Breath Figure Patterns Prepared by Spin Coating in a Dry

May 18, 2004 - We introduce a novel method for fabricating breath figure patterns on a homopolymer film by spin coating of polymer solutions with vari...
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Breath Figure Patterns Prepared by Spin Coating in a Dry Environment Min Soo Park and Jin Kon Kim* Department of Chemical Engineering and Polymer Research Institute, Electric and Computer Engineering Division, Pohang University of Science and Technology, Kyungbuk 790-784, Korea Received October 13, 2003. In Final Form: April 1, 2004 We introduce a novel method for fabricating breath figure patterns on a homopolymer film by spin coating of polymer solutions with various solvents. The homopolymers employed in this study were cellulose acetate butyrate, monocarboxylated end-functional polystyrene, and poly(methyl methacrylate). Breath figure patterns were generated even when a water-miscible solvent such as tetrahydrofuran (THF) was used as a solvent. We even succeeded in generating breath figure patterns by spin coating even under a dry environment (relative humidity less than 30%), when water was directly added into THF solution. With the combination of the spin coating method, pores with a few hundred nanometers to several micrometers have been generated. We found that the pore size becomes larger with increasing water content in THF solution and decreasing rotating speed. This is equivalent to increasing humidity and decreasing evaporation speed, respectively, in the conventional method, which is direct solvent evaporation under a humid environment. Thus, compared with the conventional method for making breath figure patterns, this method would be very convenient for fabricating large-scale films with various pore sizes.

I. Introduction Porous polymer films have long been studied because of their possible use in catalysts,1 antireflection coatings,2-4 template for inorganic growth masks,5 membranes,6 and cell culture media.7-13 Among many methods14-16 for fabricating porous polymer film, the breath figure method7-13,17-26 has attracted much attention due to the simple process for making pores. It employs solvent* To whom correspondence should be addressed (FAX: +8254-279-8298; E-mail: [email protected]). (1) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321-323. (2) Walheim, S.; Schaffer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520-522. (3) Hiller, J. A.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59-63. (4) Ibn-Elhaj, M.; Schadt, M. Nature 2001, 410, 796-799. (5) Li, R. R.; Dapkus, P. D.; Thompson, M. E.; Jeong, W. G.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Appl. Phys. Lett. 2000, 76, 1689-1691. (6) Yang, G.; Xiong, X.; Zhang, L. J. Membr. Sci. 2002, 201, 161173. (7) Maruyama, N.; Koito, T.; Nishida, J.; Sawadaishi, T.; Cieren, X.; Ijiro, K.; Karthaus, O.; Shimomura, M. Thin Solid Films 1998, 327329, 854-856. (8) Nishikawa, T.; Nishida, J.; Ookura, R.; Nishimura, S. I.; Wada, S.; Karino, T.; Shimomura, M. Mater. Sci. Eng. C 1999, 10, 141-146. (9) Nishikawa, T.; Nishida, J.; Ookura, R.; Nishimura, S. I.; Wada, S.; Karino, T.; Shimomura, M. Mater. Sci. Eng. C 1999, 8-9, 495-500. (10) Karthaus, O.; Maruyama, N.; Cieren, X.; Shimomura, M.; Hasegawa, H.; Hashimoto, T. Langmuir 2000, 16, 6071-6076. (11) Nishikawa, T.; Ookura, R.; Nishida, J.; Arai, K.; Hayashi, J.; Kurono, N.; Sawadaishi, T.; Hara, M.; Shimomura, M. Langmuir 2002, 18, 5734-5740. (12) Nishikawa, T.; Nonomura, M.; Arai, K.; Hayashi, J.; Sawadaishi, T.; Nishiura, Y.; Hara, M.; Shimomura, M. Langmuir 2003, 19, 61936201. (13) Yabu, H.; Tanaka, M.; Ijiro, K.; Shimomura, M. Langmuir 2003, 19, 6297-6300. (14) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372-375. (15) Yi, D. K.; Kim, D. Y. Nano Lett. 2003, 3, 207-211. (16) Mulder, M. In Basic Principles of Membrane Technology; Kluwer Academic Publishers: The Netherlands, 1996; Chapter 3. (17) Gau, H.; Herminghaus, S. Phys. Rev. Lett. 2000, 84, 4156-4159. (18) Beysens, D.; Knobler, C. M. Phys. Rev. Lett. 1986, 57, 14331436. (19) Family, F.; Meakin, P. Phys. Rev. Lett. 1988, 61, 428-431. (20) Limaye, A. V.; Narhe, R. D.; Dhote, A. M.; Ogale, S. B. Phys. Rev. Lett. 1996, 76, 3762-3765.

evaporative cooling on the surface of a solution under humid conditions on which water vapor is condensed and water droplets are formed at the interface of the solution and water vapor. Then, water droplets interact with one another and are finally ordered in a hexagonal lattice. After complete evaporation of the solvent and water, traces of water droplets remain in the polymer film and become pores with a honeycomb structure. The key step to preparing ordered porous structure by breath figure on a polymer film involves stabilizing condensed water droplets on the volatile solution. It is known that condensed water droplets can be stabilized either by a complexstructured polymer or by homopolymers with functional groups. For instance, Franc¸ ois and co-workers21-24 prepared honeycomb porous structure from carbon disulfide (CS2) solution of poly(p-phenylene)-block-polystyrene (PPP-PS), star polystyrene, and associative polystyrenes under flow of humid gas. Shimomura and co-workers7-13 found that condensed water droplets are stabilized when amphiphilic block copolymer, amphiphilic polyion complexes, and organic/inorganic hybrids are used. Srinivasarao et al.25 showed that uniform pores with threedimensional ordering were achieved for monocarboxylated end-functional polystyrene (PS-mCOOH). Rabolt and coworkers27,28 showed that PS fiber prepared by electrospinning of THF solution under humid conditions has porous surfaces. Although all of these methods available in the literature to generate breath figure patterns should use a humid air (21) Widawski, G.; Rawiso, M.; Franc¸ ois, B. Nature 1994, 369, 387389. (22) Franc¸ ois, B.; Pitois, O.; Franc¸ ois, J. Adv. Mater. 1995, 7, 10411044. (23) Pitois, O.; Franc¸ ois, B. Eur. Phys. J. B 1999, 8, 225-231. (24) Pitois, O.; Franc¸ ois, B. Colloid Polym. Sci. 1999, 277, 574-578. (25) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79-83. (26) Shah, P. S.; Sigman, M. B., Jr.; Stowell, C. A.; Lim, K. T.; Johnston, K. P.; Korgel, B. A. Adv. Mater. 2003, 15, 971-974. (27) Megelski, S.; Stephens, J. S.; Chase, D. B.; Rabolt, J. F. Macromolecules 2002, 35, 8456-8466. (28) Casper, C. L.; Stephens, J. S.; Tassi, N. G.; Chase, D. B.; Rabolt, J. F. Macromolecules 2004, 37, 573-578.

10.1021/la035915g CCC: $27.50 © 2004 American Chemical Society Published on Web 05/18/2004

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environment (or flow of humid gas), many processes for dealing with polymeric films need dry conditions in a clean room. Furthermore, it is not easy to prepare porous polymer films with a large area and uniform thickness by the above-mentioned methods. In this paper, we introduce a novel method to fabricate breath figure patterns on a homopolymer film of cellulose acetate butyrate (CAB), monocarboxylated end-functional polystyrene (PS-mCOOH), and poly(methyl methacrylate) (PMMA) by spin coating under dry condition. To mimic a humid environment, we added a small amount of water to a water-miscible solvent of tetrahydrofuran (THF) which is a good solvent of CAB, PS-mCOOH, and PMMA. For the first time, we succeeded in generating breath figure patterns under a dry environment, and we controlled pore sizes from hundreds of nanometers to several micrometers by varying water content in THF solution and rotating speed of a spin coater. Since CAB has now been used as a coating material and a membrane because of high clarity, mechanical strength, and good biocompatibility, CAB films with uniform pores could be used as biomaterials such as cell culture substrate. II. Experimental Section Homopolymers employed in this study were cellulose acetate butyrate (CAB) monocarboxylate end-functional polystyrene (PSmCOOH), and poly(methyl methacrylate) (PMMA). CAB, whose chemical structure is given as follows

Figure 1. SEM images of film surfaces at low magnification (a and b) and high magnification (c and d), and cross sections (e and f) of breath figure patterns prepared by direct evaporation of CAB in two solvents (0.05 g/cm3) under a humid condition (RH ) 95%): (a, c, e) chloroform; and (b, d, f) THF.

III. Results and Discussion was purchased from Acros Co. in which R represents either H, COCH3, or COC3H7. According to the information of the company, the butyryl and hydroxyl contents in CAB are 52 and 1.5 wt %, respectively, and the acetyl content is less than 4 wt %. The number-average molecular weight (Mn) and polydispersity index (PDI) were 11000 and 2.3, respectively. PS-mCOOH (Scientific Polymer Product Co.) was synthesized by anionic polymerization, and Mn and PDI were 30000 and 1.06, respectively. PMMA (Scientific Polymer Product Co.) was used as purchased, and Mn and PDI were 26000 and 1.54, respectively. Mn and PDI of all the polymers employed in this study were determined by gel permeation chromatography (Waters Co.) with polystyrene standard. Various solutions of CAB, PS-mCOOH, and PMMA using chloroform and tetrahydrofuran (THF) were prepared. Cover glasses as a substrate were placed inside an acryl box where a hot water beaker controlled relative humidity (RH) determined by a hygrometer. CAB (or PS-mCOOH and PMMA) solutions were dropped on a cover glass and directly evaporated under humid condition, which is the same technique available in the literature.7-11,19-24 To prepare large porous films with uniform thickness, polymer solutions were spin-coated on a cover glass at two rotating speeds of 1000 and 3000 rpm. The spin coater was placed inside an acryl box where a hot water beaker was placed to control RH. Finally, to avoid a humid environment, a small amount of water was added to THF solution. We found that CAB did not macrophase-separate from water (or THF/water) mixture up to 15 wt % of water. But, when the water content was 10 wt % in THF solution, we found that CAB films became dewetted on the glass substrate, even though the solution became transparent. Spin coating of THF solution including water was performed under a dry atmosphere (RH ) 30%). When polymer films were prepared by spin coating, no further drying was done. The generated pores in polymer films were observed by field emission scanning electron microscopy (FE-SEM; Hitachi S-4200).

3.1. Breath Figure Patterns Prepared under Humid Condition. Figure 1 shows SEM images for breath figure patterns produced by direct evaporation of CAB in two solvents of chloroform and THF under humid conditions (relative humidity (RH) ) 95%). Uniform pores with hexagonal packing (or honeycomb structures) were generated in water-immiscible solvent of chloroform, and the number-averaged diameter (Dn) of pores was 1.63 ( 0.05 µm. This indicates that CAB solution can successfully stabilize condensed water droplets without coalescence though CAB homopolymer does not have an associative functional group, amphiphilic moiety, and star-shaped molecular architecture, which are known to be essential for generating breath figure patterns for homopolymers.7-13,21-25 Meanwhile, Srinivasarao and coworker25 stated that homopolymers such as cellulose acetate and poly(methyl methacrylate) could stabilize condensed water droplets in water-immiscible solution. Very recently, Peng et al.29 showed that well-ordered porous structures by the breath figure method were prepared by evaporating polystyrene (PS) solution in toluene under humid conditions when the proper molecular weight (Mw) of PS (thus proper solution viscosity) was used. But, for a PS with small Mw, the solution could not prevent coalescence between condensed water droplets because of lower solution viscosity, and well-ordered pores were not obtained. On the other hand, for another PS with very high Mw, water droplets could not sink into the solution, resulting in no formation of porous structures. We found that well-ordered porous structures were generated by the breath figure for CAB in various solvents (benzene, toluene, dichloroethane, and ethyl acetate) and (29) Peng, J.; Han, Y.; Yang, Y.; Li, B. Polymer 2004, 45, 447-452.

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Figure 2. SEM images of breath figure patterns prepared by direct evaporation of CAB in THF (0.05 g/cm3) under two values of RH: (a) 80%; (b) 65%.

concentrations (0.01-0.1 g/cm3). However, for high concentrations (0.15-0.2 g/cm3) in chloroform, breath figure patterns were not obtained, suggesting that a proper viscosity of CAB in solution becomes important to generate well-ordered pores, similar to results in ref 29. Interestingly, we found that CAB in THF, a watermiscible solvent, can also play a role as an effective stabilizer for water droplets (parts b and d of Figure 1). This is not expected since condensed water droplets might mix with THF solution. Until now, it has been considered that a water-miscible or partially miscible solvent has the disadvantage of stabilizing condensed water droplets because of its affinity with water. There has been no report on generating breath figure patterns prepared by a watermiscible (or partially miscible) solvent. But, very recently, Rabolt and co-workers27,28 showed that when PS fiber was electrospun from THF solution under humid conditions, it had porous structures although the regularity of pores was not good. They suggested that breath figure would be one of main mechanisms in generating these pores. As shown in parts b and d of Figure 1, we have clearly demonstrated that CAB film prepared by direct evaporation of THF solution under humid conditions has uniform pore sizes with Dn ) 2.28 ( 0.09 µm and good packing of pores. This strongly supports that breath figure patterns could be generated even if a water-miscible solvent was employed. But inner structures or cross-sectional images of CAB films prepared by water-immiscible chloroform solution are different from those prepared by watermiscible THF solution. For instance, when CAB films were prepared by chloroform solution, multilayers of pores were seen as shown in Figure 2e, although they do not have three-dimensionally-ordered structures reported by Srinivasarao et al.25 Below the multilayered pores, bulk CAB layers were formed. On the other hand, when THF solution was used, only one layer of pores was formed (Figure 2f), even though larger pores were often seen in the CAB film below the layer of pores. These differences could be explained as follows. For the case of chloroform solution, when condensed water droplets formed on the surface of chloroform solution sink down,25 the coalescence between water droplets was severely hampered by water-immiscible chloroform as well as the stabilization effect by CAB. After complete drying of chloroform and water, multilayered pores are generated in the upper part of film, whereas a compact bulk layer of CAB was formed below the layer of pores because water does not mix with chloroform solution. For the case of THF solution, on the other hand, when condensed water droplets formed on the surface of THF solution sink down, these water droplets can coalesce and even mix with THF solution. But the polymer concentration near the surface increased and solidification occurred faster than at the inner part of films; thus concentrated polymer solution can prevent condensed water droplets from coalescing and

Figure 3. SEM images of breath figure patterns prepared by spin coating of CAB in two solvents (0.1 g/cm3) (A and a, chloroform; B and b, THF) under a humid condition (RH ) 95%) using two rotation speeds. The rotation speed in the left panels (A, B) was 1000 rpm, and that in right panels (a, b) was 3000 rpm.

mixing with polymer solution. After complete drying of water and solvent, only one layer of pores can be formed at the film surface. However, large pores can be generated below this layer of pores because of the coalescence of water droplets at inner part of the film. Figure 2 shows breath figure patterns prepared by direct evaporating CAB in THF solution (0.05 g/cm3) under two values of RH (80% for Figure 2a and 65% for Figure 2b), which indicates that Dn of pores decreased from 1.62 ( 0.10 µm to 0.88 ( 0.13 µm with decreasing RH. We found that for RH ) 40%, no pores were observed after complete evaporation of THF solvent. Although the exact mechanism to form breath figure patterns under THF solution is not clear, we explain this phenomenon as follows. It is known that the same liquids but having different density do not mix rapidly with each other especially in the absence of stirring. Thus, condensed water droplets do not mix with a water-miscible solvent within a short time. Also, the increase in CAB concentration near the surface due to rapid evaporation of THF helps the stabilization of water droplets, which retards the mixing of the water droplets with the THF solution. Since CAB is also strongly immiscible with water even though exhibiting hydrophilicity, CAB chains are easily located near the water droplets and precipitated around condensed water droplets. This thin CAB solid film can stabilize the water droplets, which is similar to the phenomenon suggested by Franc¸ ois and co-workers.21-24 Finally, a rapid evaporation of THF prevents water droplets from coalescing because of the short time available.10 Even though breath figure patterns can be generated from CAB solution in water-miscible THF under a direct evaporation method under humid environments, this method is not so convenient for preparing breath figure patterns in large size films with uniform thickness. It is well-known that film of uniform thickness is easily prepared by spin coating. However, in this situation, solvent should be evaporated very fast. Figure 3 shows SEM images for breath figure patterns prepared by spin coating of CAB in two different solvents (0.1 g/cm3) of chloroform and THF at two different rpm under a humid condition (RH ) 95%). Since uniform pores such as

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honeycomb structures were generated, we consider that condensed water droplets grow enough to interact with each other for a fast evaporating solvent because of effective surface cooling. When the rotating speed was increased, the pore sizes became smaller. At 1000 rpm, Dn of pores prepared by chloroform and THF were 0.39 ( 0.06 and 1.06 ( 0.16 µm. At 3000 rpm, those were 0.27 ( 0.04 and 0.33 ( 0.03 µm. The decrease in pore size with increasing rotating speed is because of insufficient time for growth of nucleation of condensed droplets resulting from faster evaporation at a higher speed, even though the nucleation of water droplets was enhanced. 3.2. Breath Figure Patterns Prepared under a Dry Environment. Since breath figure patterns were obtained from a water-miscible solvent of THF, the humidity might have been controlled when different amounts of water were added directly to the THF solution and evaporated fast. In this situation, the evaporating water vapor, even if the amount was small compared with THF vapor, can make a humid condition near the substrate even under a dry environment. We found that when the amount of water was less than 15 wt % in THF solution, CAB does not macrophase separate with THF/water mixture. Once THF and water evaporates fast during spin coating, the water vapors in humid layers could condense on the surface of the polymer solution, which was cooled by evaporative cooling by THF. In this situation, a humid condition, which is not suitable for dealing with polymer thin films in a clean room and deteriorates wetting and adhesion between polymer and substrate, can be avoided. Figure 4 shows breath figure patterns prepared by spin coating of CAB in THF (0.1 g/cm3) with different amounts of water under a dry condition (RH ) 30%). We observed that only a flat film without exhibiting any pores was prepared by spin coating of CAB solution in THF with water content less than 1 wt % under a dry condition (RH ) 30%). When the added amount of water in THF solution and rotating speed were 1.5 wt % and 1000 rpm, small pores with Dn of 0.41 ( 0.05 µm were generated, although the packing of pores was not good. When water content was increased to 3 wt % and rotating speed was 1000 rpm, pores with Dn of 0.68 ( 0.05 µm were seen. With water amount increasing to 5 wt % and the same rotating speed of 1000 rpm as the former, Dn of pores became 1.05 ( 0.09 µm, and well-ordered honeycomb structures were observed. The film obtained from the above method exhibited milky-white and iridescent color because of uniform size and well-ordered pores. Also, it showed a good gloss because of the flatness and uniform thickness of the polymer films prepared by spin coating. Thus, we successfully generated various sizes of uniform pores in CAB films under a dry condition. In Figure 4, it is also seen that the faster the rotating speed, the smaller the pore size. More quantitative analysis of Dn with different amounts of water in THF at two rotating speeds is shown in Figure 5. Dn increased linearly with increasing water content regardless of rotating speeds. Peng et al.29 reported that pore size prepared by breath figure from PS solution in toluene increased linearly with relative humidity. For spin coating of polymer solution under humid conditions, water droplets were supplied from the vapor phase. But, for spin coating of THF solution with water at dry condition, evaporating waters are directly condensed onto polymer solution. We found that well-ordered breath figure patterns were also prepared from THF solution of CAB under a very dry condition (RH ∼ 10%) when the amount of water added to THF solution was larger than 5 wt %. Thus, we can conclude that porous structures shown in Figure 4 originated from the condensation of

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Figure 4. SEM images of breath figure patterns prepared by spin coating of CAB in THF (0.1 g/cm3) with various amounts of water under a dry condition (RH ) 30%). The amounts of water in THF solution were as follows: (A and a) 1.5 wt %; (B and b) 3 wt %; (C and c) 5 wt %; (D and d) 9 wt %. The rotation speed in left panels (A, B, C, D) was 1000 rpm, and 3000 rpm in right panels (a, b, c, d).

Figure 5. Plots of Dn versus water content at two rotating speeds for breath figure patterns prepared under a dry condition (RH ) 30%).

water droplets, not by phase separation between water and polymer solution.

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Figure 6. SEM images of surface and cross section of breath figure patterns prepared by spin coating of CAB in THF (0.1 g/cm3) with 5 wt % of water under a dry condition (RH ) 30%). (a, c): 1000 rpm, (b, d): 3000 rpm.

Figure 8. SEM images of breath figure patterns prepared by spin coating at 1000 rpm of PS-mCOOH (a, b) and PMMA (c, d, e, f) in THF (0.1 g/cm3) with various amounts of water under a dry condition (RH ) 30%). The amounts of water in THF solution: (a and c) 1.5 wt %; (b and d) 3 wt %; (e) 5 wt %; (f) 9 wt %.

Figure 7. SEM images (a, c, e) and corresponding Voronoi polygon (b, d, f) of breath figure patterns prepared by different methods: (a, b) prepared by direct evaporation of solvent from 0.05 g/cm3 CAB in chloroform under humid condition(RH ) 95%); (c, d) prepared by direct evaporation of solvent from 0.05 g/cm3 CAB in THF under humid conditions (RH ) 95%); (e, f) prepared by spin coating of CAB in THF (0.1 g/cm3) with 5 wt % of water under a dry condition (RH ) 30%) at 1000 rpm. Pores with five- and seven-nearest neighbors are colored red and green, respectively.

Figure 6 shows SEM images for surface and cross section of porous films prepared by spin coating of CAB in THF (0.1 g/cm3) with 5 wt % of water at two different rotating speeds under a dry condition (RH ) 30%). We found that pores covering a large area up to several cm2 were prepared by spin coating of solution containing water under dry condition. As shown in Figure 6c, one layer of pores was

formed at the top of the films, and there exist incomplete pores between the top layer and the substrate. This is because of the combination effect of three-dimensional ordering of breath figure and fast solidification during spin coating. Recently, Srinivasarao et al.25 suggested that three-dimensional ordering of condensed water droplets was formed because condensed water droplets sink down from the top layer, and this top layer is covered by newly nucleated water droplets. However, during spin coating, films were solidified very fast compared with the direct evaporation method; thus it is less possible for pores formed at the top layer to sink down before films are completely solidified. With increasing spin coating speed, only one layer of pores with smaller sizes was formed, as shown in Figure 6d. Packing of pores can be quantitatively analyzed by Voronoi polygon construction.20 Parts b and d of Figure 7 are Voronoi polygons obtained based on SEM images of parts a and c of Figure 7, which were prepared by direct evaporation of 0.05 g/cm3 CAB solution in chloroform and THF, respectively, under humid condition (RH ) 95%). For water-immiscible solvent of chloroform, the probabilities of pores with five-, six-, and seven-nearest neighbors (P5, P6, and P7) are 0.061, 0.893, and 0.046, respectively, which gives the conformational entropy (S ) -Pn ∑ ln Pn) of 0.41. For the case of water-miscible solvent of THF, P5, P6, and P7 are 0.060, 0.869, and 0.071, which gives S of 0.48. These two values are much less than 1.71 for random packing of pores,20 suggesting that hexagonally ordered pores were prepared by both chloroform and THF. Parts e and f of Figure 7 give a SEM image and a Voronoi polygon of porous film prepared by spin coating of 0.1 g/cm3 CAB solution in THF with 5 wt % of water under

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dry environment at 1000 rpm. The values of P5, P6, and P7 are 0.194, 0.670, and 0.136, respectively; thus S becomes 0.86. This indicates that the breath figure pattern prepared under dry conditions has more defects in hexagonal packing compared with that prepared under humid conditions. This is due to very fast evaporation of solvent during spin coating. However, since the value of S ) 0.86 is still smaller than 1.71, we consider that good packing of pores could be obtained even if the breath figure was prepared under dry condition. We also observed that porous film of CAB was also obtained when a small amount of water (3-5 wt %) was added to another water-miscible solvent of acetone for CAB, followed by spin coating at a dry condition (RH ) 30%). Finally, we employed other homopolymers (PSmCOOH and PMMA) in THF solution and spin coated at a dry condition (RH ) 30%). When water was added to THF solution, breath figure patterns were formed for both polymers, as shown in Figure 8. Also, pore sizes increased with increasing water amounts in THF. Although breath figure patterns were formed at relatively higher amounts of water (up to 9 wt % in THF) for PMMA, we could not increase the amount of water in THF to 5 wt % for PSmCOOH. This is because the dewetting of PS-mCOOH film from the substrate occurred at water content higher

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than 5 wt % due to the hydrophobic properties of the PS main chain. The above results indicate that breath figure patterns can be obtained by spin coating of polymer solutions in a water-miscible solvent (THF or acetone) at a lower humidity condition, when small amounts of water are added. IV. Conclusion In this study, we have shown for the first time that porous films induced by breath figure patterns were successfully prepared by spin coating of CAB, PS-mCOOH, and PMMA solutions under a dry condition (RH ) 30%) when small amounts of water were added to water-miscible solvents of THF and acetone. This method becomes very convenient and useful for preparing porous materials covering large areas up to several cm2 compared to previous direct evaporation methods. Since CAB is biocompatible, CAB films with uniform pores could be used as biomaterials such as cell culture substrate. Acknowledgment. This work was supported National RND Program for Nano Science and Technology (M1-021400-0230). LA035915G