domains, theoretically, smaller amounts
219
of water (lower relative humidities) would be required to assist the proton transport.
Moreover, extensive swelling, which is very normal for most ion containing hydrocarbon
random copolymers, can be minimized. This series of multiblock copolymers showed
surprising low water uptake values of only 7%~10% (Table 13). These values are not
only lower than most ion-containing random copolymers such as poly(arylene ether
sulfone)s (BPSH-series, where BP stands for biphenol, S is for sulfonated, and H denotes
the proton form of the acid)89, but even lower than highly hydrophobic perfluorinated
copolymers like Nafion®. The reason behind this interesting phenomenon is not yet clear,
but one possible explanation is the rigid hydrophobic phase may be important.
The proton conductivities of these multiblock copolymer membrane samples were
measured under full hydrated condition in water at room temperature. The acid form
membranes showed conductivity of 0.03, 0.04 and 0.06 S/cm, for PAES-3k-PBP-3k (IEC
0.95 meq/g), PAES-6k-PBP-6k (IEC 1.05 meq/g), PAES-10k-PBP-10k (IEC 1.10 meq/g),
respectively. These values are considered to be good considering low IEC and very low
water uptake. Thus, these multiblock copolymers are promising PEM materials for fuel
cells. However, for all three multiblock copolymers the experimental IEC values were
lower than the theoretical values (1.57 meq/g) calculated for an ideal alternating
multiblock, indicating that there was somewhat less than quantitative incorporation of
sulfonated telechelics. Future studies will investigate higher IEC systems, longer block
length and higher molecular weight.
220
CONCLUSIONS
Multiblock copolymers derived from fluorine-terminated poly(2,5-benzophenone),
as the hydrophobic blocks, and hydroxyl functional sulfonated poly(arylene ether sulfone)
as hydrophilic blocks were successfully prepared by nucleophilic step copolymerization.
Transparent and creasable films were successfully solvent-cast from these PAES-PBP
multiblock copolymers. Two separate Tgs were observed via DSC for multiblock
copolymers when the block length was longer than 6,000 g/mol. Tapping mode AFM
studies also showed clear nanophase separation between hydrophilic and hydrophobic
domains. This series of PAES-PBP multiblock copolymers showed moderate
conductivities up to 0.06 S/cm with very low water uptake of 7-10%. Therefore, they are
considered to be promising PEM materials in fuel cells.
ACKNOWLEDGEMENTS
The authors would like to thank the National Science Foundation “Partnership for
Innovation” Program (HER-0090556) and the Department of Energy (DE-FG36-
06G016038) for support of this research effort.
221
Chapter 5
5 Summary and Conclusions
This research focused on synthesis and characterization of poly (2,5-
benzophenone) (PBP) and Disulfonated Poly (arylene ether sulfone) (PAES) multiblock
copolymers for Proton Exchange Membranes in fuel cells. The main objective of this
research was to synthesize nanophase-separated multiblock copolymers containing
hydrophilic and hydrophobic blocks and study the effect of chemical composition and
block length on morphology, water uptake and proton conductivity. The thermal
stabilities of these multiblock copolymers were also evaluated.
Four arylene chlorides monomers (2,5-Dichlorobenzophenone and its derivatives)
were first synthesized from aluminum chloride-catalyzed, Friedel-Crafts acylation of
benzene and other aromatic compounds with 2,5-dichlorobenzoyl chloride. All four
monomers were further purified to yield high purity monomers in high yields. The
benzoyl pendant group is known to increase the solubility of the wholly aromatic poly(p-
phenylene) material, while maintaining the outstanding thermal and mechanical
properties of the conjugated backbone.
These monomers were then polymerized via Ni (0)-catalyzed coupling reaction to
form various substituted poly(2,5-benzophenone)s. It was very efficient to use dry
tetrahydrofuran (THF) as solvent with the catalyst mixture as: NiCl2(PPh3)2 (0.1 equiv),
PPh3 (0.2 equiv) , Zn (3.1 equiv), 2,2’-bipyridyl (0.1 equiv) and dichloride monomer ( 1.0
equiv) in Ni (0)-catalyzed polymerization. As a result, a series of amorphous substituted
poly(2,5-benzophenone)s with intrinsic viscosity as high as 0.96 dL/g were successfully
222
synthesized and they all showed excellent thermal stabilities. Great care must be taken to
minimize the content of water and oxygen during the reaction.
Then, a series of poly(2,5-benzophenone) activated aryl fluoride telechelic
oligomers with different block molecular weights were successfully synthesized by Ni
(0)- catalyzed coupling of 2,5-dichloro-benzophenone and the end-capping agent 4-
chloro-4'-fluorobenzophenone. The molecular weights of these oligomers are controlled
by altering the amount of 4-chloro-4’-fluorobenzophenone. These telechelic oligomers
(hydrophobic) were then copolymerized with phenoxide terminated disulfonated poly
(arylene ether sulfone)s (hydrophilic) by nucleophilic aromatic substitution to form
hydrophilic-hydrophobic multiblock copolymers.
High molecular weight multiblock copolymers with number average block
lengths ranging from 3,000 to 10,000 g/mol were successfully synthesized. Two separate
Tgs were observed via DSC in the transparent multiblock copolymer films when each
block length was longer than 6,000 g/mol (6k). Tapping mode atomic force microscopy
(AFM) also showed clear nanophase separation between the hydrophilic and hydrophobic
domains and the influence of block length, as one increased from 6k to 10k. Transparent
and creasable films were solvent-cast and exhibited moderate proton conductivity and
low water uptake. These PAES-PBP multiblock copolymers also showed much less
relative humidity (RH) dependence than random sulfonated aromatic copolymers BPSH
35 in proton conductivity, almost the same as Nafion with decreasing RHs. This
phenomenon lies in the fact that this multiblock copolymer possesses a co-continuous
nanophase separated morphology, as confirmed previously by AFM and DSC data. Since
this unique co-continuous morphology (interconnected channels and networks)
223
dramatically facilitate the proton transport (increase the diffusion coefficient of water),
improved proton conductivity under partially hydrated conditions becomes feasible.
These multiblock copolymers are therefore considered to be promising candidates for
high temperature proton exchange membranes in fuel cells.
224
Chapter 6
6 Suggested Future Research
Results from previous chapters demonstrated that nano-phase separated
hydrophilic-hydrophobic multiblock copolymers are promising candidates for high
temperature PEMFCs. They exhibit low water absorption and much less relative
humidity (RH) dependence than random sulfonated aromatic copolymers BPSH 35 in
proton conductivity, almost the same as Nafion with decreasing RHs. This phenomenon
lies in the fact that this multiblock copolymer possesses a co-continuous nanophase
separated morphology, which dramatically facilitates the proton transport (increase the
diffusion coefficient of water) under partially hydrated conditions.
However, the proton conductivities of these multiblock copolymers are still not as
good as Nafion, probably because the experimental IEC values are still pretty low (1.0
meq/g). Therefore, future studies in multiblock copolymer systems with higher IEC
systems (1.5-2.0 meq/g), longer block length (15k-20k) and higher molecular weight are
highly recommended.
Another approach of synthesizing segmented multiblock copolymers is to utilize
two monomers and one oligomer to prepare segmented copolymers. That is, instead of
using one PAES telechelic oligomer and one PBP telechelic oligomer, one can use two
monomers, BP and SDCDPS, directly react with a PBP telechelic oligomer to synthesize
segmented copolymers (Figure 94). This method has been used widely in preparing
polyurethane segmented copolymers.266, 267 The advantage of this method is that it not
only cuts one step of preparing PAES a telechelic oligomer, but also makes it much easier
225
to synthesize high molecular weight segmented copolymers since stoichiometry is not
very critical for this method. Recall that in the synthesis of multiblock copolymers one
must keep the stoichiometry of each telechelic oligomers to be close to 1:1 to ensure high
molecular weight multiblock copolymer. This requirement is not that serious here in the
segmented copolymer synthesis because the concentration of functionality in the
telechelic oligomer is very small compared with two monomers. Therefore, controlling
the stoichiometry is much easier and simpler than in the synthesis of multiblock
copolymer.
Figure 94 Synthetic scheme of PAES-PBP segmented copolymers
The resultant segmented copolymers should have the similar nanophase
separation behavior as multiblock copolymers. Thus, one might expect that segmented
copolymer membrane would as well have good properties such as low water absorption
OHHO
OO
FF
C O
PBP Oligomers
m
K2CO3DMAc/Toluene Reflux 150oC for 4 h
SO
OO
MO3S
SO3M
OO OOO
C O
n m
S
O
ONaO3S
SO3Na
Cl Cl
+
165 o C72 h
Segmented Copolymers
226
and low relative humidity (RH) dependence on conductivity. This segmented copolymer
research has been initiated in McGrath group at Virginia Tech and we have prepared a
few segmented copolymer samples. Further studies should continue to test whether this
approach would be better than multiblock copolymer method.
Recently, Zhang et al.268 reported using sulfonated poly(p-phenylene)s (SPPs) as
PEM and proton conductivity as high as 0.34 S/cm was reached at 120 oC. Sulfonated
poly(p-phenylene)s were directly synthesized by Ni(0) catalytic coupling of sodium 3-
(2,5-dichlorobenzoyl) benzenesulfonate and 2,5-dichlorobenzophenone (same monomer
we used in hydrophobic blocks). Although no molecular weight data was reported, the
sulfonated copolymers displayed excellent film forming abilities and mechanical
properties due to the increased interaction between chains induced by the --SO3H groups.
Significant hydrophilic/hydrophobic microphase-separated structures were observed for
these membranes, which were favorable for water keeping, proton transport and
limitations of swelling. This is very interesting and more research on synthesis of
sulfonated poly(p-phenylene)s direct from sulfonated dichloride monomers by Ni (0)
catalyzed coupling reaction are also needed.
227
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