Modulation of Phase Separation Between Spherical and Rodlike Molecules
Using Geometric Surfactancy
Lichang Zeng,† Thomas N. Blanton,§ and Shaw H. Chen*,†,‡
†Department of Chemical Engineering, and ‡Laboratory for Laser Energetics, University of Rochester,Rochester, New York 14623-1212, and §Corporate Research and Engineering, Eastman Kodak Company,
Rochester, New York 14650-2106
Received April 14, 2010. Revised Manuscript Received May 28, 2010
A Dyad consisting of C60 linked to a π-conjugated oligomer by a propylene spacer was synthesized to explore itsability to modulate phase separation between OFTB and PCBM using differential scanning calorimetry, hot-stagepolarizing optical microscopy, X-ray diffraction, and atomic force microscopy techniques. Upon thermal annealing at10 �C above itsTg for 12-48 h, the 1:1 blend ofOFTB andPCBM resulted in a eutectic mixture. Thermal annealing of aOFTB:Dyad:PCBM film with a 9:2:9 mass ratio at 10 �C above its Tg for 12 h resulted in an amorphous film. Its AFMphase image indicated phase separation into two interspersed 30 nm amorphous domains at approximately equalfractions. Geometric surfactancy is inferred from the formation of microemulsions in analogy to widely reportedtraditional oil-surfactant-water systems and ternary polymer blends. In contrast, thermal annealing of a 7:6:7 filmunder a similar condition resulted in an amorphous film with compositional uniformity.
Introduction
Traditional amphiphiles comprising polar and nonpolar moi-eties are capable of serving not only as cosolvents for hydrophobicsolutes in water1 but also as surfactants for the creation ofmicrostructures involving water and oil, such as bilayers, micelles,droplet, and bicontinuous microemulsions.2 In addition, macro-molecular surfactancy originating from the Flory-Huggins inter-action is exemplified by diblock copolymers that mediate theformation of similar microstructures between two parenthomopolymers.3-7 A geometric amphiphile consisting of a disk-like moiety linked to a rodlike moiety with an undecylene spacerhas been demonstrated for cosolvency.8
In analogy to traditional surfactancy, the present study wasmotivated to demonstrate the feasibility of geometric surfactancyin terms of microstructure formation using a dyad comprisinggeometrically dissimilar spherical and rodlike moieties. To con-struct an example geometric surfactant, a propylene linker wasinserted between an electron-donating conjugated oligomer andan electron-accepting C60-derivative relevant to organic photo-voltaics. The flexible linker is essential to promoting preferentialpacking of the conjugated oligomer and the C60-derivative asseparate chemical entities. Both geometric packing and electronicinteraction should be accounted for geometric surfactancy. As
demonstrated for traditional oil-surfactant-water systems andternary polymer blends, the formation of stable microstructurescan be expected under appropriate conditions, considering theenthalpy and entropy changes accompanying molecular self-organization in addition to interfacial energy. Three model com-pounds were employed to prepare blends for the characterizationof powder and film morphologies by differential scanning calo-rimetry (DSC), hot-stage polarizing optical microscopy (POM),X-ray diffraction (XRD), and atomic forcemicroscopy (AFM) toascertain the formation of microemulsions.
Experimental Section
Materials Synthesis and Characterization. The targetcompounds, 4,7-bis(5-(9,9-bis(2-methylbutyl)fluoren-2-yl)thien-2-yl)-2,1,3-benzothiadiazole, OFTB, and N-methyl-20-(4-(3-(7-(5-(7-(5-(9,9-bis(2-methylbutyl)fluoren-2-yl)thien-2-yl)-2,1,3-ben-zo-thiadiazol-4-yl)thien-2-yl)-9,9-bis(2-methylbutyl)fluoren-2-yl)-propyl)phen-2-yl)pyrrolidino-[30,40:1,2][60] fullerene, Dyad, weresynthesized and purified according to reaction Scheme S.1 follow-ing the procedures as described in the Supporting Information.Also included in the Supporting Information are the 1H NMRspectra acquired in CDCl3 at 298 K with an Avance-400 spectro-meter (400MHz), Figures S.1 (OFTB) and S.2 (Dyad). Elementalanalysis was carried out by Quantitative Technologies, Inc. Mole-cular weights were measured with a TofSpec2E MALD/I TOFmass spectrometer (Micromass, Inc., Manchester, U.K.) using2-[(2E)-3-(4-tert-butylphenyl)-2-methylpropenylidene] malanoi-trile (DCTB) as the matrix. [6,6]-Phenyl C61-butyric acid methylester, PCBM, at a purity level of 99.5% was purchased fromNano-C (MA).
Bulk-Phase Thermal Transitions andMorphologies. Ther-mal transition temperatures were determined by DSC (Perkin-Elmer DSC-7) with a continuous N2 purge at 20 mL/min. Thethree pure componentsOFTB, PCBM, andDyadwere preheatedto 310 �C followed by quenching to -30 �C for thermal analysisby DSC at a heating rate of 20 �C/min. One set of blends waspreheated to 310 �C followedby quenching to-30 �Cbefore theirheating scans were recorded at 20 �C/min. The other set of blendswas preheated to 310 �C followed by quenching to -30 �C, andthen they were annealed at 10 �C above their respective glass
*Author to whom correspondence should be addressed. E-mail: [email protected].(1) Bauduin, P.; Renoncourt, A.; Kopf, A.; Touraud, D.; Kunz, W. Langmuir
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P. F. Macromolecules 2009, 42, 3063–3072.(8) Date, R. W.; Bruce, D. W. J. Am. Chem. Soc. 2003, 125, 9012–9013.
transition temperatures for 12h.Heating scanswere also collectedat 20 �C/min after the blends were cooled to room temperature.The nature of phase transitions was characterized by hot-stagePOM (DMLM, Leica, FP90 central processor and FP82 hotstage, Mettler Toledo). The powder samples for XRD analysiswere preheated to 310 �C followed by quenching to roomtemperature and then annealing at 103 �C for 12-48 h withsubsequent cooling to room temperature. X-ray diffraction datafor powder samples on (100) silicon wafer were collected inreflection mode geometry using a Rigaku D2000 Bragg-Brenta-no diffractometer equipped with a copper rotating anode, dif-fractedbeamgraphitemonochromator tuned toCuKR radiation,and scintillation detector. Diffraction patterns were collected at2θ from 2 to 30� with a step size of 0.02�.Film Preparation and Characterization. Sample mixtures
at predetermined mass ratios were spin-coated onto microscopeglass slides from 1 wt % chloroform solutions at 1000 rpmfollowed by drying under vacuum. The thicknesses of the resul-tant films were determined by optical interferometry (Zygo NewViews 5000). Thermal annealing was performed under argon at10 �C above glass transition temperatures for 12 h followed bycooling to room temperature. The phase and topography imagesofboth the pristine and thermally annealed filmswere recordedonaNanoscope III (Digital Instrument, CA) AFM in tapping modewith aNT-MDTNSG01 cantilever at a typical scan rate of 0.5Hzunder ambient condition. X-ray diffraction data for thin films onmicroscope glass slides were collected using the same instrumentunder the same conditions as described above.
Results and Discussion
Depicted in Chart 1 are the molecular structures of the threecomponents employed in this study. The synthesis and purifica-tion procedures togetherwith the characterization data forOFTB
and Dyad are described in the Supporting Information, whilePCBMwas used as received from a commercial source at a puritylevel of 99.5%. We chose to work withOFTB in light of a recentreport on organic photovoltaics using its long-chain polymeranalogue as the electron-donor and PCBM as the electron-acceptor.9 As indicated by the DSC data accompanying mole-cular structures, OFTB has a glass transition temperature, Tg,at 81 �C followed by crystallization at 165 �C and a crystallinemelting point,Tm, at 222 �C.Dyad has aTg at 181 �C, andPCBMhas a crystalline transformation at 274 �C and a Tm at 292 �C.
Three blends, OFTB:Dyad:PCBM at 10:0:10, 9:2:9 and 7:6:7mass ratios, were prepared by codissolution in chloroform
followed by evaporation of the solvent for an investigation oftheir morphologies. Thoroughly dried powders were character-ized by DSC for their thermal transition temperatures, and theresults in which phases were identified by POMmicrographs (seeSupporting Information, Figure S.3) are shown in Figure 1.Presented as dashed curves, preheated and thermally quenchedsamples of the 10:0:10, 9:2:9, and 7:6:7 blends exhibit single Tg
values at 93, 105, and 118 �C, respectively,which is consistentwithcompositional uniformity as will be corroborated by AFM ofspin-cast films shown in Figure 4 below.
The preheated and thermally quenched blends were thenannealed at 10 �C above their respective Tg values for 12 hfollowed by cooling to room temperature. The acquired DSCthermograms are shown as solid curves in Figure 1. Thermalannealing of the 9:2:9 and 7:6:7 blends resulted in DSC thermo-grams identical to those of unannealed samples, whereas thermalannealing of the 10:0:10 blend led to partial crystallization with asharpmelting peak at 205 �C followed by a complete transition toan isotropic liquid at 242 �Cas observed under POM.The coolingscans were also gathered at -20 �C/min for all three blendsfollowing the heating scans. The absence of crystallization duringcooling to room temperature suggests that the crystals thatmeltedfrom 205 to 242 �C have resulted exclusively from thermalannealing. The origin of the sharp crystalline melting at 205 �Cwas further examined by powder XRD as shown in Figure 2 todifferentiate between cocrystallization and the formation of aeutectic mixture.
Partial crystallization in the annealed 10:0:10blend is evidencedby the presence of several narrowdiffraction peaks attributable tocrystalline components on top of two broad amorphous peakscentered at 10.4 and 19.6 degrees inferred from a control experi-ment with an unannealed blend. Although amorphous phaseshave no crystalline short or long-range order, there still exists arange of most probable distances between neighboring atoms aswell as defined bond distances for functional groups, such asfullerene, phenyl ring, and so on. This semishort-range order isresponsible for the broad peaks observed indiffractionpatterns ofamorphous materials. The powder XRD pattern of the semicrys-talline binary blend represents a superposition of diffractionpatterns of OFTB and PCBM, indicating that these two purecomponents crystallized independently during thermal annealing.Using the Scherrer technique,10 the average crystallite size was
Figure 1. DSC heating scans of OFTB:Dyad:PCBM at threecompositions as indicated. Dashed curves: samples preheated to310 �C and quenched to-30 �C before heating at 20 �C/min. Solidcurves: samples preheated to 310 �C and quenched to -30 �Cbefore annealing at 10 �Cabove respectiveTg values, as determinedwith dashed curves, for 12 h and then cooled to room temperaturebeforeheating at 20 �C/min.Complete transition to isotropic liquidfor the annealed10:0:10blendoccurredat242 �Cas indicatedbyanarrow. Symbols: G, Glassy; K, Crystalline; I, Isotropic.
Chart 1. Molecular Structures of OFTB, PCBM, and Dyad Used in
This Study. Symbols: G, Glassy; K, Crystalline; I, Isotropic
(9) Svanstr€om, C. M. B.; Rysz, J.; Bernasik, A.; Budkowski, A.; Zhang, F. L.;Ingan€as, O.; Andersson,M. R.;Magnusson, K. O.; Benson-Smith, J. J.; Nelson, J.;Moons, E. Adv. Mater. 2009, 21, 4398–4403.
estimated at 26 and 23 nm for OFTB and PCBM, respectively,values reasonably close to 19 and 21 nm obtained from thethermally annealed powders of pure components. The observedcrystallite sizes are greater than the minimum size of 2.5 nm for acrystallite to exhibit its characteristic XRD pattern.11 It is con-cluded thatOFTB and PCBM comprise a eutectic mixture with aTm at 205 �C, below those of the two pure components, 222 �C(OFTB) and 292 �C (PCBM). The phase diagram was con-structed for the OFTB:PCBM binary system as shown in Sup-porting Information, Figure S.4, a behavior similar to that of thebinary blend of poly(3-hexylthiophene) with PCBM.12
Approximately 100-nm-thick films of the three ternary blendswere prepared by spin coating from chloroform followed bydrying in vacuo at room temperature. The pristine films appearedamorphous, according to POM micrographs (Supporting Infor-mation, Figure S.3a-c). The combination of XRD pattern andPOM micrograph shown in Figure 3a indicates that the 10:0:10film turned polycrystalline after thermal annealing at 10 �C aboveitsTg for 12 h. The diffraction peak at 2θ=4.37� appearing in thefilm XRD pattern is also present in the powder XRD patterns ofOFTB and the 10:0:10 blend identified by arrows in Figure 2.Furthermore, the average crystallite size in the annealed film wasestimated at 30 nm, in agreement with 26 nm estimated forOFTB
crystallites in the annealed powders of 10:0:10 blend. Comparedto powder XRD, film XRD produced a pattern with fewerdiffraction peaks at 2θ less than 10� and no peaks between 10and 30�. This result is attributed to the likely preferred orientationof crystallites such that one set of lattice planes lies parallel to thesample surface thus in the right geometry to be observed duringdata collection. The reduced sample volume of the thin filmcompared to that of the powder and the limited scattering powerof organic materials comprised of low atomic-number elements,such as hydrogen and carbon, are additional contributing factorsto fewer diffraction peaks in the 10:0:10 thin film than its powder.As revealed by XRD pattern and POM micrograph shown inFigure 3b, the 9:2:9 and 7:6:7 films remained amorphous afterthermal annealing at 10 �Cabove their respectiveTg values for 12h.
Spin-cast films were further characterized by AFM for theircompositional and/ormorphological characteristics. In principle,
phase contrast can be attributed to phase separation arising fromlateral variations in composition or morphology regardless oftopography.13 Empirically, the features observed under transmis-sion electron microscopy have been employed to establish AFMphase contrast as a valid tool for elucidating phase separationin block copolymers,14,15 binary polymer blends,15 and PCBM-polymer blends.16 Phase images are compiled in Figure 4 for filmsbefore and after thermal annealing at 10 �CaboveTg for 12 hwithsubsequent cooling to room temperature. Without encounteringcrystallization, the cooling process served to kinetically trap insolid state the film morphologies prevailing under the annealingconditions. The pristine films are compositionally uniform, accord-ing to Figure 4 panels a, b, and c, in addition to being amorphousunder POM. Upon thermal annealing, the 10:0:10 film was foundto crystallize, appearing as phase contrast shown in Figure 4d, witha melting range from 208 to 242 �C under hot-stage POM, anobservation in agreement with the bulk phase diagram within ex-perimental error. The 7:6:7 film remained amorphous after thermalannealing, according to its DSC thermogram, POM micrograph,and XRD pattern, and was compositionally uniform according toFigure 4f versus 4c.
Because of the difference both in geometry andmolecular-levelinteraction, OFTB and PCBM are expected to be partially mis-cible at best. As indicated by the POM micrograph and XRDpattern, the 9:2:9 film remained amorphous after thermal anneal-ing.Hence, the phase contrast shown inFigure 4e is interpreted ascompositional separation into two interspersed amorphous do-mains. Section analysis was performed to yield about 30 nmas theaverage sizes for both amorphous domains at approximatelyequal fractions (see Supporting Information,Figure S.6).Whereasthe observed film morphology remained intact when left at roomtemperature for up to six months, its long-term morphologicalstability has yet to be systematically investigated in terms ofchemical composition and thermal treatment prior to cooling toroom temperature. The AFM phase images shown in Figure 4eresemble the transmission-electron and optical micrographs ofmicroemulsions in ternary polymer blends5,6 and the cryo-trans-mission electronmicrographs of traditional oil-surfactant-water
Figure 2. Samples were preheated to 310 �Cand then quenched toroom temperature before annealing at 103 �C for 48 h followed bycooling to room temperature for powder XRD analysis. Weakercrystalline diffractionpeaks resulted froma shorter annealing time,for example, 12 h. A quenched but unannealed 10:0:10 blend wasalso characterized for identification of broad amorphous peaks.
Figure 3. XRDpatterns (collected at 0.02�/step), 10 s per step, andPOMmicrographs as the insets for 100-nm-thick spin-cast films ofOFTB:Dyad:PCBM blends at (a) 10:0:10 mass ratio and (b) 9:2:9mass ratio after thermal annealing at 10 �C above their respectiveTg values for 12 h followed by cooling to room temperature.Essentially the same XRD and POM results were observed forthe 7:6:7 film as reported in panel b for the 9:2:9 film. Preheating to310 �C as conducted for powders was avoided to preserve filmintegrity. No diffraction peaks are visible at 2θ between 10 and 30�as shown in Supporting Information, Figure S.5.
(10) Jenkins, R. Snyder, R. L. Introduction to X-ray Powder Diffractometery;John Wiley & Sons Inc.: New York, 1996: pp 90-91.(11) Bartram, S. F. InHandbook of X-rays. Kaelble, E. F., Ed.; McGraw-Hill, Inc:
New York, 1967: pp 17-9.(12) M€uller, C.; Ferenczi, T. A. M.; Campoy-Qiles, M.; Frost, J. M.; Bradley,
D. D. C.; Smith, P.; Stingelin-Stutzmann, N.; Nelson, J. Adv. Mater. 2008, 20,3510–3515.
(13) Garcia, R.; Magerle, R.; Perez, B. Nat. Mater. 2007, 6, 405-411.(14) Zhang, Q. L.; Tsui, O. K. C.; Du, B. Y.; Zhang, F. J.; Tang, T.; He, T. B.
Macromolecules 2000, 33, 9561–9567.(15) Linder, S. M.; H€uttner, S.; Chiche, A.; Thelakkat, M.; Krausch, G. Angew.
Chem., Int. Ed. 2006, 45, 3364–3368.(16) van Duren, J. K. J.; Yang, X. N.; Loos, J.; Bulle-Lieuwma, C.W. T.; Sieval,
A. B.; Hummelen, J. C.; Janssen, R. A. Adv. Funct. Mater. 2004, 14, 425–434.
microemulsions17 including domain sizes, all observed at a surfac-tant content of about 10wt%.At sucha lowcontent, it is plausiblethat facilitated by the flexible spacer,Dyadmolecules orient them-selves at the interface between the OFTB-rich and the Dyad-richamorphous domains as visualized in Figure 5. The envisionedmolecular self-organization is rationalized by geometric packingof Dyad molecules at the interface for the sake of enthalpy at theexpense of entropy of mixing with an optimum interfacial energy,thus rendering a negativeGibbs free energy change accompanyingthe formation of microemulsions.
This behavior is akin to that of a traditional surfactantpromoting the formationofmicroemulsionson the basis of polar-nonpolar amphiphilicity.2,17 Another analogy pertains to micro-emulsion formation induced by diblock copolymer as a macro-molecular surfactant between two parent homopolymers throughFlory-Huggins interaction.3-7 That a single Tg emerged from thethermally annealed 9:2:9 blend (see Figure 1) is understandablewith the reported lower limit of detectable phase separation,20-50 nm, afforded by the conventional DSC instrument18-20
employed in the present study. The results reported here representthe first demonstration of phase separation between a conjugatedoligomer and aC60-derivativemodulated byadyadwith geometricamphiphilicity. It is remarked in passing that the AFM topo-graphic images compiled in Supporting Information, Figure S.7mirror the phase images in Figure 4. In particular, it is noted that
nanoscale phase separation shown in Figure 4e is responsible forthe surface roughness observed in Figure S.7e.
Conclusions
Based on polar-nonpolar amphiphilicity, traditional surfac-tants are capable of solubilizing hydrophobic solutes in water andmediatingmicrostructure formation between oil andwater. Froman entirely different perspective, diblock copolymers can induceself-organization between the two parent homopolymers intosimilarmicrostructures by exploitingFlory-Huggins interaction.Comprising a rodlike and a disklike moiety, a geometric amphi-phile has beendemonstrated for cosolvency. In the present study ageometric amphiphile, Dyad, consisting of C60 and π-conjugatedoligomer moieties linked by a propylene spacer was tested for itsability to modulate phase separation betweenOFTB and PCBM
usingDSC, POM,XRD, andAFMtechniques.Key observationsare recapitulated as follows. Powder XRD patterns indicate thatOFTB and PCBM crystallized independently from the 1:1 blendupon thermal annealing at 10 �Cabove itsTg for 12-48 h.A eutecticmixture emergedwith aTmat 205 �C,which is definitively lower thanthose ofOFTB andPCBM at 222 and 292 �C, respectively. Thermalannealingofa spin-cast filmofOFTB:Dyad:PCBMat amass ratioof9:2:9 at 10 �C above its Tg for 12 h produced an amorphous filmaccording to POM andXRD. The analysis of its AFMphase imageconcluded phase separation into two interspersed 30 nm domains atapproximately equal fractions. Geometric surfactancy is thereforeinferred from the formation of microemulsions in analogy to widelyreported traditional oil-surfactant-water systems and ternary poly-mer blends. In contrast, thermal annealing of a 7:6:7 film under asimilar condition resulted in compositional uniformity across anamorphous film as shown by POM, XRD, and AFM.
Acknowledgment. The authors thank Mitchell Anthamatten,MatthewZ.Yates, Lewis J Rothberg, andYongli Gao for helpfuldiscussions, Semyon Papernov for assistance in the characteriza-tion of thin films by AFM, and Andrew Hoteling at the EastmanKodak Company for MALDI/TOF analysis. They are gratefulfor the financial support provided by the Department of EnergyOffice of Inertial Confinement Fusion under Cooperative Agree-ment No. DE-FC52-08NA28302 with the Laboratory forLaser Energetics and the New York State Energy Research and
Figure 4. AFMphase images of 100-nm-thick spin-cast films ofOFTB:Dyad:PCBM at three compositions before (a, b, c) and after (d, e, f)thermal annealing for 12 h at 10 �C above their respective Tg values followed by cooling to room temperature. Preheating to 310 �C asconducted for powders was avoided to preserve film integrity. Featureless phase images of panels a-c and f represent the absence of phaseseparation down to about 1 nm.
Figure 5. A schematic diagram of Dyad acting as a geometricsurfactant to modulate phase separation between OFTB andPCBM.
(17) Belkoura, L.; Stubenrauch, C.; Strey, R. Langmuir 2004, 20, 4391–4399.(18) Kammer, H. W.; Kressler, J.; Kummerloewe, C. Prog. Polym. Sci. 1993,
106, 31–85.(19) Cheunga, M. K.; Wang, J.; Zheng, S.; Mi, B. Polymer 2000, 41, 1469–1474.(20) Rathore, O.; Winningham, M. J.; Sogah, D. Y. J. Polym. Sci. A., Polym.
DevelopmentAuthority. The support ofDOEdoes not constitutean endorsement by DOE of the views expressed in this article.
Supporting Information Available: Experimental proce-dures formaterial synthesis, purification, characterization of
OFTB, Dyad, PCBM and the mixtures thereof, opticalmicrographs, the OFTB:PCBM binary phase diagram,AFM phase and topographic images, and section analysisof an AFM phase image. This material is available free ofcharge via the Internet at http://pubs.acs.org.
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