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Langmuir 2008, 24, 8898-8903
Periodic Porous Stripe Patterning in a Polymer Blend Film Induced by Phase Separation during Spin-Casting Jae-Kyung Kim, Kentaro Taki, Shinsuke Nagamine, and Masahiro Ohshima* Department of Chemical Engineering, Kyoto UniVersity, Kyoto 615-8510, Japan ReceiVed January 6, 2008. ReVised Manuscript ReceiVed April 13, 2008 A periodic striping pattern with microscale pore size is observed on the surface of thin films prepared by spin-casting from a polystyrene (PS) and polyethylene glycol (PEG) blend solution. The pattern is created by the convection generated by thermal gradients in the solution between the substrate and film solution during solvent evaporation, the radial flow of the spin-coated solution, and the primary and secondary phase separation of the PS and PEG solutions. The formation mechanism of the periodic porous stripe pattern is discussed, wherein the effects of the polymer blend weight ratio, polymer concentration, and drying rate on the formation of the periodic porous striping pattern are investigated using scanning electron and atomic force microscopy.
Introduction Over the past decade, polymer blended films have attracted substantial interest for applications in optical devices1–3 and paints.4 Polymer blended films are especially applicable in modern technological applications.5 While there are many polymer blend film studies focusing on phase-separating phenomena in the molten state,6–8 there are few investigations for polymer solutions, despite the fact that in practice polymer films are prepared by solvent-casting.9 Due to the intrinsic immiscibility of most polymers, polymer blends often phase-separate during the solventcasting or during solvent evaporation.9 As such, the immiscible polymer blends generally exhibit phase-separated morphologies with domains dispersed in the polymer matrix.10 For polymer blend films obtained by solvent-casting, the control of surface morphology is extremely relevant to many potential applications.11–15 The morphological evolution of polymer blend films during spin-casting has been investigated extensively.16–23 A hill-and* To whom correspondence should be addressed. Phone: +81-75-3832666. Fax: +81-75-383-2646. E-mail:
[email protected]. (1) Burn, P. L.; Holmes, A. B.; Kraft, A.; Bradley, D. D. C.; Brawn, A. R.; Friend, R. H.; Gymer, R. W. Nature 1992, 356, 47. (2) Healey, B. G.; Foran, S. E.; Walt, D. R. Science 1995, 269, 1078. (3) Xia, Y.; Kim, E.; Zhao, X. M.; Rogers, J. A.; Prentiss, M.; Whitesides, G. M. Science 1996, 273, 347. (4) Kikuchi, T.; Kudo, M.; Jing, C.; Tsukada, T.; Hozawa, M. Langmuir 2004, 20, 1234–1238. (5) Walheim, S.; Ramstein, M.; Steiner, U. Langmuir 1999, 15, 4828. (6) Lee, J. K.; Han, C. D. Polymer 1999, 40, 6277. (7) Lee, C. F. Polymer 2000, 41, 1337. (8) Fekete, E.; Fo¨ldes, E.; Puka´nszky, B. Eur. Polym. J. 2005, 41, 727. (9) Walheim, S.; Boltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macromolecules 1997, 30, 4995. (10) Chang, L. L.; Woo, E. M. Polymer 2003, 44, 1711. (11) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (12) Rockel, H.; Huber, J.; Gleiter, R.; Schuhmann, W. AdV. Mater. 1994, 6, 568. (13) Netz, R. R.; Andelman, D. Phys. ReV. E 1997, 55, 687. (14) Onda, T.; Shibuichi, S.; Satoh, N. Langmuir 1996, 12, 2125. (15) Service, R. F. Science 1997, 278, 383. (16) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1996, 29, 3232. (17) Kang, H.; Lee, S. H.; Kim, S.; Char, K. Macromolecules 2003, 36, 8579. (18) Prosycevas, I.; Tamulevicius, S.; Guobiene, A. Thin Solid Films 2004, 453-454, 304. (19) Wang, P.; Koberstein, J. T. Macromolecules 2004, 37, 5671. (20) Heriot, S.; Jones, R. Nat. Mater. 2005, 4, 782. (21) Boltau, M.; Walheim, S.; Mlynek, J.; Krausch, G.; Steiner, U. Nature 1998, 391, 877. (22) Li, X.; Xing, R.; Zhang, Y.; Han, Y.; An, L. Polymer 2004, 45, 1637. (23) Geoghegan, M.; Krausch, G. Prog. Polym. Sci. 2003, 28, 261.
valley periodic striping pattern is often observed on the surface of the spin-cast films in the literature.24–26 It is concluded that this phenomenon is induced by capillary instabilities generated by both thermal and compositional gradients, in addition to the radial flow of a polymer and volatile solvent mixture.26–28 Capillary instability is collectively known as Be´nard-Marangoni convection,29 and this convection is governed by a balance of the surface-tension-driven forces that are easily generated by solvent evaporation during spin-casting.28 Similar periodic striping patterns are observed in spin-cast thin films of singleor multiple-component polymer systems. Liu et al.30 developed a thin film with a self-assembled periodic pattern of polyetherimide and polycaprolactone via spin-casting from a dichloromethanepolymer solution. Wu et al.31 showed the formation of parallel striped regions produced by spin-casting films from alternating polystyrene (PS) and poly(vinylpyrrolidone) (PVP). They found that the ratio of the square of the film thickness to the viscosity is loosely related to the Marangoni number and derived the conclusion that there exists an optimal concentration at which the striping patterns are well-developed. Muller-Buschbaum et al.32 showed the formation of striation patterns over a spin-cast PS film and correlated the surface morphology to the vapor pressures of the solvents. The resulting films are homogeneous when using solvents with low or high vapor pressure. However, when solvents with intermediate vapor pressures are used, striation patterns appear on the films. In our previous paper,33 we reported the formation of porous structures in a phase-separating polymeric blend film of polystyrene and polyethylene glycol 200 (PS/PEG 200) during solvent-casting. We showed that only circular pores without any periodic striping pattern are uniformly created on the surface of the film. To explain the formation of the porous structure, we (24) Du, X. M.; Orignac, X.; Almeida, R. M. J. Am. Ceram. Soc. 1995, 78, 2254. (25) Weh, L. Mater. Sci. Eng., C 1999, 8, 463. (26) Haas, D. E.; Birnie, D. P.; Zecchino, M. J.; Figueroa, J. T. J. Mater. Sci. Lett. 2001, 20, 1763. (27) Birnie, D. P. J. Mater. Res. 2001, 16, 1145. (28) Haas, D. E.; Birnie, D. P. J. Mater. Sci. 2002, 37, 2109. (29) Normand, C.; Pomeau, Y.; Velarde, M. G. ReV. Mod. Phys. 1977, 49, 581. (30) Liu, T.; Ozisik, R.; Siegel, R. W. Thin Solid Films 2007, 515, 2965. (31) Wu, K. H.; Lu, S. Y.; Chen, H. L. Langmuir 2006, 22, 8029. (32) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Wolkenhauer, M.; Kraus, J.; Stamm, M.; Smilgies, D.; Petry, W. Macromolecules 2001, 34, 1369. (33) Kim, J. K.; Taki, K.; Ohshima, M. Langmuir 2007, 23, 12397.
10.1021/la8000398 CCC: $40.75 2008 American Chemical Society Published on Web 07/19/2008
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Table 1. Characteristics of the Polymers and Solvents jw jn j w/M jn M M M δa (MPa0.5)
material
PEG 200 PS benzene toluene chloroform a
280 222 000
260 99 000
1.07 2.24
26.134 18.635 18.835 18.235 19.035
Solubility parameter for the polymer and solvent.
suggested a two-step phase separation process, which consists of a primary phase separation into PEG-rich and PEG-poor phases and a secondary phase separation into solvent-rich and PEG-rich domains within the PEG droplets. In this study, a periodic porous striping pattern was prepared by controlling the convection in the film during the spin-casting of the PS and PEG blended solution. The periodic porous striping pattern was created in PS/PEG/solvent systems from the combination of solvent evaporation-induced convection, radial flow of the spinning solution, and two-step phase separation. The phase separation of polymer blends in spin-cast films is complex, and the formation of the final surface morphology is affected by the polymer composition, polymer molecular weight, polymer concentration, drying rate, substrate, and spin-casting process parameters.32 The effects of the polymer blend weight ratio, concentration of the polymer, and drying rate on the formation of periodic porous striping were investigated using scanning electron (SEM) and atomic force (AFM) microscopy.
Figure 1. SEM micrographs of a PS/PEG 200 (70/30, w/w) film made from a 90 wt % benzene solution: (a) edge, (b) center. Inset: SEM micrograph magnifying a porous stripe.
Experimental Section Materials. PS and PEG 200 were purchased from Aldrich Chemical Co. and Wako Pure Chemical Industries Ltd., Japan, j w), the number respectively. The weight average molecular weight (M j n), and the polydispersity (M j w/M j n) of average molecular weight (M the polymers were measured by gel permeation chromatography (Shimadzu model DGU-20A3; column, Shodex GPC KF-806L; eluent, chloroform). The retention time and molecular weight were calibrated using PS standards. Benzene (dehydrated), toluene (dehydrated), and chloroform (dehydrated) (Wako Pure Chemical Industries Ltd., purity 99.5%) were used as solvents without further purification. The characteristics of the materials used in this study are summarized in Table 1. Film Preparation. PS and PEG 200 were dissolved and mixed in a solvent by a magnetic stirrer until the blended polymer solution became a single phase. The solutions were prepared with polymer concentrations of 1 and 10 wt % and with blend ratios of PS to PEG 200 of 90/10, 80/20, and 70/30. Thin films were obtained by dropping the solution onto a substrate and sequentially performing the spincasting. Glass slides (2.5 × 2.5 cm) were used as the substrate in the spin-casting process. The volume of solution placed on the substrate was 100 µL in all experiments. The spin-casting was performed at room temperature, at a casting speed of 3000 rpm, and with 10 s of processing time. The spin-casting equipment was a 1H-D7 spin coater manufactured by Mikasa, Japan. Observation of the Film Morphology. The surface morphology of the resulting films was observed by SEM (Tiny-SEM 1540, Technex Laboratory Co. Ltd.). The dried film was coated with a gold-palladium film and placed on the SEM stage under a vacuum atmosphere. AFM (SPM-9500J3, Shimadzu, Japan) was also used to observe the surface morphology at the nanoscale size. AFM images were taken in tapping mode at room temperature. The surface morphology in low magnification was observed by a digital camera connected by an R-step surface profiler. In addition, the surface height profile was measured by confocal laser microscopy (CLM) (Keyence, VK-8510) operating at 685.0 nm. (34) Kia, S.; Samuel, H. Y. AAPS PharmSciTech. 2006, 7, E26. (35) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; WileyInterscience: NewYork, 1989.
Figure 2. Surface morphology (a) and surface height profile (b) of a PS/PEG 200 (70/30, w/w) film made from a 90 wt % benzene solution.
Results and Discussion Resulting Film. A single-phase solution of PS and PEG 200 at a ratio of 70/30 in a 90 wt % benzene solvent was cast on a glass slide. SEM micrographs were taken both at the center and at the edge of the film to show the overall surface morphology. Figure 1 shows SEM micrographs of the surface of the resulting PS/PEG films. As shown in Figure 1a, a periodic stripe pattern is formed. The periodically repeating stripes are composed of fine pores in single micrometer sizes (inset of Figure 1), and the distance between two stripes is about 20 µm. However, as shown in Figure 1b, there exists a porous, cellular pattern at the center of the film. At the center, the pore distribution is random. Figure 2a shows the image of the periodic porous stripe pattern taken by CLM. Figure 2b shows the height profiles of the film surface between the two points indicated by “A” and “B” in Figure 2a. In Figure 2, the surface morphology consists of hills (indicated by “a”) and valleys (indicated as “b”) with some periodicity. Fine pores, indicated by small fluctuations, are located on the hills of the film.
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Figure 3. Schematic of the formation mechanism for the periodic porous striping pattern.
Figure 4. Photomicrographs of the surface morphology of a binary PS/benzene film: (a) center, (b) edge.
Formation Mechanism of the Periodic Porous Striping Pattern. We speculate that the formation of the periodic porous striping patterns consists of two steps as illustrated in Figure 3. The first step is the formation of the periodic hill-and-valley pattern on the film surface: At the initial stage of the spin-casting, the solution flows onto the substrate in a radial direction due to centrifugal forces. Solvent evaporation occurs from the surface and induces a temperature gradient between the surface and the bottom of the cast solution (Figure 3, panel 1). Figure 4 shows the surface morphology of a binary PS/benzene film. As shown in Figure 4, a cellular pattern is formed at the center of the film and hill-and-valley stripes circumferentially propagate. Such cellular and striping patterns are reported by several researchers, and their results are similar to the patterns observed in Figure 4.25–27,30,31 The formation of the hill-andvalley pattern results from regular surface undulations created by convection rolls. The convection rolls are established by buoyancy and thermocapillary effects when a temperature gradient exists across a thin fluid layer.26,36,37 For the convection-induced undulations, the surface tension at the hills is higher than that of the valleys. Higher surface tension in the hill pulls material up further from neighboring regions where the surface tension is lower (Figure 3, panel 2). This formation mechanism develops a hill-and-valley morphology on the casting film.27,31 Simultaneously, hill-and-valley patterns are stretched in the radial direction by the spinning centrifugal force, and the stretching effect creates periodic striations. This formation mechanism is originally proposed by Haas.26 We speculate that the same mechanism occurs in the first step of our PS/PEG system. Figure 5 shows the surface morphology of a binary PS/toluene system. In contrast to the PS/benzene system in Figure 6, no striping pattern is observed at the center and edge of the resulting film. As previously explained, the convection and the periodic striping (36) Nield, D. A. J. Fluid Mech. 1964, 19, 341. (37) Block, M. J. Nature 1956, 178, 650.
Figure 5. Photomicrographs of the surface morphology of a binary PS/toluene film: (a) center, (b) edge.
Figure 6. SEM micrographs of a PS/benzene film made from a 90 wt % benzene solution.
pattern occur when a larger temperature gradient exists. The enthalpy of vaporization and the evaporation rate of the solvent determine the degree of temperature gradient between the top layer and the layer below it.38 Because toluene shows a lower enthalpy of vaporization and lower drying rate than benzene,39,40 it can be said that a higher temperature gradient was established in benzene solution and it enhanced the convection and more easily leads to the creation of a periodic striping pattern. In the second step, solvent evaporation induces phase separation twice in the PS/PEG/solvent ternary solution.33 Since the solubilities of PS and PEG in benzene are different (see Table 1), PEG, which is poorly soluble in benzene, precipitates from the solution and forms PEG-rich and PEG-poor domains. This is the first phase separation. Due to solvent-evaporation-induced convection, as well as a faster drying rate on the hill, phaseseparated PEG-rich domains (droplets) flow preferentially to the (38) Cui, L.; Wang, H.; Ding, Y.; Han, Y. Polymer 2004, 45, 8139. (39) Limaye, A. V.; Narhe, R. D.; Dhote, A. M.; Ogale, S. B. Phys. ReV. Lett. 1996, 76, 3762. (40) Ian, M. Smallwood SolVent RecoVery Handbook; Hodder & Stoughton: London, 1993.
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Figure 7. Micrographs of a PS/PEG 200 film made from a benzene solution: 10 wt % concentration of polymer for the upper pictures (SEM images) and 1 wt % concentration of polymer for the lower pictures (AFM height image, scan area 7.5 µm × 7.5 µm).
hills, where the evaporation rate is faster (Figure 3, panel 3). As drying proceeds, the viscosity of the polymer solution increases, and the convection and subsequent droplet movement and coalescence are stopped. Secondary phase separation then occurs in the PEG droplet, which forms solvent-rich and PEG-rich domains. Further solvent evaporation creates a cavity in the PEG droplets (Figure 3, panel 4). The interfacial energy between immiscible polymers would play an important role in phase separation of the thin films since the constituent polymers flow to the preferable regions to achieve the lowest total interfacial energy of the system as the phase separation proceeds. Wu et al.31 explained that each constituent of the polymer blend solution takes a preferential location during spin-casting so that the total interfacial energy of the system becomes lower: For the case of PS and PVP polymer blend solutions, due to the surface tension of PS (42 mJ/m2) and PVP (56 mJ/m2), PS tended to flow to the hill region where surface tension was high and PVP tended to flow to the valleys where the surface tension was lower. However, this is not our case. The surface tension of PEG was 51 mJ/m2,41 and that of PS was 45.5 mJ/m2.42 If the system were to follow Wu’s rule, PEG would flow to the valley and PS to the hill. However, our experiments showed that the PEG-rich region was found in the hill region and PS was in the valley as shown in Figure 3. Thus, it could be concluded that, in our system, the difference in surface tension between PS and PEG did not largely affect largely the formation of PEG- and PS-rich regions in the spin-cast film. Figure 6 shows the SEM micrograph of a PS/benzene film surface. Here, a porous structure is not formed on the film surface. In our previous study,33 we showed that the porous structure is prepared by drying a solution of PS, PEG 200, and toluene. The pores are created by two-step phase separation via solvent evaporation. Once a primary phase separation into PS-rich and PEG-rich phases is completed, a secondary phase separation occurs in the PEG droplet and forms solvent-rich and PEG-rich domains with a highly ordered structure. A cavity is created at each PEG droplet by further solvent evaporation. The periodic pore striping structure observed in spin-casting is created by a (41) Zhou, G.; Chen, X.; Smid, J. AdV. Chem. Ser. 1996, 248, 31. (42) Zhao, J.; Jiang, S.; Ji, X.; An, L.; Jiang, B. Polymer 2005, 46, 6513.
Table 2. Thickness of the Polymer Films Prepared from a PS/PEG 200/Benzene Solution PS/PEG blend ratio (wt %) polymer concn (wt %) thicknessa (nm) 70/30 80/20 90/10 a
10 1 10 1 10 1
180 ( 50 25 ( 15 310 ( 70 33 ( 20 410 ( 60 38 ( 22
Errors were calculated by the standard deviation.
combination of convection-induced hill-and-valley formation, stripe formation due to radial flow, and primary/secondary phase separations in PS/PEG/solvent systems. Effect of the Polymer Composition. The effects of the polymer composition and concentration on the formation of periodic porous striping patterns were investigated by preparing films from solutions with weight ratios of PS to PEG 200 of 90/10, 80/20, and 70/30 and polymer concentrations of 1 and 10 wt %. Figure 7 shows SEM micrographs and AFM images of the surfaces of the resulting films. The AFM surface images are for 1 wt % polymer films. As shown in the upper pictures of Figure 7, pores are aligned along a line forming a striping pattern. The number densities of pores on the line in the different composition films are 0.041, 0.078, and 0.103 µm-2 for 90/10, 80/20, and 70/30 wt %, respectively. The width of the line decreases with decreasing PEG weight fraction. The line widths of the differently composed films are 20, 14, and 8 µm for 70/30, 80/20, and 90/10 wt %, respectively. Similar phenomena are observed by Wu et al.31 They report that the nonporous PVP stripe becomes thinner with decreasing PVP weight fraction in PS/PVP/chloroform systems. The increase in PEG weight fraction lowered the viscosity of the total system owing to the low molecular weight of PEG. The lower viscosity enhanced convection and made phase-separated PEG-rich domains (droplets) flow to the hills. The increase in PEG weight fraction corresponded to the increase in the amount of PEG-rich domains in the hill. Therefore, as shown in Figure 7, the width of the periodic porous striping pattern increased as the PEG weight fraction increased. The morphology changes drastically when the polymer concentration decreases from 10 to 1 wt %. The periodic striping
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Figure 8. SEM micrographs of the surface morphology of the films made from (a) toluene, (b) benzene, and (c) chloroform solutions and (d) a benzene solution under a saturated benzene atmosphere. Table 3. Parameters39,44 and Estimates of the Temperature Gradient parameter
toluene
benzene
chloroform
drying ratea, r [g/(cm2 s)] enthalpy of vaporization, ∆Hvap (kJ/g) heat flux, q ) r∆Hvap [kJ/(cm2 s)] thermal conductivity, k [kJ/(cm s °C)] initial thickness of the solution,bn (cm) estimated temp gradient, ∆T ) (qn)/k (°C)
0.73 × 10-4 0.413 3.01 × 10-5 1.338 × 10-6 0.031847 0.716
2.24 × 10-4 0.433 9.70 × 10-5 1.456 × 10-6 0.031847 2.122
4.63 × 10-4 0.262 12.1 × 10-5 1.17 × 10-6 0.031847 3.294
a Measurement of the drying rate during the spin-casting process could not be conducted. The drying rate for the specialized PS/PEG/solvent drying process was used. b The film thickness was estimated from the amount of injected solution. The volume of the solution was 100 µL in the nominal case.
Figure 9. Cloud point temperature of three different PS/PEG/solvent systems vs the polymer concentration in solution with a weight ratio of PS to PEG of 70/30 wt %: toluene (b), benzene (O), and chloroform (9).
pattern disappears in films with different PS/PEG ratios. Pores appear uniformly on the entire surface of the films as shown in the lower pictures of Figure 7. The pore size decreases from micrometer size to submicrometer or nanoscale order. The polymer concentration in solution directly affects the thickness of the film as well. When the film is thin, the convection roll cannot be established since a large and stable temperature gradient is poorly established in the thin film. The film thickness was measured using CLM and AFM. The results are shown in Table 2. The film thickness decreases with decreasing polymer
concentration. The decrease in film thickness promptly establishes a uniform temperature, which dissipates the capillary force at the free surface and suppresses liquid convection.43 Therefore, the convection-induced periodic striping pattern disappears. Effect of the Drying Rate. The effect of the drying rate on the formation of the periodic porous striping pattern was investigated by using different solvents. Toluene, benzene, and chloroform, all with different vapor pressures, were used as the solvent for the PS/PEG mixture. The measurement of the drying rate was inaccurate during the spin-casting process. Therefore, it was measured using a specialized solution drying process. A solution was cast by a microinjector onto an aluminum Petri dish, 2 cm in radius and 0.5 cm in depth. The weight change during drying was measured with a balance at room temperature. The initial drying rate was calculated from the slope of the weight change per unit area of the solution. As predicted, the drying rates are 7.30 × 10-5 g/cm2 · s for PS/PEG 200/toluene systems, 2.24 × 10-4 g/cm2 · s for benzene systems, and 4.63 × 10-4 g/cm2 · s for chloroform systems. Figure 8 shows SEM micrographs of PS/PEG 200 films made from the aforementioned different solvent systems. Figure 8 shows a clear difference in the surface morphology among the three films. In the case of toluene systems (Figure 8a), where the drying rate is the slowest, the periodic striping pattern does not appear on the surface and pores are uniformly scattered on the surface of the film. In contrast, parts b and c of Figure 8 show porous striping patterns are wellorganized in benzene and chloroform systems. A temperature gradient between the surface and bottom of the film creates convection and liquid motion within the solution. (43) Yamamura, M.; Horiuchi, K.; Kajiwara, T.; Adachi, K. AIChE J. 2002, 48, 2711.
Periodic Stripe Patterning in a Polymer Blend Film
Convection develops the hill-and-valley profile on the film surface and eventually creates the striping pattern of pores via the aforementioned mechanism. Because of the difficulty in measuring the temperature gradient in the thin transparent solution, the temperature gradients were roughly estimated from the drying rate, enthalpy of vaporization, thermal conductivity, and film thickness with the assumptions that a linear temperature gradient was established and the substrate temperature was kept constant. Table 3 shows the estimated temperature gradients in ternary systems with different solvents. Measurement of the drying rate during the spin-casting process could not be conducted. Thus, the drying rate for the specialized solution drying process was used for estimation of temperature gradients. Since the actual drying rate during spin-casting would be higher, the actual temperature gradient would be larger. The degree of temperature gradient was mainly determined by the drying rate due to the small difference in the enthalpy of vaporization and thermal conductivity of the solvents. The temperature gradient in the toluene system is the lowest, and thus, the convection is suppressed in the system. As a result, any periodic striping pattern does not appear in the polymer/toluene solution. Table 3 also shows the larger temperature gradients in the benzene and chloroform solvent systems than the toluene system. It can be said that higher drying rates cool the liquid surface more due to latent heat removal. The larger temperature difference enhances the hill-and-valley formation and leads to phase separation of PEG on the hills. To confirm the impact of the drying rate on the film morphology, an additional drying experiment was performed under air for a PS/PEG200/benzene system. The drying rate was controlled by preparing a saturated benzene atmosphere for drying, meaning the drying was conducted in a saturated benzene atmosphere, which reduced the driving force of solvent evaporation. The result is shown in Figure 8d. Due to a slower drying rate and subsequent smaller temperature gradient in the thickness direction, pores are scattered and no periodic porous striping pattern is observed. Formation of the Porous Structure. When the viscosity of the solution is high, convection is suppressed and the deformation of hill-and-valley structures to the striping pattern is also delayed. Furthermore, the size of the pores created by the phase separation of PEG from the solution is reduced when the viscosity of the solution at the onset of PEG precipitation is increased.33 To investigate the solution viscosity at the onset of PEG separation, cloud point temperatures of the polymer solution with different PEG concentrations and different solvents were measured by monitoring the intensity of the laser beam shining through the solution. The detail of this experimental method is described in the literature.33 The resulting cloud point temperatures are plotted in Figure 9. Figure 9 shows the composition in the solution at the cloud point at room temperature (25 °C) estimated for the three different polymer-solvent systems. The PEG concentration (44) Xu, S.; Li, M.; Mitov, Z.; Kumacheva, E. Prog. Org. Coat. 2003, 48, 227.
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in solutions at the onset of the cloud point, i.e., PEG phase separations, at room temperature are 6.06 wt % for the PS/PEG200/toluene system, 8.70 wt % for the PS/PEG200/ benzene system, and 9.60 wt % for the PS/PEG200/chloroform system. In other words, the concentration of PS at the onset is 14.14 wt % for the PS/PEG200/toluene system, 20.30 wt % for the PS/PEG200/benzene system, and 22.40 wt % for the PS/ PEG200/chloroform system. The viscosity of the solution increases as the PS concentration in the solution increases. The high viscosity of the solution at the onset of phase separation of PEG suppressed the convection, and there is not sufficient time to form PEG-rich domains in the periodic stripe pattern. It also suppressed coalescence of the PEG-rich domains after the phase separation. The lower drying rate in the valleys made the viscosity at the valley lower than that at the hill. Due to the lower viscosity, droplet coalescence increased at the valley and the droplet size became larger. As drying proceeded, the viscosity of the polymer solution increased. The convection stopped before all PEG-rich domains moved to the hills when the viscosity at the onset was high. Therefore, PEG-rich domains formed not only at the hill but also at the valley as shown in Figure 1a. In the case of chloroform where the temperature gradient in the solution between the substrate and film surface is highest as estimated in Table 3, the convection could be enhanced. However, as shown in Figure 9, phase separation occurred at high concentration due to the higher solubility of PEG in chloroform (see Table 1). Thus, the viscosity at the onset of PEG phase separation became high. As a result, the striping pattern was established by small pores and pores connecting the stripe lines as illustrated in Figure 8c.
Conclusion A polymeric film having a periodic porous striping pattern surface morphology was prepared by spin-casting via phase separation of ternary polymer-solvent systems. As the PEG weight fraction increases in PS/PEG200, the width of the periodic porous striping pattern increases due to the low viscosity of the solution and the subsequent enhancement of convection. In addition, it is found that the periodic porous striping pattern strongly depends on the thickness of the film and drying rate. Changing the film thickness and drying rare affects the establishment of the convection. The formation mechanism of the porous striping pattern is proposed as a two-step process. In the first step, a hill-and-valley undulation is created on the surface by the combined effects of solvent-evaporation-induced convection and radial flow during spinning. In the second step, primary phase-separated PEG-rich domains (droplets) are pulled and ordered on the hill, which is a region of high surface tension and high drying rate. Pores in the PEG-rich domain are created by secondary phase separation between PEG-rich and solvent-rich phases. LA8000398
