Morphological Evolution in Dewetting Polystyrene ... - ACS Publications

The silicon wafers were boiled in a 1:1:5 (by volume) solution of ammonium ..... a key feature is that the brittle upper layers do not undergo plastic...
0 downloads 0 Views 1MB Size
Download PDF
4676

Langmuir 2008, 24, 4676-4684

Morphological Evolution in Dewetting Polystyrene/Polyhedral Oligomeric Silsesquioxane Thin Film Bilayers Rituparna Paul, Ufuk Karabiyik, Michael C. Swift, John R. Hottle,† and Alan R. Esker* Macromolecules and Interfaces Institute and the Department of Chemistry (0212), Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061 ReceiVed June 2, 2007. In Final Form: October 23, 2007 Morphological evolution in dewetting thin film bilayers of polystyrene (PS) and a polyhedral oligomeric silsesquioxane (POSS), trisilanolphenyl-POSS (TPP), was studied as a function of annealing temperature and annealing time. The results demonstrate unique dewetting morphologies in PS/TPP bilayers at elevated temperatures that are significantly different from those typically observed in dewetting polymer/polymer bilayers. During temperature ramp studies by optical microscopy (OM) in the reflection mode, PS/TPP bilayers form cracks with a weak optical contrast at ∼130 °C. The crack formation is attributed to tensile stresses within the upper TPP layer. The weak optical contrast of the cracks observed in the bilayers for annealing temperatures below ∼160 °C is consistent with the cracking and dewetting of only the upper TPP layer from the underlying PS layer. The optical contrast of the morphological features is significantly enhanced at annealing temperatures of >160 °C. This observation suggests dewetting of both the upper TPP and the lower PS layers that results in the exposure of the silicon substrate. Upon annealing the PS/TPP bilayers at 200 °C in a temperature jump experiment, the upper TPP layer undergoes instantaneous cracking as observed by OM. These cracks in the upper TPP layer serve as nucleation sites for rapid dewetting and aggregation of the TPP layer, as revealed by OM and atomic force microscopy (AFM). X-ray photoelectron spectroscopy (XPS) results indicated that dewetting of the lower PS layer ensued for annealing times >5 min and progressed up to 90 min. For annealing times >90 min, OM, AFM, and XPS results revealed complete dewetting of both the layers with the formation of TPP encapsulated PS droplets.

Introduction Thin film bilayers consisting of different polymers as well as polymeric and non-polymeric materials, such as nanoparticles on solid substrates, have immense potential for applications in thin film transistors,1 light emitting diodes,2 sensors,3 biocompatible coatings,4 and drug delivery systems.5 These future highend applications require the reduction of multilayer thicknesses to the nanoscale regime while maintaining stability and adhesion at the interfaces. Furthermore, the stability of multilayer films depends on the stability of each layer and the interaction between these layers. Spontaneous hole formation or dewetting in amorphous polymer thin films above their glass transition temperature (Tg) is a major problem for the fabrication and stability of polymer/polymer (non-polymer) multilayers. Amorphous polymers, although not obvious liquids, can be regarded as liquids above their Tg and hence may undergo dewetting at elevated temperatures. Dewetting may proceed either via nucleation and growth or spinodal dewetting mechanisms. Hole formation is initiated by an impurity in the film and/or a defect on the substrate, for the nucleation and growth mechanism.6 In contrast, hole formation during spinodal dewetting is initiated by capillary waves on the surface * To whom correspondence should be addressed. E-mail: [email protected]; fax: (540) 231-3255. † Present address: Department of Chemistry, University of Wisconsin, Madison, WI 53706. (1) Garnier, F. Chem. Phys. 1998, 227, 253-262. (2) Liu, B.; Niu, Y. H.; Yu, W. L.; Cao, Y.; Huang, W. Synth. Met. 2002, 129, 129-134. (3) Thomas, S. W., III; Amara, J. P.; Bjork, R. E.; Swager, T. M. Chem. Commun. 2005, 36, 4572-4574. (4) Ratner, B. Biomed. Mater. Res. 1993, 27, 837-850. (5) Grayson, A. C. R.; Voskerician, G.; Lynn, A.; Anderson, J. M.; Cima, M. J.; Langer, R. J. Biomater. Sci., Polym. Ed. 2004, 15, 1281-1304. (6) Lorenz-Haas, C.; Mu¨ller-Buschbaum, P.; Kraus, J.; Bucknall, D. G.; Stamm, M. Appl. Phys. A 2002, 74, 383-385.

that arise from density fluctuations in the polymer.7 The capillary waves of varying amplitudes can initiate holes upon contact with the substrate. Upon heating the polymer films above Tg, holes are formed, and the polymer collects in rims surrounding the holes, irrespective of the hole initiation mechanism. Recently, the dewetting behavior in polymer/polymer thin film bilayers has gained considerable attention that can be attributed to the widespread use of polymers in thin film applications that require film stability and the potential application of controlled dewetting in polymer films for the fabrication of patterned surfaces. Theoretical8-17 and experimental18-44 studies on the stability of polymeric bilayers have demonstrated a variety of (7) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Phys. ReV. Lett. 1998, 81, 1251-1254. (8) Bandyopadhay, D.; Gulabani, R.; Sharma, A. Ind. Eng. Chem. Res. 2005, 44, 1259-1272. (9) Bandyopadhay, D.; Sharma, A. J. Chem. Phys. 2006, 125, 54711/1-54711/ 13. (10) Brochard-Wyart, F.; Martin, P.; Redon, C. Langmuir 1993, 9, 36823690. (11) Craster, R. V.; Matar, O. K. Phys. Fluids 2005, 17, 32104/1-32104/17. (12) Fisher, L. A.; Golovin, A. A. J. Colloid Interface Sci. 2005, 291, 515528. (13) Kumar, S.; Matar, O. K. J. Colloid Interface Sci. 2004, 273, 581-588. (14) Matar, O. K.; Gkanis, V.; Kumar, S. J. Colloid Interface Sci. 2005, 286, 319-332. (15) Pototsky, A.; Bestehorn, M.; Merkt, D. Phys. ReV. E: Stat., Nonlinear, Soft Matter Phys. 2004, 70, 025201/1-025201/4. (16) Pototsky, A.; Bestehorn, M.; Merkt, D. Thiele, U. J. Chem. Phys. 2005, 122, 224711/1-224711/13. (17) Shankar, V.; Sharma, A. J. Colloid Interface Sci. 2004, 274, 294-308. (18) Ade, H.; Winesett, D. A.; Smith, A. P.; Anders, S.; Stammler, T.; Heske, C.; Slep, D.; Rafailovich, M. H.; Sokolov, J.; Stoehr, J. Appl. Phys. Lett. 1998, 73, 3775-3777. (19) David, M. O.; Reiter, G.; Sitthaie, T.; Schultz, J. Langmuir 1998, 14, 5667-5672. (20) Faldi, A.; Composto, R. J.; Winey, K. I. Langmuir 1995, 11, 4855-4861. (21) Harris, M.; Appel, G.; Ade, H. Macromolecules 2003, 36, 3307-3314. (22) Higgins, A. M.; Sferrazza, M.; Jones, R. A. L.; Jukes, P. C.; Sharp, J. S.; Dryden, L. E.; Webster, J. Eur. Phys. J. E 2002, 8, 137-143. (23) Higgins, A. M.; Jones, R. A. L. Nature (London, U.K.) 2000, 404, 476478.

10.1021/la701625g CCC: $40.75 © 2008 American Chemical Society Published on Web 04/10/2008

Dewetting PS/POSS Thin Film Bilayers

initial instability modes that evolve into different dewetting pathways and morphologies as dewetting proceeds. Some of the factors that affect dewetting behavior for polymer/polymer bilayers include interfacial energies, viscosities, film thicknesses of the bilayer components, and the bounding media. Spin-coating is commonly used to prepare polymer films for dewetting studies, and dewetting is probed shortly after film preparation by annealing the films near and above Tg. Spincoated, amorphous polymer films typically undergo structural relaxations when heated near Tg that result in the shrinkage of the films and the generation of residual stresses.45-48 Furthermore, spin-coated films of glassy polymers may retain residual solvents for long periods of time, and changes in film thickness associated with solvent loss may occur over several hours under ambient conditions.47,49 Recent studies have shown that structural relaxation and slow evaporation of residual solvents in spincoated, glassy films may complicate and alter the dewetting behavior of these films upon annealing near and above Tg.48-50 The dewetting behavior in polymer/nanoparticle thin film systems has gained attention recently but is far from being completely understood. Most of these studies focused on the suppression of dewetting by the addition of small amounts of nanoparticles,51-57 and a few have shed light on utilizing dewetting (24) Hu, X.; Narayanan, S.; Lurio, L. B.; Lal, J. J. Non-Cryst. Solids 2006, 352, 4973-4976. (25) Kang, H.; Lee, S.-H.; Kim, S.; Char, K. Macromolecules 2003, 36, 85798583. (26) Krausch, G. J. Phys.: Condens. Matter 1997, 9, 7741-7752. (27) Lambooy, P.; Phelan, K. C.; Haugg, O.; Krausch, G. Phys. ReV. Lett. 1996, 76, 1110-1113. (28) Lee, W.-K.; Ryou, J.-H.; Cho, W.-J.; Ha, C.-S. Polym. Test. 1998, 17, 167-177. (29) Li, Y.; Yang, Y.; Yu, F.; Dong, L. J. Polym. Sci., Part B: Polym. Phys. 2005, 44, 9-21. (30) Lin, Z.; Tobias, K.; Russell, T. P.; Schaeffer, E.; Steiner, U. Macromolecules 2002, 35, 6255-6262. (31) Morariu, M. D.; Voicu, N. E.; Schaffer, E.; Lin, Z.; Russel, T. P.; Steiner, U. Nat. Mater. 2003, 2, 48-52. (32) Mueller-Buschbaum, P.; Wunnicke, O.; Stamm, M.; Lin, Y.-C.; Mueller, M. Macromolecules 2005, 38, 3406-3413. (33) Pan, Q.; Winey, K. I.; Hu, H. H.; Composto, R. J. Langmuir 1997, 13, 1758-1766. (34) Qu, S.; Clarke, C. J.; Liu, Y.; Rafailovich, M. H.; Sokolov, J.; Plelan, K. C.; Krausch, G. Macromolecules 1997, 30, 3640-3645. (35) Reiter, G.; Sharma, A.; Casoli, A.; David, M.-O.; Khanna, R.; Auroy, P. Langmuir 1999, 15, 2551-2558. (36) Renger, C.; Mueller-Buschbaum, P.; Stamm, M.; Hinrichsen, G. Macromolecules 2000, 33, 8388-8398. (37) Sferrazza, M.; Heppenstall-Butler, M.; Cubbit, R.; Bucknall, D.; Webster, J.; Jones, R. A. L. Phys. ReV. Lett. 1998, 81, 5173-5176. (38) Slep, D.; Asselta, J.; Rafailovich, M. H.; Sokolov, J.; Winesett, D. A.; Smith, A. P.; Ade, H.; Anders, S. Langmuir 2000, 16, 2639-2375. (39) Wang, C.; Krausch, G.; Geoghagen, M. Langmuir 2001, 17, 6269-6274. (40) Wang, X.; Tvingstedt, K.; Ingana¨s, O. Nanotechnology 2005, 16, 437443. (41) Wei, B.; Genzer, J.; Spontak, R. J. Langmuir 2004, 20, 8659-8667. (42) Wunnicke, O.; Lorennz-Haas, C.; Mueller-Buschbaum, P.; Leiner, V.; Stamm, M. Appl. Phys. A 2002, 74, 445-447. (43) Yoon, B. K.; Huh, J.; Kim, H.-C.; Hong, J.-M.; Park, C. Macromolecules 2006, 39, 901-903. (44) Yuan, C.; Quyamg, M.; Koberstein, J. T. Macromolecules 1999, 32, 23292333. (45) Croll, S. G. J. Appl. Polym. Sci. 1979, 23, 847-858. (46) Kawana, S.; Jones, R. A. L. Eur. Phys. J. E 2003, 10, 223-230. (47) Richardson, H.; Sferrazza, M.; Keddie, J. L. Eur. Phys. J. E 2003, 12, 87-91. (48) Richardson, H.; Carelli, C.; Keddie, J. L.; Sferrazza, M. Eur. Phys. J. E 2003, 12, 437-441. (49) Reiter, G.; de Gennes, P. G. Eur. Phys. J. E 2001, 6, 25-28. (50) Bollinne, C.; Cuenot, S.; Nysten, B.; Jonas, A. M. Eur. Phys. J. E 2003, 12, 389-396. (51) Barnes, K. A.; Douglas, J. F.; Liu, D.-W.; Karim, A. AdV. Colloid Interface Sci. 2001, 94, 83-104. (52) Barnes, K. A.; Karim, A.; Douglas, J. F.; Nakatani, A. I.; Gru¨ll, H.; Amis, E. J. Macromolecules 2000, 33, 4177-4185. (53) Hosaka, N.; Tanaka, K.; Otsuka, H.; Takahara, A. Compos. Interfaces 2004, 11, 297-306. (54) Hosaka, N.; Torikai, N.; Otsuka, H.; Takahara, A. Langmuir 2007, 23, 902-907.

Langmuir, Vol. 24, No. 9, 2008 4677

Figure 1. (a) Initial configuration of the PS/TPP bilayer films used in this study. XPS studies show that ∼1 in 5 Si atoms are at least partially oxidized to SiOx (x ) 0.5-2) when annealed at 200 °C for ∼15-120 min (the time scale of the dewetting experiments in this paper). (b) TPP where R ) phenyl. (c) PS.

in polymer/nanoparticle films to create patterned surfaces.58-63 A fundamental understanding of dewetting mechanisms and their morphological evolution in polymer/nanoparticle bilayer films is essential for controlling the final properties and surface morphologies of these systems. This study explored the dewetting behavior and pattern formation in thin film bilayers of polystyrene (PS) and a polyhedral oligomeric silsesquioxane (POSS), trisilanolphenyl-POSS (TPP), with Si//PS/TPP//air configurations (Figure 1). Here, a double slash (//) signifies a distinct interface, while a single slash (/) is used to depict a dynamic interface between mobile components. The ability to craft bilayers of a glassy polymer and a nanoparticle allow us to begin from a kinetically stable state below Tg for PS and probe how pattern evolution in PS/TPP bilayers during annealing at temperatures in excess of Tg for PS differs from pattern formation in poly(t-butyl acrylate) (PtBA)/TPP bilayers,60 as well as studies of mechanically confined polymers.19,64,65 The POSS component of the bilayers is an organic/inorganic hybrid material (a molecule) consisting of a rigid, highly symmetric three-dimensional inorganic core (Si-O cage) and a flexible organic corona, thereby serving as a model monodisperse nanofiller.66,67 The organic corona allows processability and (55) Krishnan, R. S.; Mackay, M. E.; Hawker, C. J.; Van Horn, B. Langmuir 2005, 21, 5770-5776. (56) Sharma, S.; Rafailovich, M. H.; Peiffer, D.; Sokolov, J. Nano Lett. 2001, 1, 511-514. (57) Xavier, J. H.; Sharma, S.; Seo, Y. S.; Isseroff, R.; Koga, T.; White, H.; Ulman, A.; Shin, K.; Satija, S. K.; Sokolov, J.; Rafailovich, M. H. Macromolecules 2006, 39, 2972-2980. (58) Huang, J.; Kim, F.; Tao, A. R.; Connor, S.; Yang, P. Nat. Mater. 2005, 4, 896-900. (59) Lee, L.-T.; Leite, C. A. P.; Galembeck, F. Langmuir 2004, 20, 44304435. (60) Paul, R.; Esker, A. R. Langmuir 2006, 22, 6734-6738. (61) Qi, Y.; Chen, P.; Wang, T.; Hu, X.; Zhou, S. Macromol. Rapid Commun. 2006, 27, 356-360. (62) Rana, N.; Yau, S.-T. Nanotechnology 2004, 15, 275-278. (63) Rezende, C. A.; Lee, L.-T.; Galembeck, F. Langmuir 2007, 23, 28242828. (64) Dalnoki-Veress, K.; Nickel, B. G.; Dutcher, J. R. Phys. ReV. Lett. 1999, 82, 1486-1489. (65) Murray, C. A.; Kamp, S. W.; Thomas, J. M.; Dutcher, J. R. Phys. ReV. E: Stat., Nonlinear, Soft Matter Phys. 2004, 69, 61612/1-61612/11. (66) Baney, R. H.; Itoh, M.; Sakakibara, A.; Suzuki, T. Chem. ReV. 1995, 95, 1409-1430.

4678 Langmuir, Vol. 24, No. 9, 2008

compatibility with polymers, and the rigid inorganic core provides mechanical strength and oxidative stability. Over the past decade, POSS has been suggested for a variety of applications that include templates for catalysts,68 low-k dielectric materials,69 flame retardants,70 high-temperature lubricants,71 resist coatings for lithography,72 and space-survivable coatings.73 Furthermore, Langmuir-Blodgett (LB) films of TPP can adsorb organophosphonate based chemical warfare agent (CWA) simulants and even decompose chlorinated-organophosphate CWA simulants at elevated temperatures.74,75 The pattern formation described here may be one way to create high surface area substrates for CWA sensors. Experimental Procedures Materials. PS (number average molar mass, Mn ) 20.0 kg mol-1; polydispersity index, Mw/Mn ) 1.25) and TPP 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. Thermogravimetric analysis of PS and TPP (Supporting Information Figure S1) revealed that the TPP cage and PS began to degrade at ∼240 and ∼280 °C, respectively. Hence, the maximum annealing temperature for any experiment was selected to be ∼210 °C to avoid sample degradation. Cleaning Procedure for Silicon Substrates. The silicon wafers were boiled in a 1:1:5 (by volume) solution of ammonium hydroxide, hydrogen peroxide, and ultrapure water (Millipore, Milli-Q Gradient A-10, 18.2 MΩ, bulk Tg of PS) for 45 h under vacuum before the deposition of TPP layers to allow enough time for the removal of residual stresses that may have resulted from structural relaxations near and above Tg and slow residual solvent loss and (2) after the deposition of TPP layers via Y-type LB deposition, PS/TPP bilayers were annealed at 60 °C for 24 h under vacuum to remove any residual stresses that may be associated with the double layer structure of the TPP LB film. The temperature at which the loss of the double layer structure (Tdl) occurs for pure TPP LB films was ∼40 °C. Dewetting Studies. An optical microscope (OM) operating in the reflection mode (Axiotech Vario 100 HD, Carl Zeiss, Inc.) with a Linkam temperature controlled stage was used to study dewetting. All optical micrographs were captured at 20× magnification. The bilayers were annealed on the heating stage from 30 to 210 °C at a heating rate of 1 °C min-1 in air, and dewetting as a function of annealing temperature was viewed with Scion Image software. Furthermore, additional bilayer samples were annealed at 200 °C on the heating stage and viewed with Scion Image software to monitor dewetting as a function of annealing time. Atomic force microscopy (AFM) in the tapping mode (Dimension 3000 scope with a Nanoscope IIIa controller, Digital Instruments, set point of ca. 0.6) was used to study the surface morphology of the bilayers after annealing them at 200 °C under vacuum for variable times. The surface composition of the samples annealed under weak vacuum (pressure ∼80 mmHg) was determined by XPS (PHI 5400, PerkinElmer, Mg- KR radiation) at a takeoff angle of 45°. XPS for HF etched silicon substrates under air and vacuum annealing conditions revealed that ∼1 in 4 and ∼1 in 5 silicon atoms are oxidized to SiOx (x ) 0.5-2), respectively, during annealing at 200 °C for as little as 15 min and that this ratio does not increase further for annealing times up to 120 min. Hence, the elemental silicon peak, if present, is used to determine the amount of the silicon bound to oxygen, Si(O), arising from the oxidized substrate, and this value is subtracted from the Si(O) peak to determine the contribution from POSS alone. Complete details are provided in the Supporting Information (section VI). To explore the effect of residual stresses on the morphological evolution in PS/TPP bilayers, dewetting as a function of temperature (30-210 °C at 1 °C min-1) in PS/TPP bilayers after the removal of residual stresses was monitored using a reflected light OM. Furthermore, the morphological evolution of spin-coated PS films (Si//PS//air) and TPP LB films (Si//TPP//air) on hydrophobic silicon substrates was explored using an OM in the reflection mode upon annealing from 30 to 210 °C at a heating rate of 1 °C min-1 as control experiments. Selective Removal of TPP from Dewet PS/TPP Bilayers. The dewet PS/TPP bilayers (annealed at 200 °C for 5 min) were immersed in ethanol for 24 h at room temperature to selectively remove TPP. After TPP removal, the samples were rinsed with ethanol and Millipore water, and the films were dried in a vacuum oven at 60 °C before AFM analysis. (76) Karabiyik, U.; Esker, A. R. Proc. Annu. Meet. Adhes. Soc. 2005, 28, 253-254.

Dewetting PS/POSS Thin Film Bilayers

Langmuir, Vol. 24, No. 9, 2008 4679

Figure 2. Optical micrographs of the morphological evolution in a PS (∼30 nm)/TPP (∼25 nm) bilayer film as a function of temperature at a heating rate of 1 °C min-1: (a) 30, (b) 140, (c) 160, (d) 180, (e) 200, and (f) 210 °C. All images are 0.57 mm × 0.76 mm.

Results and Discussion Morphological Evolution as a Function of Temperature. The PS/TPP bilayers were heated from 30 to 210 °C at a heating rate of 1 °C min-1 to determine the temperature range over which dewetting occurs. A uniform film was observed by optical microscopy at the initial temperature of 30 °C (Figure 2a). The upper TPP layer of the PS/TPP bilayers underwent cracking at ∼128 °C (optical micrograph is not shown here), and the cracks were presumably due to tensile stresses within the TPP film. The weak optical contrast of the cracks that was observed in the PS/TPP bilayers for annealing temperatures up to 160 °C (Figure 2b,c) is consistent with cracking and dewetting of only the upper TPP layer from the underlying PS layer without exposing the silicon substrate. Further support for this conclusion will be presented next for temperature jump experiments at 200 °C. For annealing temperatures >160 °C, the optical contrast afforded by the morphological features was significantly enhanced (Figure 2d-f). This observation may suggest dewetting of both the upper TPP and the lower PS layers that led to the exposure of the

silicon substrate. Again, further evidence for this conclusion will come from temperature jump experiments at 200 °C. At this point, it is important to discuss several control experiments. As noted in the Experimental Procedures, thermogravimetric analysis revealed that all annealing temperatures used in this study were below the thermal decomposition temperatures for PS and TPP (Supporting Information Figure S1). Furthermore, HF etched silicon wafers were annealed in air and under vacuum. The initial elemental Si surface (with a presumable Si-H surface layer) was verified through the absence of a Si(O) peak in the XPS spectra of the as-prepared substrates. During annealing at 200 °C, an oxide layer rapidly forms, ∼15 min, and does not change significantly during annealing up to 120 min. XPS on these films revealed that ∼1 in 5 and ∼1 in 4 silicon atoms were oxidized for samples annealed in vacuum (pressure ∼80 mmHg) and air, respectively. The formation of the oxide layer has no effect on the stability of the PS layer. Spin-coated PS films (∼30 nm) on HF etched silicon substrates did not dewet in the absence of TPP nanoparticles upon annealing

4680 Langmuir, Vol. 24, No. 9, 2008

Paul et al.

Figure 3. Optical micrographs of the morphological evolution in a PS (∼30 nm)/TPP (∼25 nm) bilayer film annealed at 200 °C for (a) 1, (b) 15, (c) 60, and (d) 120 min. All images are 0.57 mm × 0.76 mm.

over the temperature range shown in Figure 2. This behavior may be attributed to the affinity of hydrophobic PS films to the mostly hydrophobic silicon substrates (water contact angles of 67.3 ( 1.2° before any annealing vs 57.9 ( 1.3° after annealing for 120 min at 200 °C, in contrast to the combined ammonia and piranha treatments of the native SiO2-coated Si wafer that yield surfaces that are completely wet by water, 0° contact angle). However, 30 layers (∼25 nm) of TPP directly deposited on HF etched silicon substrates cracked at lower temperatures, ∼112 °C (Supporting Information Figure S2). The crack formation of TPP during annealing may be attributed to tensile stresses within the film that arise from a mismatch of thermal expansion coefficients (R) between the TPP film [R ) (4.33 ( 0.02) × 10-4 K-1] and the silicon substrate (R ) 2.60 × 10-6 K-1).77 The details for the determination of thermal expansion coefficients of TPP and PS films are provided in the Supporting Information (Figure S3). It is evident from the behavior of TPP films on silicon substrates that the thermal expansion coefficient mismatch between TPP and silicon governs the initial morphological evolution in the PS/TPP bilayers. However, the crack formation in the PS/TPP bilayers ensues at slightly higher temperatures (∼128 °C) as compared to TPP films directly deposited onto HF etched silicon substrates (∼112 °C). This phenomenon suggests that the PS layer [R ) (5.87 ( 0.06) × 10-4 K-1, T > Tg] helps to dissipate some stress but cannot completely screen the upper TPP layer from the hydrophobic silicon substrate. To ascertain the role residual stresses play on the morphological features of Figure 2, special PS/TPP bilayers were prepared. After spin-coating, the PS films were annealed under vacuum at 120 °C (>bulk Tg of PS) for 45 h. Next, the TPP layer was added as a Y-type LB multilayer film. Finally, the PS/TPP bilayers were annealed at 60 °C (>Tdl of TPP) for 24 h under vacuum. This process should allow for structural relaxations and complete (77) Gadre, K. S.; Alford, T. L. Thin Solid Films 2001, 394, 125-130.

removal of residual solvent from the bilayers. The morphological evolution of dewetting in these bilayers (Supporting Information Figure S4) is essentially identical to the behavior observed for PS/TPP bilayers in which the PS films were annealed at 95 °C for 2 h with no further annealing after the deposition of the TPP layers. On the basis of Figure S4, we conclude that residual stresses arising from spin-coating and Y-type LB deposition do not play a significant role in the dewetting exhibited by the PS/TPP bilayer systems. This observation is consistent with previous studies that suggest film shrinkage and that the generation of residual stresses associated with structural relaxations near and above Tg and slow residual solvent loss are not a significant problem for spin-coated PS films.78-80 Morphological Evolution as a Function of Annealing Time at 200 °C. On the basis of the morphological regimes observed in Figure 2, it was desirable to find a suitable annealing temperature for temperature jump experiments. A temperature of 200 °C was chosen as it afforded the ability to observe both the early and the late stage annealed structures. Figure 3 shows optical micrographs of PS/TPP bilayers annealed at 200 °C as a function of annealing time. The upper TPP layer undergoes instantaneous cracking (Figure 3a). The crack formation in the TPP layer is most likely caused by stresses generated in the TPP layer that arise from a 2 orders of magnitude thermal expansion coefficient difference between the TPP layer and the hydrophobic silicon substrate. We propose that these cracks in the upper TPP layer act as nucleation sites for the dewetting and aggregation of the TPP layer and dewetting of the PS layer. The optical contrast of the morphological features is enhanced with increasing annealing times (Figure 3b,c). The strong optical contrast suggests (78) Kanaya, T.; Miyazaki, T.; Watanabe, J.; Nishida, K.; Yamano, H.; Tasaki, S.; Bucknall, D. B. Polymer 2003, 44, 3769-3773. (79) Mukherjee, M.; Bhattacharya, M.; Sanyal, M. K.; Geue, T.; Grenzer, J.; Pietsch, U. Phys. ReV. E: Stat., Nonlinear, Soft Matter Phys. 2002, 66, 61801/ 1-61801/4. (80) Reiter, G. Macromolecules 1994, 27, 3046-3052.

Dewetting PS/POSS Thin Film Bilayers

that dewetting of both the TPP and the PS layers (to be discussed shortly) results in the exposure of the silicon substrate with longer annealing times. Furthermore, annealing the bilayers for >90 min results in complete dewetting of both the TPP and the PS layers (Figure 3d) with the formation of TPP encapsulated PS droplets (to be discussed shortly). With the exception of the droplet morphology (Figure 3d), the morphological features observed for PS/TPP bilayers upon annealing at 200 °C are consistent with temperature ramp experiments (Figure 2). The droplet morphology can also be seen in temperature ramp experiments if the films are annealed for longer times at higher temperatures. It is reasonable to assume that the morphologies and time scales for their evolution are dependent upon the transport properties of the system (viscosities, diffusion coefficients, etc.) and hence on temperature. To test this hypothesis, the morphological evolution of PS/TPP bilayers was also studied at temperatures Tg] cannot completely screen out the presence of the silicon substrate from the upper TPP layer. With increasing annealing times, it is observed that TPP aggregates rapidly along the cracks (Figure 5b). This observation most likely suggests that the cracks in the upper TPP layer act as nucleation sites for the dewetting and aggregation of the TPP layer and subsequent dewetting of the lower PS layer. The formation of TPP aggregates is consistent with strong interparticle interactions.51,52 Moreover, Figure 5a,b appears to support the conclusion that dewetting initiates and grows outward from the cracks. Root-mean-square (rms) surface roughness values were calculated for the AFM images shown in Figure 5a,b and can provide further insight into the observed dewetting behavior in PS/TPP bilayers. The rms surface roughness of the uniform areas (representative black box in Figure 5a) observed for PS/TPP bilayers annealed for