Synthesis and properties of highly branched sulfonated poly(arylene ether)s as proton exchange membranes Lei Wang ⇑ , Dagang Wang, Guangming Zhu, Junqin Li Shenzhen Key Laboratory of Special Functional Materials and College of Materials Science and Engineering, Shenzhen University, Shenzhen, Guangdong 518060, China article info Article history: Received 28 December 2010 Received in revised form 12 June 2011 Accepted 14 July 2011 Available online 25 July 2011 Keywords: Branching agent Branched polymer Sulfonated poly(arylene ether)s Proton exchange membrane Oxidative stability abstract A series of branched sulfonated poly(arylene ether)s were successfully synthesized using 1,3,5-tris[4-(4-fluorobenzoyl)phenoxy]benzene (B 3 ) as branching agent. The synthesized branched copolymers were soluble in polar organic solvents, such as N-methyl-2-pyrroli- done (NMP), N,N-dimethylacetamide (DMAc) and dimethylsulfoxide (DMSO), and could be cast to form tough and smooth films. The effect of degree of branching (DB) on the pro- ton conductivity, swelling ratio and oxidative stability of the membranes was investigated. With increasing DB value, the proton conductivity and oxidative stability of the mem- branes increased. A maximum oxidative stability of the branched membrane with 4% of DB value was determined to be 3.4 times larger than that of the linear membrane. In addi- tion, as the DB value increased, the swelling ratio of the membranes decreased from 13.51% to 9.09% at 80 °C. The results indicated that increasing DB value might be an effective way to improve the properties of proton exchange membranes. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Proton exchange membrane fuel cells (PEMFCs) are believed to have a potential to alleviate the major problems associated with the energy production and con- sumption [1]. Large interest in using PEMFCs as power sources, such as vehicles and portable applications has greatly promoted the fundamental research in the field of proton-conducting polymers over the past few years [2–6]. Great progress has been made on the development of proton-exchange membranes. However, there are still two major remaining challenges that current PEMFCs face: cost and lifetime [3]. Perfluorinated membranes are regarded as the most preferable candidate from the stand- point of chemical and thermal stabilities, physical and pro- ton-conducting properties, but they are too expensive which is a major obstacle for their widespread application in fuel cells [7,8]. As one of alternative proton exchange membranes, sulfonated poly(arylene ether)s [9–14] have been developed. The sulfonated poly(arylene ether)s were found to possess good thermal stability, appropriate mechanical strength and high proton conductivity. How- ever, most of them failed to be used as proton-exchange membranes since their short lifetime due to a combination of hydrolysis and oxidative degradation [3]. Therefore, ef- forts to improve the stability of the membranes are eagerly requested. It has been reported by several research groups that the poly(arylene ether)s containing sulfonic acid groups on the pendant side chains exhibit good durability [15–22]. Unfortunately, the mechanical properties of these linear sulfonated polymers tend to deteriorate progressively with sulfonation, which makes the long-term stability of highly sulfonated polymers questionable [3,23]. Recently, cross- linked sulfonated poly(arylene ether)s for fuel cell applica- tions have attracted increasing interest [24–29] due to their effective improvement on the properties of the mem- branes. Guiver et al. [23] had synthesized the cross-linked polymer electrolytes with polyatomic alcohols, which made the polymer mechanically stronger and reduced its 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.07.016 ⇑ Corresponding author. Tel.: +86 775 26538537; fax: +86 775 26536239. E-mail address: [email protected](L. Wang). European Polymer Journal 47 (2011) 1985–1993 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Synthesis and properties of highly branched sulfonatedpoly(arylene ether)s as proton exchange membranes
Lei Wang ⇑, Dagang Wang, Guangming Zhu, Junqin LiShenzhen Key Laboratory of Special Functional Materials and College of Materials Science and Engineering, Shenzhen University, Shenzhen,Guangdong 518060, China
a r t i c l e i n f o
Article history:Received 28 December 2010Received in revised form 12 June 2011Accepted 14 July 2011Available online 25 July 2011
A series of branched sulfonated poly(arylene ether)s were successfully synthesized using1,3,5-tris[4-(4-fluorobenzoyl)phenoxy]benzene (B3) as branching agent. The synthesizedbranched copolymers were soluble in polar organic solvents, such as N-methyl-2-pyrroli-done (NMP), N,N-dimethylacetamide (DMAc) and dimethylsulfoxide (DMSO), and couldbe cast to form tough and smooth films. The effect of degree of branching (DB) on the pro-ton conductivity, swelling ratio and oxidative stability of the membranes was investigated.With increasing DB value, the proton conductivity and oxidative stability of the mem-branes increased. A maximum oxidative stability of the branched membrane with 4% ofDB value was determined to be 3.4 times larger than that of the linear membrane. In addi-tion, as the DB value increased, the swelling ratio of the membranes decreased from 13.51%to 9.09% at 80 �C. The results indicated that increasing DB value might be an effective wayto improve the properties of proton exchange membranes.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Proton exchange membrane fuel cells (PEMFCs) arebelieved to have a potential to alleviate the majorproblems associated with the energy production and con-sumption [1]. Large interest in using PEMFCs as powersources, such as vehicles and portable applications hasgreatly promoted the fundamental research in the field ofproton-conducting polymers over the past few years[2–6]. Great progress has been made on the developmentof proton-exchange membranes. However, there are stilltwo major remaining challenges that current PEMFCs face:cost and lifetime [3]. Perfluorinated membranes areregarded as the most preferable candidate from the stand-point of chemical and thermal stabilities, physical and pro-ton-conducting properties, but they are too expensivewhich is a major obstacle for their widespread applicationin fuel cells [7,8]. As one of alternative proton exchange
. All rights reserved.
37; fax: +86 775
. Wang).
membranes, sulfonated poly(arylene ether)s [9–14] havebeen developed. The sulfonated poly(arylene ether)s werefound to possess good thermal stability, appropriatemechanical strength and high proton conductivity. How-ever, most of them failed to be used as proton-exchangemembranes since their short lifetime due to a combinationof hydrolysis and oxidative degradation [3]. Therefore, ef-forts to improve the stability of the membranes are eagerlyrequested.
It has been reported by several research groups that thepoly(arylene ether)s containing sulfonic acid groups on thependant side chains exhibit good durability [15–22].Unfortunately, the mechanical properties of these linearsulfonated polymers tend to deteriorate progressively withsulfonation, which makes the long-term stability of highlysulfonated polymers questionable [3,23]. Recently, cross-linked sulfonated poly(arylene ether)s for fuel cell applica-tions have attracted increasing interest [24–29] due totheir effective improvement on the properties of the mem-branes. Guiver et al. [23] had synthesized the cross-linkedpolymer electrolytes with polyatomic alcohols, whichmade the polymer mechanically stronger and reduced its
1986 L. Wang et al. / European Polymer Journal 47 (2011) 1985–1993
swelling by water. Meng et al. [30] had prepared thecross-linked membranes, which exhibited good chemicalresistance and oxidative stability from Fenton’s tests. InGeneral, cross-linking is an effective way to improve oxida-tive stability and reduce swelling by water. However, thecross-linked membranes are usually insoluble in commonorganic solvents and difficult to process. Hay et al. [31]gave a method to prepared PEMs and reported the firstexamples of branched poly(ether-ketones)s with sulfonicacid groups on the end groups synthesized by specialmonomers. The branched membranes cast from the poly-mer solution showed proton conductivities comparableto that of Nafion. More recently, branched sulfonatedpoly(arylene ether ketone sulfone)s [32] containingbranching agent lower than 0.4% were prepared. The resul-tant polymers with high solvent power had a positive ef-fect on the oxidative stability and mechanical strength.However, when the branching agent was increased tohigher than 0.4%, the polymers were cross-linked by itselfduring the polymerization and thus the highly branchedpolymers could not be obtained.
In order to obtain highly branched polymers with goodsolubility for casting to form the films with desirable prop-erties, appropriate monomers, such as bisphenol fluoreneand B3, were chosen. It is reasonable to believe that theintroduction of bisphenol fluorene with large steric hin-drance and B3 scaffold with long and hard arm can reducecross-linked opportunities and highly branched polymersmay be obtained. In this work, a series of linear andbranched sulfonated poly(arylene ether)s were synthe-sized. The highest degree of branching was obtained inthe branched polymer with 4% of B3. The resultant poly-mers were soluble in polar organic solvents and could becast to form tough and smooth films. The properties,including hydrolysis and oxidative stabilities, ion-ex-change capacity (IEC), proton conductivity, water uptake,thermal stability and swelling ratio of the membranes,were investigated.
2. Experimental
2.1. Materials
Bisphenol fluorene, phloroglucinol and 4,4-difluoroben-zophenone (DFBP) were obtained from commercial sourcesand used without further purification. DMAc was driedwith 4 Å molecule sieves and toluene was dried with so-dium wire prior to use. Anhydrous potassium carbonatewas dried at 200 �C for 10 h prior to use. Sulfonated dif-luorobenzo phenone was prepared according to the litera-ture [33].
2.2. Measurements
1H NMR spectra, reported in ppm, were recorded on aVarian 400-Hz NMR instrument using DMSO-d6 as the sol-vent with tetramethylsilane (TMS) as the internal stan-dard. The glass transition temperatures (Tg) weredetermined on a TA instrument Q200 DSC at a heating rateof 10 �C/min under the protection of nitrogen. The second
scan was immediately initiated just after the sample wascooled to room temperature. Thermal stability of the poly-mers was investigated at a rate of 10 �C/min during thetemperature range of 30–600 �C on a Q50 TGA instrumentunder a nitrogen environment with a flow of 50 mL/min.
2.3. Synthesis of 1,3,5-tris[4-(4-fluorobenzoyl)phenoxy]benzene (B3)
B3 was synthesized from phloroglucinol and DFBP usinga modified method reported in the literature [34]. To a250 ml four-neck flask equipped with a mechanical stirrer,nitrogen inlet, thermometer and a condenser was charged36.6 g (0.168 mol) of DFBP, 70 ml of DMAc and 13 g ofK2CO3. The mixture was heated to 165 �C with constantstirring, together with adding the solution of 1.51 g(0.012 mol) of phloroglucinol in 10 mL DMAc dropwise.After the adding the solution, the temperature was keptat 165 �C for 6 h. The resulting product was cooled to roomtemperature and then poured into the aqueous solution ofhydrochloric acid. The precipitates were filtered, washedwith ethanol to remove excess DFBP and dried. The crudeproduct was crystallized from ethanol and water to givewhite powders (m.p. 130–131 �C; 52.5% yield). The struc-ture of B3 was shown in Scheme 1 and confirmed by 1HNMR (Fig. 1). 1H NMR (CDCl3, ppm): 6.61 (S, 3 H), 7.14–7.20 (m, 12 H), 7.79–7.84 (m, 12 H).
2.4. Synthesis of the linear and branched polymers
The synthesis procedure of the linear polymer 5a wasdescribed by Chen et al., using bisphenol fluorene 1(0.70 g, 2.0 mmol), sulfonated difluorobenzophenone 2(0.5064 g, 1.2 mmol), 4,40-Difluorobenzo phenone 3(0.1744 g, 0.8 mmol) [33]. The fibrous polymer 5a was col-lected and dried at 110 �C under vacuum for 24 h (yield:93%).
The polymerization procedure for the polymer 5g, a typ-ical example for synthesis of the branched polymers 5b–5g,is described below. The bisphenol fluorene 1 (0.70 g,2.0 mmol), sulfonated difluorobenzophenone 2 (0.5064 g,1.2 mmol), 4,40-Difluoro benzophenone 3 (0.1483 g,0.68 mmol), B3 (5.76 � 10�2 g, 8.0 � 10�2 mmol), potas-sium carbonate (0.416 g, 3.0 nmmol), DMAc (10 mL) andtoluene (6.0 mL) were carefully introduced into a 50 mLthree-neck round bottom flask equipped with a Dean–Starktrap and condenser under nitrogen protection. Toluene wasused as azeotropic solvent to remove the water formed dur-ing the reaction. The reaction mixture was heated at 140–150 �C for 4.0 h to remove the water produced and then in-creased the temperature to 170 �C (oil bath temperature) todistill the toluene off. The reaction mixture was kept at thistemperature for 4 h. After cooled, the resulting viscous mix-ture was diluted with 5 mL DMAc and poured slowly into100 mL of mixture of deionized water and methanol con-taining 2 mL of concentrated HCl to precipitate the formedpolymer. The precipitates were filtered and washed with
Scheme 1. Synthesis of 1,3,5-tris[4-(4-fluorobenzoyl)phenoxy] benzene(B3).
Fig. 1. 1H NMR spectrum for B3.
L. Wang et al. / European Polymer Journal 47 (2011) 1985–1993 1987
water for three times to remove inorganic salts. The fibrouspolymer was collected and dried at 110 �C under vacuum for24 h (Yield: 89%).
The branched polymers 6a–6c were synthesized by thesimilar procedure of 5g, as shown in Scheme 3. 1,1,1-tris-p-Hydroxyphenylethane (THPE) was used as a branchingagent instead of B3. The fibrous polymer 6c was obtainedin a yield of 91%.
Polymer films were prepared by casting from a solutionof 5 wt% polymer in DMAc on the glass slide at 60 �C in adust-free environment, and then dried at 100 �C under vac-uum to remove the residue solvent. The membrane wasacidified with 2 M HCl solution overnight to exchange so-dium ion with proton, then rinsed with deionized water
to get rid of excess HCl and finally dried under vacuumat 110 �C for 24 h.
2.6. Water uptake, dimensional stability and density
After vacuum-dried at 110 �C for 24 h, the membraneswere weighed, and then immersed in deionized water atroom temperature for 24 h. The films were taken out,wiped, dried and quickly weighed again. The water uptake(S) was calculated by the following equation:
S ¼ ðWs �WdÞ=Wd � 100ð%Þ
where Ws and Wd are the weights of wet and dry mem-brane, respectively.
Dimensional change was investigated by immersing themembrane into water at different temperature for 24 h, thechange of length was calculated from the equation:
L ¼ ðLs � LdÞ=Ld
where Ls and Ld are the length of wet and dry membrane,respectively.
Membrane density was calculated from measurementsof membrane dimensions and weight after drying at110 �C for 24 h according to the method reported by theliteratures [35,36]. The volume-based water uptake (WU)was calculated by the following equation:
WUðVol%Þ ¼ ðWs �WdÞ=dw=ðWd=dmÞ � 100 ð%Þ
where Ws and Wd are the weights of the wet and dry mem-branes, respectively; dw is the density of water (1 g/cm3),and dm is the membrane density in the dry state.
2.7. Ion-exchange capacity
The titration technique was used to determine the ion-exchange capacity (IEC) of the membranes. The mem-branes were first converted to acid form and immersedin a 1 M NaCl solution for 24 h to exchange H+ ions withNa+ ions. Then, the exchanged H+ ions within the solutionswere titrated with a 0.01 M NaOH solution using phenol-phthalein as an indicator. The IEC values were calculatedaccording to the following equation:
1988 L. Wang et al. / European Polymer Journal 47 (2011) 1985–1993
IECðmequiv=gÞ¼ consumed NaOH
�molarity NaOH=weight of dried membrane
2.8. Proton conductivity
Proton conductivity measurement was conducted onthe hydrated film using an impedance analyzer (Solartron1260A) with an oscillating voltage of 10 mV and frequencyrange from 10 MHz to 500 Hz. Prior to the measurement,the membrane was immersed in 1 M H2SO4 at 80 �C for12 h and then washed to a pH of 7 with deionized water.After kept in deionized water over 12 h, the membranewas tightly clamped and placed in a closed container withthe relative humidity of 100%. The whole container wasplaced in a temperature-controlled water bath during themeasurement. The proton conductivity was calculatedfrom the impedance data according to following equation
r ¼ d=RS
where d and S are the thickness and the cross section areaof the specimen, respectively, and R is the membrane resis-tance measured by impedance analyzer.
2.9. Oxidative and hydrolysis stabilites
The oxidative stability of the branched sulfonatedcopolymers was investigated by immersing the
Scheme 2. Synthesis of linear and
membranes into Fenton’s reagent (2 ppm FeSO4 in 3%H2O2) at 80 �C. The oxidative stability of the membraneswas characterized by the expended time that the mem-branes started to break into pieces. The hydrolytic stabilitywas also investigated by treating membrane samples inboiling water.
3. Results and discussion
3.1. Synthesis and characterization of monomer and polymers
B3 was synthesized from phloroglucinol and DFBP usinga modified method reported in the literature [34]. Fig. 1depicts the 1H NMR spectrum of B3. The signals of B3 wereobserved at 6.61 ppm, 7.14–7.20 ppm, 7.79–7.84 ppm,respectively. As shown in Scheme 2, a series of linear andbranched sulfonated poly(arylene ether)s were synthe-sized by reaction of bisphenol fluorene, 4,40-Difluoroben-zophenone, sulfonated difluorobenzophenone in theabsence or presence of B3. The sulfonated polymers com-position was set at 60% according to the literature [33](using 60% of sulfonated difluorobenzophenone relativeto mol% of bisphenol fluorene employed) since the mem-branes with this sulfonation degree exhibited good protonconductivity. The polymerization was studied by changingthe amount of B3 from 0 to 4 mol% relative to bisphenolfluorene employed. The feed amount of B3 and monomer3 were calculated by formula (1) and (2), respectively.
branched polymers 5a-5g.
Table 1The polymerization results and properties of membranes 5a–5g.
a Mol% of B3 relative to mol% of bisphenol fluorine.b Measured at 30 �C in DMAc.c Theoretical IEC value.d Determined by titration with 0.01 M NaOH aq.e Measured at 80 oC.
L. Wang et al. / European Polymer Journal 47 (2011) 1985–1993 1989
The feed amount and polymerization results are listedin Table 1. The sulfonated polymers were obtained in goodyields (>88%). The intrinsic viscosities were measured withUbbelohde viscometer at 30 �C. The intrinsic viscosities de-creased from 1.49 dL/g to 0.49 dL/g with increasing degreeof branching. The lower viscosities of branched polymerscould be a result of fewer chain entanglements by polymerchain because of the increase in branch points [37]. Bycomparing the 1H NMR spectra shown in Fig. 2 of the linearand branched polymers, we found that the signal of B3 oc-curred at 6.66 ppm in the branched polymers, suggestingthat the B3 moiety was introduced into the copolymers.The polymers containing 0–4 mol% B3 could be success-fully synthesized, however, when the amount of B3 wasover 4 mol%, the copolymers were crosslinked during thepolymerization and became insoluble in polar organic sol-
Fig. 2. 1H NMR spectrum for linear polymer 5a and branched polymers5d, 5g. The arrow indicates that the branched polymers have beensuccessfully synthesized.
vents. Compared with the branched polymers recently re-ported by the literature [32] which would be crosslinkedduring the polymerization containing over 0.4 mol% ofbranching agent, the branched sulfonated poly(aryleneether)s we synthesized exhibited higher DB values. Thehigh DB value was mainly due to the introduction ofbisphenol fluorene and B3 moieties. The large steric hin-drance of fluorine ring might cause the polymer chain torepulse each other, which would lead to a reduced cross-linked opportunities and improve DB value accordingly.Furthermore, the B3 scaffold with long and hard arm mightalso reduce cross-linked opportunities of the branchedpolymers. In order to evaluate the hypothesis, thebranched polymers 6a–6c were synthesized using THPEas branching agent which has been used by Park [32] fora contrast (Scheme 3). Bisphenol fluorine was used to syn-thesize the branched polymers 6a–6c instead of bisphenolA. We found that the branched polymers began to crosslinkwhen the branching agent content was over 2%, whichproved that DB value could be improved by only changingthe structure of the bisphenol monomer. The results sug-gested that the DB value could be further increased byusing special monomers.
The solubility of the synthesized polymers is listed inTable 2. The linear and branched polymers 5a–5g contain-ing 0–4 mol% B3 were soluble in common polar aprotic sol-vents, such as DMF, DMSO, DMAc and NMP, and insolublein H2O and CH3OH. The branched polymers could be read-ily cast into tough and smooth films from their solutions.
3.2. Thermal properties
The thermal properties of the polymers were investi-gated by TGA and DSC. In order to reduce the effect ofwater, polymers 5a–5g were preheated from 50 to 200 �Cand then TGA experiment was carried out from 50 to600 �C at a heating rate of 10 �C/min under the protectionof nitrogen. Fig. 3 shows the TGA spectra of linear andbranched sulfonated polymers 5a–5g. The results indicatedthat branched sulfonated polymers 5b–5g exhibitedsimilar thermal stability as that of the linear polymer 5a.Two consecutive weight loss steps were observed. The firstweight loss beginning at about 250 �C could be attributedto the weight loss of –SO3H by the desulfonation. The sec-ond weight loss over 400 �C was due to the thermal
Fig. 3. TGA curves of sulfonated poly(arylene ether)s 5a–5g.
Fig. 4. Water uptake of sulfonated poly(arylene ether)s 5a–5g.
1990 L. Wang et al. / European Polymer Journal 47 (2011) 1985–1993
decomposition of polymer main chains, in which the poly-mer residuals were further degraded. DSC measurement ofthe polymers at a temperature range from 30 to 300 �C did
not show glass transition temperature (Tg) before thermaldecomposition. The high Tg of the polymers were attrib-uted to their ionomeric nature, which was analogous tothe results described in our previous publication [38].The high glass transition and high decomposition temper-ature of the sulfonated polymers indicate that thesebranched sulfonated polymers have the potential applica-tion for high temperature (>100 �C) PEMFC.
3.3. Water uptake, density and dimensional stability
Fig. 4 shows the water uptake of sulfonated poly(aryl-ene ether)s 5a–5g at different temperatures. Comparedwith the linear polymer, the branched polymers exhibitedhigher water uptake. The water uptake of the branched
Table 3Mechanical properties, density and water uptake of membranes 5a–5g.
a Based on dry state.b Measured at 30 �C.c Data taken from Ref. [35,36].
Fig. 5. Dimensional stability of sulfonated poly(arylene ether)s 5a–5g.
L. Wang et al. / European Polymer Journal 47 (2011) 1985–1993 1991
polymer membranes increased with increasing DB valueand temperature. It is believed that different domains inlinear polymer membranes can be formed by micro-phaseseparation due to the mutual repulsion between hydro-philic sulfonate groups and hydrophobic blocks, and watermainly aggregated near the hydrophilic sulfonate groups.The hydrophobic regions in linear polymer membrane aredifficult to absorb water. However, the branched mem-branes are different and water can filter into the hydropho-bic region in the three-dimensional structure near thebranch points [39], which might contribute to the highwater uptake of the branched polymers. These results ofthe density provide further support for the inference.The densities of the membranes were demonstrated inTable 3. The densities decreased with increasing DB value.The volume-based water uptake was calculated and thehighly branched polymer membranes have large free vol-ume, which could help trap water in the branched poly-mers membranes.
Sulfonated poly(arylene ether)s exhibited good poten-tial as proton exchange membrane. However, these sulfo-nated aromatic polymer membranes required a highsulfonation level to achieve sufficient proton conductivity.Unfortunately, for the linear polymers, such a high sulfona-tion level made them excessively water-swollen anddimensional unstable in water [40]. In order to evaluatewhether the branched polymers synthesized could relievethese problems, the dimensional stability of which wereevaluated at 30 and 80 �C, respectively, by comparing theirhydrated state with their dry state. The results were illus-trated in Fig. 5. It was found that the swelling ratio ofbranched polymer membranes decreased significantlywith increasing DB value at same temperature. At 80 �C,the swelling ratio of the membranes decreased from13.51% to 9.09% with increasing DB values from 0% to 4%.We also saw from Fig. 5 that the swelling ratio of all themembranes at high temperature was higher than that ofthe membranes at low temperature and the increase ofswelling ratio of branched polymer membranes was smal-ler than that of the linear polymer membrane with temper-ature, as deduced from the distance between two lines inFig. 5. The results indicated that the branched polymermembranes could exhibit better dimensional stability than
that of the linear polymer membrane, which was in excel-lent agreement with the results reported by the literature[32]. The high dimensional stability of branched polymermembranes might be contributed to the increased branch-ing points, which limit the movement of polymer chain inwater and prevent swelling of the membranes.
Mechanical properties are crucial to a viable fuel cellmembrane. Mechanical properties of the PEM materialsaffect manufacturing condition of MEAs and the durabilityof PEMFC operations. The mechanical properties of thebranched polymer membranes and Nafion 117 membranewere determined at 30 �C and 100% relative humidity. Ta-ble 3 listed the tensile strength and the elongation at breakof the branched polymer membranes and Nafion 117. Allthe membranes show comparable tensile strength withNafion 117. The tensile strength and the elongation atbreak decreases with increasing DB values from 0% to 4%.The decrease of densities and increase of water uptakewith increasing DB values could result in the reductionmechanical properties.
3.4. Ion-exchange capacity and proton conductivity
The ion-exchange capacities (IECs) of the membranesmeasured are almost the same or slightly smaller than thetheoretical value, as listed in Table 1. With increasing DB va-lue, the IEC values decrease slightly. The proton conductivityof the branched and linear polymer membranes was mea-sured at 80 �C and 100% relative humidity, as shown inFig. 6. In general, proton conductivity relies heavily on theIEC values and water uptake of electrolyte membranes.However, IEC values of the sulfonated polymers did notchange significantly with increasing DB value. Water uptakebecomes a critical factor in proton conductivity for polymerelectrolytes because water in a membrane acts as a trans-portation medium of protons. With increasing DB values,the proton conductivities of branched polymer membranesincreased gradually due to the increasing water uptake ofthe membranes. All the branched membranes exhibitedhigher proton conductivities (>8.0 � 10�2 S/cm), whichmeet the requirement (>10�2 S/cm) of fuel cells, than thoseof the linear polymer membranes [41].
Fig. 6. Proton conductivity of sulfonated poly(arylene ether)s 5a–5g.
1992 L. Wang et al. / European Polymer Journal 47 (2011) 1985–1993
3.5. Oxidative and hydrolysis stabilities
The oxidative stability to peroxide radical attack wasevaluated by measuring the elapsed time just before themembrane breaks after its immersion in Fenton’s reagent(2 ppm FeSO4 in 3% H2O2) at 80 �C. From the test resultsshown in Fig. 7, we can see that the branched polymermembranes exhibit superior oxidative stabilities ascompared with that of the linear polymer membrane.The oxidative stabilities of branched polymer membranesincreased gradually with increasing DB values. The mem-brane formed by the branched polymers with 4 mol% ofB3 showed the best oxidative stability and the elapsedtime was over 190 min, which is 3.4 times longer thanthat of the membrane formed by linear polymers(55 min) at the same condition. The hydrolytic stabilitywas investigated by treating the membrane samples inboiling water for more than 8 days. The membranes didnot change in shape and appearance after the treatment,implying that there was no hydrolysis occurred duringthe treatment.
Fig. 7. Oxidative stability of sulfonated poly(arylene ether)s 5a–5g.
4. Conclusions
Highly branched sulfonated poly(arylene ether)s weresuccessfully synthesized in the presence of B3 and bisphenolmonomers. Different from the cross-linked membranes, thehighly branched membranes synthesized were soluble incommon polar aprotic solvents and could be readily castinto tough and smooth films. Compared with the membraneformed by linear polymers, the highly branched membranesexhibited higher proton conductivities, better dimensionalstabilities and oxidative stabilities.
The long and rigid arm of B3 monomer and large sterichindrance of bisphenol fluorene had a positive effect onincreasing DB values of the branched polymers and theDB value could be further improved by changing the struc-ture of the monomers. The properties of the branchedmembranes could be improved by properly increasingthe DB value of the branched polymers. Further studiesare ongoing.
Acknowledgments
The authors would like to thank Chinese National Nat-ural Science Foundation (51003060) and Shenzhen Sci &Tech research Grant (JC200903130261A and CXB200903090012A) for their financial supports.
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