Thin Film Blends - American Chemical Society

Apr 10, 2008 - Macromolecules and Interfaces Institute and Department of Chemistry, Virginia Polytechnic Institute and. State UniVersity, Blacksburg, ...
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Langmuir 2008, 24, 5079-5090

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Phase Separation in Poly(tert-butyl acrylate)/Polyhedral Oligomeric Silsesquioxane (POSS) Thin Film Blends Rituparna Paul, Ufuk Karabiyik, Michael C. Swift, and Alan R. Esker* Macromolecules and Interfaces Institute and Department of Chemistry, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061 ReceiVed July 10, 2007. In Final Form: September 11, 2007 Phase separation in thin film blends of poly(tert-butyl acrylate) (PtBA) and a polyhedral oligomeric silsesquioxane (POSS), trisilanolphenyl-POSS (TPP), is studied as functions of annealing temperature and time, using reflected light optical microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The results demonstrate that the PtBA/TPP blend system confined to thin films (∼90 nm) exhibits lower critical solution temperature (LCST) behavior with a critical temperature of ∼70 °C and a critical composition of 60 wt % PtBA with insignificant dewetting at the phase boundary. Off-critical spinodal behavior is observed for 58 and 62 wt % PtBA blend films. Phase separation by nucleation and growth is observed for all compositions outside the window between 58 and 62 wt % PtBA. The temporal evolution of spinodal decomposition in 60 wt % PtBA blend films is explored at annealing temperatures of 75, 85, 95, and 105 °C. The morphological evolution in 60 wt % PtBA blend films is similar for all experimental temperatures (75, 85, 95, and 105 °C) with the expected shorter time scales for phase evolution at higher annealing temperatures. Fast Fourier transforms of optical micrographs reveal that these blend films immediately undergo phase separation by spinodal decomposition during temperature jump experiments. Power law scaling for the characteristic wavevector with time (q ∼ tn with n ≈ -1/4 to -1/3) for domain growth during the early stages of phase separation yields to domain pinning at the later stages for 60 wt % PtBA blend films annealed at 75, 85, and 95 °C. In contrast, domain growth is pinned over the entire experimental time scale for 60 wt % PtBA blend films annealed at 105 °C.

Introduction Thin film polymer blends are increasingly important for a host of technological applications that include lithography,1 dielectric layers,2 gas separation membranes,3 fabrication of microlectronics,4 biocompatible coatings,5 and drug delivery systems.6 Polymer blend films utilized for technological applications are generally composed of multiple components to achieve improved processability, and key properties such as mechanical strength, adhesion, and thermal stability.7,8 For most applications, polymer thin film blends are required to maintain their stability after being spread onto substrates. However, incompatibility between various components in polymer blends may lead to phase separation. Phase separation in polymer blends can be induced by temperature and/or pressure variations. Phase separation can occur either by spinodal decomposition (SD) or nucleation and growth (NG) mechanisms depending on the blend composition.9,10 Spinodal decomposition occurs when a polymer mixture is locally unstable without a thermodynamic barrier to phase separation * To whom correspondence should be addressed. Email: [email protected]. Fax: (01) 540-231-3255. (1) Wei, J. H.; Coffey, D. C.; Ginger, D. S. J. Phys. Chem. B 2006, 110, 24324-24330. (2) Lee, J.-W.; Sung, S.-J.; Park, J.-K. Synth. Met. 2001, 117, 271-272. (3) Ghalei, B.; Semsarzadeh, M.-A. Macromol. Symp. 2007, 249/250, 330335. (4) Moons, E. J. Phys.: Condens. Matter 2002, 14, 12235-12260. (5) Zorlutuna, P.; Tezcaner, A.; Kiyat, I.; Aydinli, A.; Hasirci, V. J. Biomed. Mater. Res., Part A 2006, 79A, 104-113. (6) Wang, L.-C.; Chen, X.-G.; Zhong, D.-Y.; Xu, Q.-C. J. Mater. Sci.: Mater. Med. 2007, 18, 1125-1133. (7) Paul, D. R. In Polymer Blends; Paul, D. R., Newman, S., Eds.; Academic Press: New York, 1982; Vol. 1, pp 1-14. (8) Utracki, L. A. Polymer Alloys and Blends: Thermodynamics and Rheology; Hanser: New York, 1988. (9) Kwei, T. K.; Wang, T. T. In Polymer Blends; Paul, D. R., Newman, S., Eds.; Academic Press: New York, 1982; Vol. 1, pp 141-183. (10) Strobl, G. R. The Physics of Polymers: Concepts for Understanding their Structure and BehaVior; Springer: Berlin, 1997.

that results in spontaneous segregation of phases. In contrast, phase separation via the nucleation and growth mechanism occurs when a polymer blend is locally stable and phase separation can only proceed by overcoming a thermodynamic barrier to phase separation through a large compositional fluctuation. These mechanisms of phase separation are not unique to polymeric systems, but are also present in small molecule,11-13 metal,14-16 inorganic glass,17,18 and colloidal systems.19,20 Figure 1a shows the important features of a temperature (T) versus volume fraction of component A (φA) phase diagram for a generic, binary blend (A + B), exhibiting a lower critical solution temperature (LCST) with a critical composition φA,c and a critical temperature, Tc. The solid (binodal) line separates the homogeneous one phase region from the two phase region, whereas the dashed (spinodal) line marks the boundary between the locally stable and unstable two phase regions. Phase separation in the generic system can be induced by temperature jumps from the one phase region into the two phase regions at a fixed initial φA. Depending on the composition of the system, phase separation can occur through different mechanisms. If the system is rapidly heated along path 1 to a temperature T1, phase separation will proceed via a nucleation and growth mechanism. For nucleation and growth along path 1, a minor phase with composition φA′′,NG at the end of the tie line (dashed and dotted line in Figure 1a) grows as discrete (typically round) domains inside a second continuous major phase with composition φA′,NG located at the opposite end (11) Wong, N. C.; Knobler, C. M. J. Chem. Phys. 1978, 69, 725-735. (12) Wong, N. C.; Knobler, C. M. Phys. ReV. A 1981, 24, 3205-3211. (13) Chou, Y. C.; Goldburg, W. I. Phys. ReV. A 1979, 20, 2105-2113. (14) Vaks, V. G. Phys. Rep. 2004, 391, 157-242. (15) Dogel, J.; Tsekov, R.; Freyland, W. J. Chem. Phys. 2005, 122, 094703/ 1-094703/8. (16) Hennion, M.; Ronzaud, D.; Guyot, P. Acta Metall. 1982, 30, 599-610. (17) Tomozawa, M.; MacCrone, R. K.; Herman, H. Phys. Chem. Glasses 1970, 11, 136-150. (18) Zarzycki, J.; Naudin, F. J. Non-Cryst. Solids 1969, 1, 215-234. (19) Dhont, J. K. G.; Duyndam, A. F. H. Phys. A 1992, 189, 532-553. (20) Buyevich, Y. A.; Ivanov, A. O. Phys. A 1993, 192, 375-390.

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Figure 1. (a) A LCST phase diagram for a generic binary blend (A + B) undergoing phase separation, (b) phase separation via a nucleation and growth mechanism where the circular domains correspond to the minor phase, and (c) bicontinuous domains formed during phase separation by spinodal decomposition where the blue features correspond to the minor phase.

of the tie line. Typical morphological features for phase separation by nucleation and growth correspond to the schematic in Figure 1b where the blue dots represent the minor phase. Alternatively, one may study path 1 with a slow heating rate (cloud point experiment). In such an experiment, one detects the onset of phase separation to locate the binodal temperature for composition φA,1 (Tbin,1). For blends with compositions matching φA,c temperature jumps into the two phase region undergo phase separation by spinodal decomposition. Rapid heating along path 2 (φA ) φA,c) finds a locally unstable blend that undergoes spontaneous phase separation into phases with compositions φA′,SD (minor) and φA′′,SD (major). During spinodal decomposition, a bicontinuous, interconnected morphology results (schematically depicted in Figure 1c where the blue regions represent the minor phase). It is also possible to enter the spinodal region of the phase diagram at off-critical compositions (path 3). Depending on the heating rate for the temperature jump relative to the relevant phase separation kinetics, elements of one phase separation mechanism, nucleation and growth (slow heating rates) or spinodal decomposition (fast heating rates), or both mechanisms (intermediate heating rates) may be observed during morphological studies. Phase separation by spinodal decomposition for bulk polymer blends has been extensively studied and distinguishable time regimes are established for its evolution.21-30 Early stage phase (21) Bates, F. S.; Wiltzius, P. J. Chem. Phys. 1989, 91, 3258-3274. (22) Binder, K. J. Chem. Phys. 1983, 79, 6387-6409. (23) de Gennes, P. G. J. Chem. Phys. 1980, 72, 4756-4763. (24) Han, C. C.; Bauer, B. J.; Clark, J. C.; Muroga, Y.; Matsushita, Y.; Okada, M.; Cong, Q. T.; Chang, T.; Sanchez, I. C. Polymer 1988, 29, 2002-2014.

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separation during bulk spinodal decomposition is well described by the linear theory of Cahn, Hillard, and Cook.31-33 The linear theory predicts an initial one phase morphology with sinusoidal composition fluctuations of a fixed wavelength (λ) that are symmetric about the average composition. The amplitude of these fluctuations grows exponentially with time and eventually results in the formation of a two phase system with a bicontinuous morphology. In the intermediate and late stages of spinodal decomposition, the time dependent evolution of phase separating domains is usually characterized by a power law, q ∼ tn, where q is the characteristic wavevector (2π/λ) and n is the scaling exponent. For intermediate and late times during the bulk spinodal decomposition process, n is predicted to be -1/3 and -1, respectively.34-37 Morphologically, the bicontinuous, two-phase spinodal domains coarsen and break-up into isolated droplets during the intermediate and late stages of spinodal decomposition, respectively.38 Phase separation of polymer blends confined to thin film geometries has gained attention recently but phase evolution in these systems is far from being completely understood. Phase separation in thin film polymer blends is complicated by polymersurface interactions that may change the thermodynamics and kinetics of phase evolution in these systems.39-45 Moreover, polymer blends confined to thin films may surface segregate and form wetting layers as a consequence of the preferential attraction of one of the components to the surface. Previous studies have reported power law growth of the thickness of the wetting layer, d ∼ tx, with growth exponents ranging from x ) 0.1 to ∼1.46-60 Preferential wetting or surface segregation may give rise to oscillatory compositional profiles perpendicular to the surface (25) Hashimoto, T.; Itakura, M.; Hasegawa, H. J. Chem. Phys. 1986, 85, 61186128. (26) Hashimoto, T.; Itakura, M.; Shimidzu, N. J. Chem. Phys. 1986, 85, 67736786. (27) Okada, M.; Han, C. C. J. Chem. Phys. 1986, 85, 5317-5327. (28) Pincus, P. J. Chem. Phys. 1981, 75, 1996-2000. (29) Sato, T.; Han, C. C. J. Chem. Phys. 1988, 88, 2057-2065. (30) Wiltzius, P.; Bates, F. S.; Heffner, W. R. Phys. ReV. Lett. 1988, 60, 1538-1541. (31) Cahn, J. W.; Hilliard, J. E. J. Chem. Phys. 1958, 28, 258-267. (32) Cook, H. E. Acta Metall. 1970, 18, 297-306. (33) Cahn, J. W. J. Chem. Phys. 1965, 42, 93-99. (34) Akcasu, A. Z.; Klein, R. Macromolecules 1993, 26, 1429-1441. (35) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35-50. (36) McMaster, L. P. AdV. Chem. Ser. 1975, 142, 43-65. (37) Siggia, E. D. Phys. ReV. A 1979, 20, 595-605. (38) Cahn, J. W. Acta Metall. 1966, 14, 1685-1692. (39) Binder, K. Acta Polym. 1995, 46, 204-225. (40) Flebbe, T.; Dunweg, B.; Binder, K. J. Phys. II 1996, 6, 667-695. (41) Kotelyanskii, M.; Kumar, S. K. Phys. ReV. Lett. 1998, 80, 1252-1255. (42) Reich, S.; Cohen, Y. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 12551267. (43) Tanaka, K.; Yoon, J.; Takahara, A.; Kajiyama, T. Macromolecules 1995, 28, 934-938. (44) Binder, K. J. Non-Equilib. Thermodyn. 1998, 23, 1-44. (45) Puri, S.; Binder, K.; Frisch, H. L. Phys. ReV. E 1997, 56, 6991-7000. (46) Chen, H.; Chakrabarti, A. Phys. ReV. E 1997, 55, 5680-5688. (47) Geoghegan, M.; Jones, R. A. L.; Clough, A. S. J. Chem. Phys. 1995, 103, 2719-2724. (48) Heier, J.; Kramer, E. J.; Revesz, P.; Battistig, G.; Bates, F. S. Macromolecules 1999, 32, 3758-3765. (49) Jones, R. A. L.; Kramer, E. J. Philos. Mag. B 1990, 62, 129-137. (50) Jones, R. A. L.; Kramer, E. J.; Rafailovich, M. H.; Sokolov, J.; Schwarz, S. A. Phys. ReV. Lett. 1989, 62, 280-283. (51) Krausch, G.; Dai, C.-A.; Kramer, E. J.; Bates, F. S. Phys. ReV. Lett. 1993, 71, 3669-3672. (52) Krausch, G.; Kramer, E. J.; Bates, F. S.; Marko, J. F.; Brown, G.; Chakrabarti, A. Macromolecules 1994, 27, 6768-6776. (53) Lipowsky, R.; Huse, D. A. Phys. ReV. Lett. 1986, 57, 353-356. (54) Puri, S.; Binder, K. Phys. ReV. E 1994, 49, 5359-5377. (55) Steiner, U.; Klein, J. Phys. ReV. Lett. 1996, 77, 2526-2529. (56) Steiner, U.; Klein, J.; Eiser, E. J.; Budkowski, A.; Fetters, L. J. Science 1992, 258, 1126-1129. (57) Tanaka, H. Phys. ReV. E 1996, 54, 1709-1714. (58) Wang, H.; Composto, R. J. J. Chem. Phys. 2000, 113, 10386-10397. (59) Wang, H.; Composto, R. J. Phys. ReV. E 2000, 61, 1659-1663. (60) Wang, H.; Composto, R. J. Interface Sci. 2003, 11, 237-248.

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leading to surface-directed spinodal decomposition.46-48,51,52,57-62 Furthermore, the kinetics and morphology of phase separation are a function of film thickness.60 The morphological features for surface induced phase separation is a topic recently reviewed by Geoghegan and Krausch.63 Ultimately, the final properties and morphologies of polymer blend films are governed by the interplay between phase separation and surface segregation. Thin film multicomponent polymer blends that are used for industrial applications generally contain solid fillers for cost reduction and enhancement of properties relative to purely polymeric blends. The incorporation of low concentrations (2-5 wt %) of nanoscopic fillers and nanoparticles into polymers can improve tensile strength, thermal stability, and barrier properties of these polymeric systems.64-66 Recently, there have been several attempts to explore the effect of solid fillers on the phase behavior and morphological evolution in bulk and thin film binary polymer/ polymer blends; however, a complete understanding of phase behavior in “filled” polymer systems is lacking. Most of the previous studies have focused on the changes in thermodynamics (phase boundary) and kinetics (temporal evolution) of phase separation and morphological evolution upon the addition of micron- and nanosized filler particles to binary polymer blends.67-79 In contrast, relatively little is known about phase behavior and morphological evolution in binary blends of a polymer and a filler.80,81 It is crucial to gain fundamental insight into phase evolution in binary polymer/nanoparticle blend systems for controlling the stability, properties, and morphologies of these systems. Polyhedral oligomeric silsesquioxanes (POSS) are organic/ inorganic hybrid materials with the empirical formula RSiO1.5, where R is hydrogen or any alkyl, aryl, arylene, or derivatives of aryl or arylene groups. POSS materials consist of threedimensional, rigid inorganic cores (Si-O cages) and flexible organic coronae, and can be considered to be the smallest particles of silica (∼1-3 nm).82,83 Figure 2b provides a generic structure for an open cage, trisilanol-POSS derivative. Previous studies (61) Jones, R. A. L.; Norton, L. J.; Kramer, E. J.; Bates, F. S.; Wiltzius, P. Phys. ReV. Lett. 1991, 66, 1326-1329. (62) Sung, L.; Karim, A.; Douglas, J. F.; Han, C. C. Phys. ReV. Lett. 1996, 76, 4368-4371. (63) Geoghegan, M.; Krausch, G. Prog. Polym. Sci. 2003, 28, 261-302. (64) Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 983-986. (65) Krishnamoorti, R.; Vaia, R. A.; Giannelis, E. P. Chem. Mater. 1996, 8, 1728-1734. (66) Yano, K.; Usuki, A.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2493-2498. (67) Balazs, A. C. Curr. Opin. Colloid Interface Sci. 2000, 4, 443-448. (68) Balazs, A. C.; Ginzburg, V. V.; Qiu, F.; Peng, G.; Jasnow, D. J. Phys. Chem. B 2000, 104, 3411-3422. (69) Chakrabarti, A. J. Chem. Phys. 1999, 111, 9418-9423. (70) Chung, H.-J.; Taubert, A.; Deshmukh, R. D.; Composto, R. J. Europhys. Lett. 2004, 68, 219-225. (71) Ginzburg, V. V.; Peng, G.; Qiu, F.; Jasnow, D.; Balazs, A. C. Phys. ReV. E 1999, 60, 4352-4359. (72) Ginzburg, V. V.; Qiu, F.; Paniconi, M.; Peng, G.; Jasnow, D.; Balazs, A. C. Phys. ReV. Lett. 1999, 82, 4026-4029. (73) Karim, A.; Douglas, J. F.; Nisato, G.; Liu, D.-W.; Amis, E. J. Macromolecules 1999, 32, 5917-5924. (74) Karim, A.; Liu, D.-W.; Douglas, J. F.; Nakatani, A. I.; Amis, E. J. Polymer 2000, 41, 8455-8458. (75) Laradji, M.; MacNevin, G. J. Chem. Phys. 2003, 119, 2275-2283. (76) Lee, B. P.; Douglas, J. F.; Glotzer, S. C. Phys. ReV. E 1999, 60, 58125822. (77) Lipatov, Y. S.; Nesterov, A. E.; Ignatova, T. D.; Nestarov, D. A. Polymer 2001, 43, 875-880. (78) Suppa, D.; Kuksenok, O.; Balazs, A. C.; Yeomans, J. M. J. Chem. Phys. 2002, 116, 6305-6310. (79) Tanaka, H.; Lovinger, A. J.; David, D. D. Phys. ReV. Lett. 1994, 72, 2581-2584. (80) Lee, J. Y.; Shou, Z.; Balazs, A. C. Phys. ReV. Lett. 2003, 91, 136103/ 1-136103/4. (81) Lee, J. Y.; Shou, Z.; Balazs, A. C. Macromolecules 2003, 36, 77307739.

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Figure 2. (a) Configuration of the PtBA/TPP blend films used in this study. TPP is depicted as the round structures in the PtBA film. Chemical structures of (b) TPP and (c) PtBA.

have shown that the incorporation of POSS nanoparticles into bulk polymers leads to improvements in thermal and mechanical properties.84-89 The structure-property relationships (thermal and mechanical) of polymer/POSS binary blends in bulk have been extensively studied.90-96 Nonetheless, the phase behavior in these systems has not been explored systematically. Moreover, the effect of POSS incorporation into polymer thin films has received less attention. This work studies phase separation and determines the phase boundary for a binary mixture of poly(tert-butyl acrylate) (PtBA) and a POSS, trisilanolphenyl-POSS (TPP), confined to thin films with Si//PtBA+TPP//Air configurations. The configuration of the PtBA/TPP blend films used in this study and the chemical structures of PtBA and TPP are depicted in Figure 2. Surface cloud point measurements via optical microscopy define the phase boundary, while atomic force microscopy (AFM) with selective solvent etching differentiates the phases, and differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS) determine where crystallization may be an important consideration and show dewetting is not (82) Haddad, T. S.; Viers, B. D.; Phillips, S. H. J. Inorg. Organomet. Polym. 2001, 11, 155-164. (83) Voronkov, M. G.; Lavrent’yev, V. I. Top. Curr. Chem. 1982, 102, 199236. (84) Bharadwaj, R. K.; Berry, R. J.; Farmer, B. L. Polymer 2000, 41, 72097221. (85) Haddad, T. S.; Lichtenhan, J. D. Macromolecules 1996, 29, 7302-7304. (86) Lee, A.; Lichtenhan, J. D. Macromolecules 1998, 31, 4970-4974. (87) Lichtenhan, J. D.; Otonari, Y. A.; Carr, M. J. Macromolecules 1995, 28, 8435-8437. (88) Romo-Uribe, A.; Mather, P. T.; Haddad, T. S.; Lichtenhan, J. D. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 1857-1872. (89) Tsuchida, A.; Bolln, C.; Sernetz, F. G.; Frey, H.; Muelhaupt, R. Macromolecules 1997, 30, 2818-2824. (90) Li, G. Z.; Wang, L.; Toghiani, H.; Daulton, T. L.; Koyama, K.; Pittman, C. U., Jr. Macromolecules 2001, 34, 8686-8693. (91) Kopesky, E. T.; Haddad, T. S.; Cohen, R. E.; McKinley, G. H. Macromolecules 2004, 37, 8992-9004. (92) Kopesky, E. T.; Haddad, T. S.; McKinley, G. H.; Cohen, R. E. Polymer 2005, 46, 4743-4752. (93) Kopesky, E. T.; McKinley, G. H.; Cohen, R. E. Polymer 2006, 47, 299309. (94) Joshi, M.; Butola, B. S.; Simon, G.; Kukaleva, N. Macromolecules 2006, 39, 1839-1849. (95) Hao, N.; Boehning, M.; Goering, H.; Schoenhals, A. Macromolecules 2007, 40, 2955-2964. (96) Zhao, Y.; Schiraldi, D. A. Polymer 2005, 46, 11640-11647.

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a significant competing mechanism, respectively. Furthermore, optical microscopy studies probe the temporal evolution of spinodal decomposition in 60 wt % PtBA blend films. Experimental Section Materials. PtBA (number average molar mass (Mn) of 5.0 kg‚mol-1; polydispersity index (Mw/Mn) of 1.25) and TPP (molar mass ) 0.931 kg‚mol-1) were obtained from Polymer Source, Inc. and Hybrid Plastics, Inc., respectively, and were used without further purification. Silicon wafers (100) were obtained from Waferworld, Inc. Cleaning Procedure for Silicon Substrates. The silicon wafers were boiled in a 1:1:5 (by volume) solution of ammonium hydroxide (28%) : hydrogen peroxide (30%) : ultrapure water (Millipore, Milli-Q Gradient A-10, 18.2 MΩ, 100 °C, only coalesced domains are observed as seen in Figure 3e. At 140 °C, the merger of phase separated domains is complete and the entire surface is covered by coalesced domains (Figure 3f). The morphological evolution with the formation of isolated, circular phase separated domains and the absence of interconnected structures in 20 wt % PtBA blend films is consistent with phase separation via the nucleation and growth mechanism. Further support for the conclusion that Figure 3 represents phase separation rather than dewetting comes from AFM and XPS experiments. AFM on films annealed to 140 °C shows that very large aggregates have formed. The root-mean-square (rms) roughnesses of the film increases from ∼1 nm (unannealed) to ∼55 nm (annealed to 140 °C at 1 °C‚min-1) (Supporting Information, Figure S12). While one may suspect dewetting to grow structures this large, XPS reveals that very little elemental silicon can be seen at the end of the cloud point experiment (elemental silicon to carbon atomic concentration ratio of Si/C ∼ 0.016). As a point for comparison, a representative Si/C ratio for a dewetting system of PS and TPP is Si/C ∼ 0.12 (see Supporting Information, Figure S20). Operating on the assumption

that Figure 3 represents phase separation by nucleation and growth, it should be possible to show that the small domains are PtBA-rich. Figure 4a and b shows height and phase AFM images, respectively, for a 20 wt % PtBA blend annealed at 85 °C (binodal temperature) for 15 min. The morphology consists of round domains (white objects) with a rms roughness of ∼23 nm, in a relatively smooth matrix (rms roughness ∼2 nm), with strong height and phase contrast. The height of the circular domain is on the order of ∼40 nm over the uniform film (line scan, Figure 4c). Selective solvent etching with water following hydrolysis of the PtBA by HCl leads to the morphologies seen in Figure 5a and b. While pits form in the continuous TPP-rich matrix (rms roughness increases to ∼42 nm) where PtBA was converted to PAA and extracted, initially PtBA-rich circular domains now become holes (line scan, 5c) with insoluble TPP frameworks inside them. The rms roughness inside the holes increases dramatically from ∼23 nm prior to etching to ∼150 nm after etching as PAA (PtBA) is removed from the holes. XPS was also performed as a control during the annealing process for the temperature jump experiment. For annealing times up to 15 min, the Si/C ratio is zero. For longer annealing times (∼60 min) Si/C ∼ 0.004 showing that dewetting is not a concern with respect to defining the binodal temperature. A similar representative

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Figure 4. 40 × 40 µm2 tapping mode AFM (a) height and (b) phase

images for a 20 wt % PtBA blend film annealed at 85 °C for 15 min. Z ranges for the height and phase images are 50 nm and 30°, respectively. (c) A representative line scan profile (along the black arrow in part a) used for the determination of domain (white object) height. The reference line (0 nm) for the line profile analysis is set in the uniform area along the black arrow in part (a). The black box in part (a) represents a uniform area used to determine rms surface roughness of the continuous phase. A box of comparable size on the white domain is used to determine the domain surface roughness. The Si/C ratio for this film is zero at this early stage of phase separation.

Figure 5. 100 × 100 µm2 tapping mode AFM (a) height and (b) phase

images for a 20 wt % PtBA blend film annealed at 85 °C for 15 min. For these images, the PtBA component of the blend film was acid hydrolyzed (gas phase) to PAA and the PAA was removed with water. Z ranges for the height and phase images are 500 nm and 100°, respectively. (c) A representative line scan profile (along the black arrow in part a) to illustrate the relative depth of a hole formed in the film after selective removal of PtBA (comparable to the film thickness of ∼90 nm). The reference line (0 nm) for the line profile analysis is set in the TPP-rich matrix along the black arrow in part (a). The black box in part (a) represents an area used to determine rms surface roughness of the TPP-rich matrix. The Si/C ratio for this film is ∼0.004.

example for the OM, AFM, and XPS analysis of a PtBA-rich blend (80 wt % PtBA) at the end of a cloud point experiment

Figure 6. Optical micrographs of the morphological evolution in a 60 wt % PtBA blend film as a function of temperature at a heating rate of 1 °C‚min-1: (a) 25, (b) 75, (c) 85, (d) 95, (e) 105, and (f) 140 °C. The insets show the FFT patterns of the corresponding optical micrographs.

is provided as Supporting Information (Figures S7 and S17, Table S1). As one might expect, isolated POSS-rich domains remain after selective solvent etching removes the continuous PtBArich phase (Figure S21) with substantial exposure of the silicon substrate (Si/C ∼ 0.06, Supporting Information, legend of Figure S21). Cloud Point Experiments at 60 wt % PtBA: Spinodal Decomposition. Annealing initially homogeneous (Figure 6a) 60 wt % PtBA blends during temperature ramp experiments reveals the formation of bicontinuous structures at ∼70 °C (optical micrograph is not shown here). The bicontinuous features coarsen noticeably with increasing annealing temperatures (Figure 6b-d) and domain growth is pinned at ∼105 °C (Figure 6e). The breakup of interconnected features and the formation of circular domains ensues at ∼115 °C. At temperatures >130 °C, only circular domains that are formed by the break-up of the bicontinuous structures are observed (Figure 6f). Fast Fourier transforms (FFTs) of the images in Figure 6 provide information about the relevant length scales for the bicontinuous structures. The FFT patterns shown in the insets of Figure 6 are circular, consistent with a laterally isotropic morphology with a dominant wavelength.58,60-62 The decrease in the dimensions of the FFT patterns with increasing annealing temperatures (insets of Figure 6b-e) indicates coarsening of the spinodal features. It is clear from the morphologies and their corresponding FFT patterns that phase separation in 60 wt % PtBA blend films proceeds via spinodal decomposition. As was the case for the 20 wt % PtBA films, AFM and XPS experiments provide conformation that the behavior in Figure 6 is not a consequence of dewetting. AFM on films annealed to 140 °C at 1 °C‚min-1 shows that large aggregates have formed. The root-mean-square (rms) roughness of the film increases from ∼1 nm (unannealed) to ∼9 nm (annealed) (Supporting Information, Figure S15). Furthermore, XPS reveals that no elemental silicon has been exposed (elemental silicon to carbon atomic

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Figure 7. 100 × 100 µm2 tapping mode AFM (a) height and (b) phase images for a 60 wt % PtBA blend film annealed at 75 °C for 30 min. Z ranges for the height and phase images are 50 nm and 50°, respectively. The overall rms surface roughness is ∼3 nm. The Si/C ratio for this film is zero.

Figure 8. 100 × 100 µm2 tapping mode AFM (a) height and (b) phase images for a 60 wt % PtBA blend film annealed at 75 °C for 30 min. For these images, the PtBA component of the blend film was acid hydrolyzed (gas phase) to PAA and the PAA was removed with water. Z ranges for the height and phase images are 400 nm and 100°, respectively. The overall rms surface roughness is ∼88 nm. The Si/C ratio for this film is ∼0.02.

ratios of Si/C ) 0, Table S1). If Figure 6 represents phase separation by spinodal decomposition, it should be possible to determine which features are PtBA-rich. Figure 7a and b shows height and phase AFM images, respectively, for a 60 wt % blend annealed at 75 °C (slightly above the critical temperature) for 30 min. The morphology consists of bicontinuous domains with a rms roughness of ∼3 nm. This value is less extreme than the example in Figure 4 for phase separation by a nucleation and growth mechanism. As seen in Figure 7, there is no detectable phase contrast and hence no phase image (b). This observation is consistent with a narrow spinodal region, whereby the two coexisting phases are much closer in composition than for films undergoing phase separation by nucleation and growth. The height differences between the two bicontinuous phases are also very small, on the order of a few nanometers as the overall rms roughness of the film is only ∼3 nm. After exposure to HCl vapor, selective solvent etching of PAA with water leads to the morphologies seen in Figure 8a and b. As one can see, the features have changed in size. As noted previously, estimates of the binodal are on the order of ∼8 to 10 °C higher than the equilibrium phase boundary. As the films are heated to 60 °C for ∼12 h during HCl vapor exposure,97 further coarsening of the spinodal features occurs (similar to the late stage images of Figure 6). Nonetheless, the TPP-rich features correspond to the higher features in Figure 7. The other interesting feature of Figure 8a and b is that phase contrast returns following extraction of the PAA formed during PtBA exposure to HCl. This contrast likely arises from different densities of voids in the two phases following etching. The phase separation mechanism (spinodal decomposition) does not seem to induce dewetting in PtBA/TPP blends for the thicknesses used in this study (∼90 nm). XPS for annealing times up to 120 min at 75 °C for the 60 wt % PtBA blend, show the Si/C ratio

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is still zero indicating that dewetting is not a concern for studying morphology and kinetics in the spinodal regime of the phase diagram for this system. Moreover, the selective solvent etching process does not lead to gross exposure of the silicon substrate. After selective solvent etching the film in Figure 7, the Si/C only increases to ∼0.02 for the film in Figure 8. Cloud Point Experiments at 58 and 62 wt % PtBA: OffCritical Spinodal Decomposition. The morphological evolution in 60 wt % PtBA blend films could be consistent with spinodal decomposition at the critical composition. To test this hypothesis, cloud point experiments for compositions close to 60 wt % PtBA (55, 58, 62, and 65 wt % PtBA) were studied. During temperature ramp experiments, 58 and 62 wt % PtBA blend films form isolated, small, circular phase separated domains at ∼71 and ∼67 °C, respectively (optical micrographs are not shown here). The density and dimensions of the nucleated domains increase as temperature increases (Figures 9b-d and 10b-d). Bicontinuous phase separated domains start to appear at ∼85 and ∼87 °C for 58 and 62 wt % PtBA blend films, respectively, and the spinodal structures coarsen (Figures 9c,d and 10c,d) with increasing annealing temperatures. The coalescence of the nucleated domains and the break-up of the spinodal features ensues for annealing temperatures >93 °C (Figures 9e and 10e) and spinodal structures are not observed at temperatures g100 °C, for 58 and 62 wt % blend films. At temperatures >110 °C, only coalesced domains with dark aggregates are observed for both 58 and 62 wt % PtBA blend films as shown in Figures 9f and 10f. Another feature of Figure 10 worth noting is the appearance of dark-field spots in the optical micrographs (eg. Figure 10e). One possible explanation for this effect is crystallization within the blend. DSC studies for amorphous TPP samples prepared from ethanol solutions, and low wt % PtBA blends with TPP show an exothermic transition that is consistent with TPP crystallization at temperatures Tx ∼ 116 °C (Supporting Information, Figures S22-S27). This transition is not present in as received (crystalline) TPP which has a melting transition at ∼ 208 °C,96 well above the temperatures in this study. It is also inconsistent with sample degradation, which can occur in excess of ∼150 °C for PtBA and ∼200 °C for TPP. Whether the dark field features that occur at higher temperatures in the two-phase region arise from crystallization or simply arise from the large size of the phase separated domains providing illumination through secondary scattering of comparably large structures in the dark field region is not critical for defining the phase boundary which occurs at significantly lower temperatures. Hence, the morphologies observed in 58 and 62 wt % PtBA blend films during temperature ramp experiments suggest that these blend films initially undergo phase separation via the nucleation and growth mechanism followed by off-critical spinodal decomposition. In order to test this hypothesis further, AFM and XPS provide additional characterization of the films shown in Figures 9f and 10f. Figure 11a and b shows height and phase images at the end of the cloud point experiment for 58 wt % PtBA. As seen in Figure 11a, the film consists of relatively high, discrete features (Z-range ) 700 nm) with strong phase contrast. Nonetheless, the features do not represent significant dewetting as Si/C ratios are still only ∼0.01. After hydrolysis of the PtBA to PAA and etching with water, one sees that the relatively high domains remain (Figure 12a); however, the phase contrast decreases (Figure 12b). Hence the elevated structures in the phase separated film (the darkest spots in Figure 9f) are TPP rich. The reason for the decrease in phase contrast arises from significant removal of the continuous PtBA-rich phase with significant exposure of the underlying silicon (Si/C ∼ 0.07 from XPS). The contrast in

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Figure 9. Optical micrographs of the morphological evolution in a 58 wt % PtBA blend film as a function of temperature at a heating rate of 1 °C‚min-1: (a) 25, (b) 80, (c) 87, (d) 90, (e) 97, and (f) 140 °C. The insets in parts (c) and (d) represent 0.14 × 0.19 mm2 sections of the original images that have been cut using imaging software to enhance the clarity of the bicontinuous domains.

the phase image decreases because the TPP and silicon are more similar in hardness than TPP and PtBA. Similar conclusions pertain to the 62 wt % PtBA blend as seen in Figures 13 (at the end of the cloud point experiment) and 14 (after selective solvent etching). Again, the relatively high features represent TPP-rich domains, while the continuous PtBA rich regions are removed exposing Si (Si/C ∼ 0.03 by XPS), and decreasing the phase contrast in Figure 14b. It is also important to note one key difference between the morphologies grown during off-critical spinodal decomposition versus those generated for the proposed critical composition (60 wt % PtBA). As seen in Figures 11-14, the relevant length scales for the morphologies before and after selective solvent etching are unaffected by the HCl treatment step. This result is consistent with imaging late stage structure in Figures 11-14 where the key features are already widely spaced. The uniform 55 and 65 wt % PtBA blend films only undergo phase separation via the nucleation and growth mechanism during temperature ramp experiments with the formation of small, circular domains starting at ∼73 and ∼65 °C, respectively. The density and dimensions of the domains increase with increasing annealing temperatures. For 55 and 65 wt % PtBA blend films, the coalescence of domains is observed for annealing temperatures g95 °C. The formation of dark aggregates within the coalesced

domains are observed at temperatures >95 °C and >110 °C for 55 and 65 wt % PtBA blend films, respectively. These features are qualitatively similar to Figures 9f and 10f. However, at this late stage of the morphological evolution, it is not possible to see if the morphology was also initiated by off-critical spinodal behavior. At 140 °C, only coalesced domains with dark aggregates are observed for both 55 and 65 wt % PtBA blend films. Optical micrographs of the phase evolution in 55 and 65 wt % PtBA blend films during cloud point measurements as a function of temperature, and AFM images of the final state are provided in the Supporting Information as Figures S5 and S6, and Figure S14 and S16, respectively. Phase Diagram from Cloud Point Measurements. Cloud point experiments like those discussed above yield a phase diagram for a specific heating rate (1 °C‚min-1). As previously discussed, this diagram is likely ∼8 to 10 °C higher than the equilibrium phase diagram. Figure 15 shows the experimentally determined binodal (open circles) and spinodal (filled diamonds with dotted line) curves. In addition, Figure 15 also contains values of the surface Tg (filled triangles), and thermal transitions associated with the loss of double layer structures (Tdl) (filled squares) for the PtBA/TPP thin film blend system obtained from ellipsometry (Supporting Information, Figure S28). The double layer structure is a characteristic of PtBA multilayer films prepared

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Figure 10. Optical micrographs of the morphological evolution in a 62 wt % PtBA blend film as a function of temperature at a heating rate of 1 °C‚min-1: (a) 25, (b) 80, (c) 90, (d) 92, (e) 97, and (f) 140 °C. The insets in parts (c) and (d) represent 0.14 × 0.19 mm2 sections of the original images that have been cut using imaging software to enhance the clarity of the bicontinuous domains.

Figure 11. 100 × 100 µm2 tapping mode AFM (a) height and (b) phase images for a 58 wt % PtBA blend film at the end of the cloud point experiment. Z ranges for the height and phase images are 700 nm and 50°, respectively. The overall rms surface roughness of the film is ∼158 nm. The Si/C ratio for this film is ∼0.01.

by Y-type LB-deposition.97 From the binodal and spinodal curves the PtBA/TPP blend system confined to thin films appears to exhibit LCST behavior with a critical temperature of ∼70 °C corresponding to a critical composition of 60 wt % PtBA. Offcritical spinodal behavior, observed for the 58 and 62 wt % PtBA blend films, provides an estimate for the width of the spinodal region. One obvious difference between Figure 1 and Figure 15 is that the phase diagram is flattened for high wt % PtBA. A less

Figure 12. 100 × 100 µm2 tapping mode AFM (a) height and (b) phase images for a 58 wt % PtBA blend film at the end of the cloud point experiment. For these images, the PtBA component of the blend film was acid hydrolyzed (gas phase) to PAA and the PAA was removed with water. Z ranges for the height and phase images are 700 nm and 50°, respectively. The overall rms surface roughness of the film is ∼139 nm. The Si/C ratio for this film is ∼0.07.

obvious feature is that the phase diagram is also shifted toward a higher wt % PtBA. If one were to assume a Flory-Huggins type free-energy relationship developed for binary polymer blends, the critical composition (volume fraction) for a binary blend would be defined as φa,c ) nb1/2(na1/2 + nb1/2), where na and nb are degrees of polymerization.10 Treating the TPP as a

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Figure 13. 100 × 100 µm2 tapping mode AFM (a) height and (b) phase images for a 62 wt % PtBA blend film at the end of the cloud point experiment. Z ranges for the height and phase images are 700 nm and 50°, respectively. The overall rms surface roughness of the film is ∼141 nm. The Si/C ratio for this film is ∼0.01.

Figure 14. 100 × 100 µm2 tapping mode AFM (a) height and (b) phase images for a 62 wt % PtBA blend film at the end of the cloud point experiment. For these images, the PtBA component of the blend film was acid hydrolyzed (gas phase) to PAA and the PAA was removed with water. Z ranges for the height and phase images are 700 nm and 50°, respectively. The overall rms surface roughness of the film is ∼175 nm. The Si/C ratio for this film is ∼0.03.

Figure 15. Experimentally determined binodal (open circles) and spinodal (filled diamonds with dotted line) curves, surface Tg (filled triangles), and thermal transitions associated with the loss of double layer structure (Tdl) (filled squares) for the PtBA/TPP thin film blend system.

polymer with nb ) nTPP ) 7, along with na ) nPtBA ) 39, φPtBA,c ∼ 0.3, or roughly half the observed experimental value of φPtBA,c ) 0.60. Part of this shift, but not all, corresponds to density differences between PtBA (∼1 g/mL) and TPP (∼1.33 g/mL), whereby 30 wt % PtBA is ∼35 vol % PtBA. Clearly, other factors need to be considered that can influence the critical compositions and shapes of phase diagrams in thin films. Some obvious choices are the interplay of dewetting with phase separation, preferential wetting of the silicon substrate or enrichment of the air/blend interface by one of the two components, and dynamic effects associated with proximity to the glass transition or other structural transition temperatures.

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Several recent studies have shown that phase separation and morphological evolution in thin film polymer blends may be affected by dewetting or spontaneous hole formation in these systems at elevated temperatures.98-102 However, AFM and XPS studies already discussed show that dewetting is unlikely to be important for the present, relatively thick (∼90 nm) PtBA/TPP blend system. A more probable contributor to the results of this study is surface interactions. Preferential wetting of the substrate by one of the two components effectively enriches the blend in the other component. While this is insignificant for bulk quantities of materials, it can have major effects on thin film systems. Previous researchers have shown that critical compositions (and phase diagrams) in thin films blends can substantially shift because of surface segregation.103-106 If surface segregation is the major contributor to the shift in the critical composition, PtBA most likely is wetting the silicon substrate, thereby having an effectively lower concentration in the blend. It is unlikely that PtBA is enriching the air/polymer interface as POSS derivatives like organosilicones tend to have lower surface energies than common purely organic polymers.107 Finally, proximity of the two phase boundary to the glass transition and double layer transition temperatures could be contributing to the flattening of the phase diagram at high wt % PtBA. For low wt % PtBA, the binodal is shifted relatively far away from both Tg and Tdl, by ∼55 and ∼35 °C, respectively. In contrast for films with critical (60 wt % PtBA) and higher PtBA compositions, the measured binodal is within 10 to 15 °C of the double layer transition, but is still ∼40 to 45 °C away from Tg. As noted previously, the binodal in Figure 15 may actually be ∼8-10 °C higher than the equilibrium curve. Hence, the nonequilibrium conformations of the molecules in the double layer structure of the film if not solely responsible, likely have a significant influence on the flattening of the binodal curve at high wt % PtBA. Morphology and Kinetics of Spinodal Decomposition at Different Temperatures. By knowing the phase diagram for PtBA/TPP blends as thin films, it is possible to address mechanistic questions for phase separation in thin films. One open question is how the characteristic wavelength (wavevector) for phase separation by spinodal decomposition depends on time. Theoretical and experimental studies at intermediate times during phase separation by spinodal decomposition show q ∼ tn where n ) -1/4 to -1/3.46,58,60,62,108 On the basis of the phase diagram (Figure 15) and the observed morphologies during cloud point experiments (Figures 6-8), 60 wt % PtBA blend films (critical composition) were studied by temperature jump experiments at 75, 85, 95, and 105 °C. Figure 16 shows optical micrographs for 60 wt % PtBA blend films during annealing at 85 °C as a function (98) Liao, Y.; Su, Z.; Sun, Z.; Shi, T.; An, L. Macromol. Rapid Commun. 2006, 27, 351-355. (99) Mueller-Buschbaum, P.; Gutmann, J. S.; Stamm, M. Macromolecules 2000, 33, 4886-4895. (100) Ton-That, C.; Shard, A. G.; Daley, R.; Bradley, R. H. Macromolecules 2000, 33, 8453-8459. (101) Mueller-Buschbaum, P.; Gutmann, J. S.; Stamm, M.; Cubitt, R. Macromol. Symp. 2000, 149, 283-288. (102) Chung, H.-J.; Ohno, K.; Fukuda, T.; Composto, R. J. Macromolecules 2007, 40, 384-388. (103) Chung, H.-J.; Composto, R. J. Phys. ReV. Lett. 2004, 92, 185704/1185704/4. (104) Jones, R. L.; Indrakanti, A.; Briber, R. M.; Muller, M.; Kumar, S. K. Macromolecules 2004, 37, 6676-6679. (105) Wen, G.; Li, X.; Liao, Y.; An, L. Polymer 2003, 44, 4035-4045. (106) Binder, K.; Muller, M. Macromol. Symp. 2000, 149, 1-10. (107) Hosaka, N.; Torikai, N.; Otsuka, H.; Takahara, A. Langmuir 2007, 23, 902-907. (108) Liao, Y.; Su, Z.; Ye, X.; Li, Y.; You, J.; Shi, T.; An, L. Macromolecules 2005, 38, 211-215.

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Figure 16. Optical micrographs of the morphological evolution in a 60 wt % PtBA blend film annealed at 85 °C for (a) 2, (b) 5, (c) 10, (d) 20, (e) 45, and (f) 120 min. The insets show the FFT images of the corresponding optical micrographs.

of time. It is observed that the blend films undergo instantaneous spinodal decomposition upon annealing. The spinodal features coarsen with increasing annealing times (Figure 16b-e) until domain growth is arrested. Finally, the spinodal structures break up to form circular domains during the late stages of phase separation for annealing times longer than ∼45 min for blend films annealed at 85 °C (Figure 16f). The FFT patterns shown in the insets of Figure 16 are circular suggesting the evolution of a laterally isotropic morphology with a dominant wavelength (wavevector) during annealing.58,60-62 Moreover, the dimensions of the FFT patterns decrease with increasing annealing times. This observation is consistent with coarsening of the spinodal features as a function of annealing time. The morphological evolution in blend films annealed at 75 and 95 °C (Supporting Information, Figures S29 and S30) is similar to that observed for 85 °C with the exception of longer and shorter time scales, respectively, for phase evolution. Spinodal decomposition and pinning of domain growth ensues immediately upon annealing the blend films at 105 °C, followed by the formation of circular domains for annealing times longer than 5 min (Supporting Information, Figure S31). The variation of the characteristic wavevector (q) with annealing time (t) for 60 wt % PtBA blend films annealed at 75, 85, and 95 °C is illustrated in Figure 17 on a log-log scale. The

Figure 17. Log-log plots of q vs t at 75 (open circles), 85 (open triangles), and 95 °C (open squares) for 60 wt % PtBA blend films. The numbers on the graph represent the scaling exponents for q ∼ tn with one standard deviation error bars obtained from a leastsquares fitting analysis (solid lines).

plots of q versus t exhibit two distinct regimes for 75, 85, and 95 °C. Initially, q ∼ t-0.242(0.008 (75 °C), q ∼ t-0.265(0.006 (85 °C), and q ∼ t-0.33(0.02 (95 °C) followed by an approximately zeroorder dependence (q ∼ t0) for all temperatures shown on the plot (error bars on the exponents represent one standard deviation).

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The values of the power law exponents, q ∼ tn, are close to n ≈ -1/3 in agreement with the Lifschitz-Slyozov law.35 The observation of n ≈ -1/3 is consistent with diffusion controlled domain growth at intermediate times during phase separation by spinodal decomposition.108 As seen from Figure 17, films annealed at both 75 and 85 °C exhibit power law exponents that are closer to n ≈ -1/4. This value is smaller than both the diffusion controlled growth exponent, n ≈ -1/3, and the growth exponent of n ≈ -0.44 reported for two-dimensional systems.62 However, a growth exponent of n ≈ -1/4 has been reported by Wang and Composto for perdeuterium-labeled polymethylmethacrylate (dPMMA) and random styrene-acrylonitrile copolymers (SAN) blend films with thicknesses less than the radius of gyration, Rg.60 The double layer structure of the Y-type LB-films and proximity of Tdl (∼49 °C for 60 wt % PtBA blend LB-films) to the annealing temperatures (75 and 85 °C) could mean that these annealing experiments (temperature jumps) are performed under conditions of nonequilibrium chain conformations (a condition similar to a film of thickness