pubs.acs.org/Langmuir © 2010 American Chemical Society
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 its ability to modulate phase separation between OFTB and PCBM using differential scanning calorimetry, hot-stage polarizing optical microscopy, X-ray diffraction, and atomic force microscopy techniques. Upon thermal annealing at 10 °C above its Tg for 12-48 h, the 1:1 blend of OFTB and PCBM resulted in a eutectic mixture. Thermal annealing of a OFTB: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 AFM phase image indicated phase separation into two interspersed 30 nm amorphous domains at approximately equal fractions. Geometric surfactancy is inferred from the formation of microemulsions in analogy to widely reported traditional oil-surfactant-water systems and ternary polymer blends. In contrast, thermal annealing of a 7:6:7 film under a similar condition resulted in an amorphous film with compositional uniformity.
Introduction Traditional amphiphiles comprising polar and nonpolar moieties are capable of serving not only as cosolvents for hydrophobic solutes in water1 but also as surfactants for the creation of microstructures involving water and oil, such as bilayers, micelles, droplet, and bicontinuous microemulsions.2 In addition, macromolecular surfactancy originating from the Flory-Huggins interaction is exemplified by diblock copolymers that mediate the formation of similar microstructures between two parent homopolymers.3-7 A geometric amphiphile consisting of a disklike moiety linked to a rodlike moiety with an undecylene spacer has been demonstrated for cosolvency.8 In analogy to traditional surfactancy, the present study was motivated to demonstrate the feasibility of geometric surfactancy in terms of microstructure formation using a dyad comprising geometrically dissimilar spherical and rodlike moieties. To construct an example geometric surfactant, a propylene linker was inserted between an electron-donating conjugated oligomer and an electron-accepting C60-derivative relevant to organic photovoltaics. The flexible linker is essential to promoting preferential packing of the conjugated oligomer and the C60-derivative as separate chemical entities. Both geometric packing and electronic interaction should be accounted for geometric surfactancy. As *Author to whom correspondence should be addressed. E-mail: shch@ lle.rochester.edu. (1) Bauduin, P.; Renoncourt, A.; Kopf, A.; Touraud, D.; Kunz, W. Langmuir 2005, 21, 6769–6775. (2) Schwuger, M. J.; Stickdorn, K.; Schomacker, R. Chem. Rev. 1995, 95, 849– 864. (3) Bates, F. S.; Waurer, W. W.; Lipic, P. M.; Hillmyer, M. A.; Almdal, K.; Mortensen, K.; Fredrickson, G. H.; Lodge, T. P. Phys. Rev. Lett. 1997, 79, 849– 852. (4) Hillmyer, M. A.; Maurer, W. W.; Lodge, T. P.; Bates, F. S.; Almdal, K. J. Phys. Chem. B 1999, 103, 4813–3824. (5) Lee, J. H.; Ruegg, M. L.; Balsara, N. P.; Zhu, Y. Q.; Gido, S. P.; Krishnamoorti, R.; Kim, M. H. Macromolecules 2003, 36, 6537–6548. (6) Wanakule, N. S.; Nedoma, A. J.; Robertson, M. L.; Fang, Z. X.; Jackson, A.; Garetz, B. A.; Balsara, N. P. Macromolecules 2008, 41, 471–477. (7) Liu, G. L.; Stoykovich, M. P.; Ji, S. X.; Stuen, K. O.; Craig, G. S. W.; Nealey, P. F. Macromolecules 2009, 42, 3063–3072. (8) Date, R. W.; Bruce, D. W. J. Am. Chem. Soc. 2003, 125, 9012–9013.
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demonstrated for traditional oil-surfactant-water systems and ternary polymer blends, the formation of stable microstructures can be expected under appropriate conditions, considering the enthalpy and entropy changes accompanying molecular selforganization in addition to interfacial energy. Three model compounds were employed to prepare blends for the characterization of powder and film morphologies by differential scanning calorimetry (DSC), hot-stage polarizing optical microscopy (POM), X-ray diffraction (XRD), and atomic force microscopy (AFM) to ascertain the formation of microemulsions.
Experimental Section Materials Synthesis and Characterization. The target compounds, 4,7-bis(5-(9,9-bis(2-methylbutyl)fluoren-2-yl)thien2-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-benzo-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, were synthesized and purified according to reaction Scheme S.1 following the procedures as described in the Supporting Information. Also included in the Supporting Information are the 1H NMR spectra acquired in CDCl3 at 298 K with an Avance-400 spectrometer (400 MHz), Figures S.1 (OFTB) and S.2 (Dyad). Elemental analysis was carried out by Quantitative Technologies, Inc. Molecular weights were measured with a TofSpec2E MALD/I TOF mass spectrometer (Micromass, Inc., Manchester, U.K.) using 2-[(2E)-3-(4-tert-butylphenyl)-2-methylpropenylidene] malanoitrile (DCTB) as the matrix. [6,6]-Phenyl C61-butyric acid methyl ester, PCBM, at a purity level of 99.5% was purchased from Nano-C (MA). Bulk-Phase Thermal Transitions and Morphologies. Thermal transition temperatures were determined by DSC (PerkinElmer DSC-7) with a continuous N2 purge at 20 mL/min. The three pure components OFTB, PCBM, and Dyad were preheated to 310 °C followed by quenching to -30 °C for thermal analysis by DSC at a heating rate of 20 °C/min. One set of blends was preheated to 310 °C followed by quenching to -30 °C before their heating scans were recorded at 20 °C/min. The other set of blends was preheated to 310 °C followed by quenching to -30 °C, and then they were annealed at 10 °C above their respective glass
Published on Web 06/28/2010
DOI: 10.1021/la1014797
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Chart 1. Molecular Structures of OFTB, PCBM, and Dyad Used in This Study. Symbols: G, Glassy; K, Crystalline; I, Isotropic
Figure 1. DSC heating scans of OFTB:Dyad:PCBM at three
transition temperatures for 12 h. Heating scans were also collected at 20 °C/min after the blends were cooled to room temperature. The nature of phase transitions was characterized by hot-stage POM (DMLM, Leica, FP90 central processor and FP82 hot stage, Mettler Toledo). The powder samples for XRD analysis were preheated to 310 °C followed by quenching to room temperature and then annealing at 103 °C for 12-48 h with subsequent cooling to room temperature. X-ray diffraction data for powder samples on (100) silicon wafer were collected in reflection mode geometry using a Rigaku D2000 Bragg-Brentano diffractometer equipped with a copper rotating anode, diffracted beam graphite monochromator tuned to Cu KR radiation, and scintillation detector. Diffraction patterns were collected at 2θ 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 microscope glass slides from 1 wt % chloroform solutions at 1000 rpm followed by drying under vacuum. The thicknesses of the resultant films were determined by optical interferometry (Zygo New Views 5000). Thermal annealing was performed under argon at 10 °C above glass transition temperatures for 12 h followed by cooling to room temperature. The phase and topography images of both the pristine and thermally annealed films were recorded on a Nanoscope III (Digital Instrument, CA) AFM in tapping mode with a NT-MDT NSG01 cantilever at a typical scan rate of 0.5 Hz under ambient condition. X-ray diffraction data for thin films on microscope glass slides were collected using the same instrument under the same conditions as described above.
Results and Discussion Depicted in Chart 1 are the molecular structures of the three components employed in this study. The synthesis and purification procedures together with the characterization data for OFTB and Dyad are described in the Supporting Information, while PCBM was used as received from a commercial source at a purity level of 99.5%. We chose to work with OFTB in light of a recent report on organic photovoltaics using its long-chain polymer analogue as the electron-donor and PCBM as the electronacceptor.9 As indicated by the DSC data accompanying molecular structures, OFTB has a glass transition temperature, Tg, at 81 °C followed by crystallization at 165 °C and a crystalline melting point, Tm, at 222 °C. Dyad has a Tg at 181 °C, and PCBM has 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 7:6:7 mass ratios, were prepared by codissolution in chloroform (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.
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compositions as indicated. Dashed curves: samples preheated to 310 °C and quenched to -30 °C before heating at 20 °C/min. Solid curves: samples preheated to 310 °C and quenched to -30 °C before annealing at 10 °C above respective Tg values, as determined with dashed curves, for 12 h and then cooled to room temperature before heating at 20 °C/min. Complete transition to isotropic liquid for the annealed 10:0:10 blend occurred at 242 °C as indicated by an arrow. Symbols: G, Glassy; K, Crystalline; I, Isotropic.
followed by evaporation of the solvent for an investigation of their morphologies. Thoroughly dried powders were characterized by DSC for their thermal transition temperatures, and the results in which phases were identified by POM micrographs (see Supporting Information, Figure S.3) are shown in Figure 1. Presented as dashed curves, preheated and thermally quenched samples 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 consistent with compositional uniformity as will be corroborated by AFM of spin-cast films shown in Figure 4 below. The preheated and thermally quenched blends were then annealed at 10 °C above their respective Tg values for 12 h followed by cooling to room temperature. The acquired DSC thermograms are shown as solid curves in Figure 1. Thermal annealing of the 9:2:9 and 7:6:7 blends resulted in DSC thermograms identical to those of unannealed samples, whereas thermal annealing of the 10:0:10 blend led to partial crystallization with a sharp melting peak at 205 °C followed by a complete transition to an isotropic liquid at 242 °C as observed under POM. The cooling scans were also gathered at -20 °C/min for all three blends following the heating scans. The absence of crystallization during cooling to room temperature suggests that the crystals that melted from 205 to 242 °C have resulted exclusively from thermal annealing. The origin of the sharp crystalline melting at 205 °C was further examined by powder XRD as shown in Figure 2 to differentiate between cocrystallization and the formation of a eutectic mixture. Partial crystallization in the annealed 10:0:10 blend is evidenced by the presence of several narrow diffraction peaks attributable to crystalline components on top of two broad amorphous peaks centered at 10.4 and 19.6 degrees inferred from a control experiment with an unannealed blend. Although amorphous phases have no crystalline short or long-range order, there still exists a range of most probable distances between neighboring atoms as well as defined bond distances for functional groups, such as fullerene, phenyl ring, and so on. This semishort-range order is responsible for the broad peaks observed in diffraction patterns of amorphous materials. The powder XRD pattern of the semicrystalline binary blend represents a superposition of diffraction patterns of OFTB and PCBM, indicating that these two pure components crystallized independently during thermal annealing. Using the Scherrer technique,10 the average crystallite size was Langmuir 2010, 26(15), 12877–12881
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Figure 3. XRD patterns (collected at 0.02°/step), 10 s per step, and Figure 2. Samples were preheated to 310 °C and then quenched to room temperature before annealing at 103 °C for 48 h followed by cooling to room temperature for powder XRD analysis. Weaker crystalline diffraction peaks resulted from a shorter annealing time, for example, 12 h. A quenched but unannealed 10:0:10 blend was also characterized for identification of broad amorphous peaks.
POM micrographs as the insets for 100-nm-thick spin-cast films of OFTB:Dyad:PCBM blends at (a) 10:0:10 mass ratio and (b) 9:2:9 mass ratio after thermal annealing at 10 °C above their respective Tg values for 12 h followed by cooling to room temperature. Essentially the same XRD and POM results were observed for the 7:6:7 film as reported in panel b for the 9:2:9 film. Preheating to 310 °C as conducted for powders was avoided to preserve film integrity. No diffraction peaks are visible at 2θ between 10 and 30° as shown in Supporting Information, Figure S.5.
estimated at 26 and 23 nm for OFTB and PCBM, respectively, values reasonably close to 19 and 21 nm obtained from the thermally annealed powders of pure components. The observed crystallite sizes are greater than the minimum size of 2.5 nm for a crystallite to exhibit its characteristic XRD pattern.11 It is concluded that OFTB and PCBM comprise a eutectic mixture with a Tm at 205 °C, below those of the two pure components, 222 °C (OFTB) and 292 °C (PCBM). The phase diagram was constructed for the OFTB:PCBM binary system as shown in Supporting Information, Figure S.4, a behavior similar to that of the binary blend of poly(3-hexylthiophene) with PCBM.12 Approximately 100-nm-thick films of the three ternary blends were prepared by spin coating from chloroform followed by drying in vacuo at room temperature. The pristine films appeared amorphous, according to POM micrographs (Supporting Information, Figure S.3a-c). The combination of XRD pattern and POM micrograph shown in Figure 3a indicates that the 10:0:10 film turned polycrystalline after thermal annealing at 10 °C above its Tg for 12 h. The diffraction peak at 2θ = 4.37° appearing in the film XRD pattern is also present in the powder XRD patterns of OFTB and the 10:0:10 blend identified by arrows in Figure 2. Furthermore, the average crystallite size in the annealed film was estimated at 30 nm, in agreement with 26 nm estimated for OFTB crystallites in the annealed powders of 10:0:10 blend. Compared to powder XRD, film XRD produced a pattern with fewer diffraction peaks at 2θ less than 10° and no peaks between 10 and 30°. This result is attributed to the likely preferred orientation of crystallites such that one set of lattice planes lies parallel to the sample surface thus in the right geometry to be observed during data collection. The reduced sample volume of the thin film compared to that of the powder and the limited scattering power of organic materials comprised of low atomic-number elements, such as hydrogen and carbon, are additional contributing factors to fewer diffraction peaks in the 10:0:10 thin film than its powder. As revealed by XRD pattern and POM micrograph shown in Figure 3b, the 9:2:9 and 7:6:7 films remained amorphous after thermal annealing at 10 °C above their respective Tg values for 12 h. Spin-cast films were further characterized by AFM for their compositional and/or morphological characteristics. In principle,
phase contrast can be attributed to phase separation arising from lateral variations in composition or morphology regardless of topography.13 Empirically, the features observed under transmission electron microscopy have been employed to establish AFM phase contrast as a valid tool for elucidating phase separation in block copolymers,14,15 binary polymer blends,15 and PCBMpolymer blends.16 Phase images are compiled in Figure 4 for films before and after thermal annealing at 10 °C above Tg for 12 h with subsequent cooling to room temperature. Without encountering crystallization, the cooling process served to kinetically trap in solid state the film morphologies prevailing under the annealing conditions. The pristine films are compositionally uniform, according to Figure 4 panels a, b, and c, in addition to being amorphous under POM. Upon thermal annealing, the 10:0:10 film was found to crystallize, appearing as phase contrast shown in Figure 4d, with a melting range from 208 to 242 °C under hot-stage POM, an observation in agreement with the bulk phase diagram within experimental error. The 7:6:7 film remained amorphous after thermal annealing, according to its DSC thermogram, POM micrograph, and XRD pattern, and was compositionally uniform according to Figure 4f versus 4c. Because of the difference both in geometry and molecular-level interaction, OFTB and PCBM are expected to be partially miscible at best. As indicated by the POM micrograph and XRD pattern, the 9:2:9 film remained amorphous after thermal annealing. Hence, the phase contrast shown in Figure 4e is interpreted as compositional separation into two interspersed amorphous domains. Section analysis was performed to yield about 30 nm as the average sizes for both amorphous domains at approximately equal fractions (see Supporting Information, Figure S.6). Whereas the observed film morphology remained intact when left at room temperature for up to six months, its long-term morphological stability has yet to be systematically investigated in terms of chemical composition and thermal treatment prior to cooling to room temperature. The AFM phase images shown in Figure 4e resemble the transmission-electron and optical micrographs of microemulsions in ternary polymer blends5,6 and the cryo-transmission electron micrographs of traditional oil-surfactant-water
(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. In Handbook 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.
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Figure 4. AFM phase images of 100-nm-thick spin-cast films of OFTB: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 as conducted for powders was avoided to preserve film integrity. Featureless phase images of panels a-c and f represent the absence of phase separation down to about 1 nm.
nanoscale phase separation shown in Figure 4e is responsible for the surface roughness observed in Figure S.7e.
Figure 5. A schematic diagram of Dyad acting as a geometric surfactant to modulate phase separation between OFTB and PCBM.
microemulsions17 including domain sizes, all observed at a surfactant content of about 10 wt %. At such a low content, it is plausible that facilitated by the flexible spacer, Dyad molecules orient themselves at the interface between the OFTB-rich and the Dyad-rich amorphous domains as visualized in Figure 5. The envisioned molecular self-organization is rationalized by geometric packing of Dyad molecules at the interface for the sake of enthalpy at the expense of entropy of mixing with an optimum interfacial energy, thus rendering a negative Gibbs free energy change accompanying the formation of microemulsions. This behavior is akin to that of a traditional surfactant promoting the formation of microemulsions on the basis of polarnonpolar amphiphilicity.2,17 Another analogy pertains to microemulsion formation induced by diblock copolymer as a macromolecular surfactant between two parent homopolymers through Flory-Huggins interaction.3-7 That a single Tg emerged from the thermally annealed 9:2:9 blend (see Figure 1) is understandable with 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 represent the first demonstration of phase separation between a conjugated oligomer and a C60-derivative modulated by a dyad with geometric amphiphilicity. It is remarked in passing that the AFM topographic images compiled in Supporting Information, Figure S.7 mirror the phase images in Figure 4. In particular, it is noted that (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. Chem. 2000, 38, 352–366.
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Conclusions Based on polar-nonpolar amphiphilicity, traditional surfactants are capable of solubilizing hydrophobic solutes in water and mediating microstructure formation between oil and water. From an entirely different perspective, diblock copolymers can induce self-organization between the two parent homopolymers into similar microstructures by exploiting Flory-Huggins interaction. Comprising a rodlike and a disklike moiety, a geometric amphiphile has been demonstrated for cosolvency. In the present study a geometric amphiphile, Dyad, consisting of C60 and π-conjugated oligomer moieties linked by a propylene spacer was tested for its ability to modulate phase separation between OFTB and PCBM using DSC, POM, XRD, and AFM techniques. Key observations are recapitulated as follows. Powder XRD patterns indicate that OFTB and PCBM crystallized independently from the 1:1 blend upon thermal annealing at 10 °C above its Tg for 12-48 h. A eutectic mixture emerged with a Tm at 205 °C, which is definitively lower than those of OFTB and PCBM at 222 and 292 °C, respectively. Thermal annealing of a spin-cast film of OFTB:Dyad:PCBM at a mass ratio of 9:2:9 at 10 °C above its Tg for 12 h produced an amorphous film according to POM and XRD. The analysis of its AFM phase image concluded phase separation into two interspersed 30 nm domains at approximately equal fractions. Geometric surfactancy is therefore inferred from the formation of microemulsions in analogy to widely reported traditional oil-surfactant-water systems and ternary polymer blends. In contrast, thermal annealing of a 7:6:7 film under a similar condition resulted in compositional uniformity across an amorphous film as shown by POM, XRD, and AFM. Acknowledgment. The authors thank Mitchell Anthamatten, Matthew Z. Yates, Lewis J Rothberg, and Yongli Gao for helpful discussions, Semyon Papernov for assistance in the characterization of thin films by AFM, and Andrew Hoteling at the Eastman Kodak Company for MALDI/TOF analysis. They are grateful for the financial support provided by the Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302 with the Laboratory for Laser Energetics and the New York State Energy Research and Langmuir 2010, 26(15), 12877–12881
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Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article. Supporting Information Available: Experimental procedures for material synthesis, purification, characterization of
Langmuir 2010, 26(15), 12877–12881
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OFTB, Dyad, PCBM and the mixtures thereof, optical micrographs, the OFTB:PCBM binary phase diagram, AFM phase and topographic images, and section analysis of an AFM phase image. This material is available free of charge via the Internet at http://pubs.acs.org.
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