Porous Structures of Polymer Films Prepared by Spin Coating with

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Langmuir 2006, 22, 4594-4598

Porous Structures of Polymer Films Prepared by Spin Coating with Mixed Solvents under Humid Condition Min Soo Park, Wonchul Joo, and Jin Kon Kim* National CreatiVe Research Center for Block Copolymer Self-Assembly, Department of Chemical Engineering and Polymer Research Institute, Pohang UniVersity of Science and Technology, Kyungbuk 790-784, Korea ReceiVed NoVember 8, 2005. In Final Form: March 7, 2006 We investigate the effects of interfacial energy between water and solvent as well as polymer concentration on the formation of porous structures of polymer films prepared by spin coating of cellulose acetate butyrate (CAB) in mixed solvent of tetrahydrofuran (THF) and chloroform under humid condition. The interfacial energy between water and the solvent was gradually changed by the addition of chloroform to the solvent. At a high polymer concentration (0.15 g/cm3 in THF), porous structures were limited only at the top surfaces of CAB films, regardless of interfacial energies, due to the high viscosity of the solution. At a medium concentration (∼0.08 g/cm3 in THF), CAB film had relatively uniform pores at the top surface and very small pores inside the film because of the mixing of the water droplets with THF solution. When chloroform was added to THF, pores at the inner CAB film had a comparable size with those at the top surface because of the reduced degree of the mixing between the water droplets and the mixed solvent. A further decrease in polymer concentration (0.05 g/cm3 in THF) caused the final films to have a two-layer porous structure, and the size of pores at each layer was almost the same.

1. Introduction Porous polymer films have attracted much attention due to their usefulness as supporting media in tissue engineering,1 membranes in separation process,2,3 templates for inorganic growth,4 dielectric materials for electronic devices5,6 and optical materials.7 Porous structures seen in the entire thickness direction in addition to the two-dimensional top surface of the film become very important to control the desirable properties of porous films, such as cell grow rate,1 selectivity of membrane,2,3 and effective refractive index of an optical film.7 Among many methods for the preparation of porous polymer films, the breath figure has been extensively investigated in that it gives porous morphology with uniform pore size and two (or three) dimensional ordered structures.8-22 It utilizes solvent-evaporative cooling on the * To whom correspondence should be addressed. Fax: +82-54-2798298. E-mail: [email protected]. (1) Cameron, N. R. Polymer 2005, 46, 1439-1449. (2) Pinnau, I.; Koros, W. J. J. Appl. Polym. Sci. 1991, 43, 1491. (3) Pinnau, I.; Koros, W. J. J. Membrane. Sci. 1992, 71, 81. (4) 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. (5) Ding, S.; Wang, P.; Wan, X.; Zhang, D. W.; Wang, J.; Lee, W. W. Mater. Sci. Eng. B 1999, 83, 130-136. (6) Schwo¨diauer, R.; Bauer, S. ReV. Sci. Instrum. 2002, 73, 1845-1852. (7) Park, M. S.; Lee Y.; Kim, J. K. Chem. Mater. 2005, 17, 3944-3950. (8) Widawski, G.; Rawiso, B.; Franc¸ ois, B. Nature 1994, 369, 387-389. (9) Pitois, O.; Franc¸ ois, B. Eur. Phys. J. B 1999, 8, 225-231. (10) Limaye, A. V.; Narhe, R. D.; Dhote, A. M.; Ogale, S. B. Phys. ReV. Letts. 1996, 76, 3762-3765. (11) Karthaus, O.; Maruyama, N.; Cieren, X.; Shimomura, M.; Hasegawa, H.; Hashimoto, T. Langmuir 2000, 16, 6071-6076. (12) Nishikawa, T.; Ookura, R.; Nishida, J.; Arai, K.; Hayashi, J.; Kurono, N.; Sawadaishi, T.; Hara, M.; Shimomura, M. Langmuir 2002, 18, 5734-5740. (13) Yabu, H.; Tanaka, M.; Ijiro, K.; Shimomura, M. Langmuir 2003, 19, 6297-6300. (14) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21, 3235-3237. (15) Yabu, H.; Shimomura, M. Langmuir 2005, 21, 1709-1711. (16) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79-83. (17) Hayakawa, T.; Shin Horiuchi, S. Angew. Chem., Int. Ed. 2003, 42, 22852289 (18) Park, M. S.; Kim, J. K. Langmuir 2004, 20, 5347-5352. (19) Park, M. S.; Kim, J. K. Langmuir 2005, 21, 11404-11408.

surface of a solution under humid conditions under which water vapors are condensed into water droplets at the solution surface. Then, water droplets interact with one another and are finally hexagonally packed. After complete evaporation of the solvent and water, traces of water droplets remain in the polymer film and become pores with honeycomb structure. The pore size formed by breath figures ranges from a few hundred nanometers to several micrometers, depending upon the amount of water condensation from the humid air (or humid air flow rate), evaporation speed, and polymer concentration. One of the most interesting features of the breath figure is multilayer stacking of pores in three-dimensions, because water droplets should be stabilized even in the solution after they sink into the solution.16-20 Srinivasarao and co-workers16 showed that three-dimensional pores were formed due to sinking of condensed water droplets into polymer solution and consecutive condensations of water vapor on the surface of the polymer solution.3 Hayakawa et al.17 prepared multilayered porous film by increasing the polymer concentration of rod/coil block copolymer solution. Very recently, Bolognesi et al.20 showed that multilayer stacking of condensed water droplets is mainly due to the interfacial energy balance between water and solvent. They considered that the interfacial energy balance parameter (zo) given by eq 1 becomes a very important parameter to determine the multilayer stacking of condensed water droplets

zo )

γw - γw/S γS

(1)

where γw, γS, and γw/S are surface energies of water and a solvent, and interfacial energy between water and the solvent. For -1 < zo < 1, water droplets are floating in polymer solution; thus, multilayer porous films could not be obtained. Only for zo > 1 (20) Bolognesi, A.; Mercogliano, C.; Yunus, S.; Civardi, M.; Comoretto, D.; Turturro, A. Langmuir 2005, 21, 3480-3485. (21) Peng, J.; Han, Y.; Yang, Y.; Li, B. Polymer 2004, 45, 447-452. (22) Cui, L.; Peng, J.; Ding, Y.; Li, X.; Han, Y. Polymer 2005, 46, 53345340.

10.1021/la053009t CCC: $33.50 © 2006 American Chemical Society Published on Web 04/05/2006

Porous Structures of Polymer Films Prepared by Spin Coating

can droplets sink into the polymer solution; thus, multilayer porous films were obtained. Previously, we showed that porous morphology could be prepared by breath figure patterning from polymer solution in a water-soluble solvent such as tetrahydrofuran (THF) under humid conditions.18 But the porous structures prepared from THF solution were different from those prepared from a waterimmiscible solvent such as chloroform. Namely, the inner part of the CAB film prepared from THF solution contained a small number of larger sized pores due to the mixing of water droplets with THF solution and coalescence with droplets. We also showed that spin-coating of polymer solution under humid conditions provided porous films with thin and uniform thickness.19 In this situation, much larger pore fractions were obtained from spincoating from THF solution compared with chloroform, because negligible interfacial energy between THF and water lowers an energy barrier for the nucleation process, thus enhancing the nucleation rate.19,23 This suggests that when interfacial energy between polymer solution and water is properly controlled, various porous structures of polymer film would be obtained. For instance, the degree of mixing between water droplets and polymer solution inside the film would decrease upon increasing the interfacial energy between two, whereas a higher interfacial energy limits the nucleation of water droplets at the top of the film. zo for water/THF solvent is estimated to be 2.76 from eq 1 with γw/THF ∼ 0, γw ) 72.8 mN/m, and γTHF ) 26.4 mN/m. This is larger than that of water/chloroform solvent (1.63) estimated by using γchloroform ) 27.5 mN/m and γw/chloroform ) 28.0 mN/ m.24,25 Therefore, both THF and chloroform solution could allow a multilayer stack of condensed water droplets to form as long as water droplets have enough time to interact with one another, a condition usually met for the commonly used drop and evaporation under humid conditions method. However, once spin-coating under humid conditions is employed for making breath figures, the nucleation rate of water droplets, and thus the in-flux of water droplets into the solution, also becomes important in determining the stacking of water droplets. Therefore, the multilayer stacking of condensed water droplets could not be determined by the value of zo alone when spin-coating is employed to make breath figures. In this paper, we report the effects of interfacial tension and polymer concentration on porous structure prepared by spincoating under humid conditions. Surface tension of solvent and interfacial tension between solvent and water are important in the formation of breath figures, because they affect condensation of water droplets on polymer solution.19,20,26 We introduced the systematic control of the interfacial tension between water and solvent, in which the amount of chloroform was gradually increased in the THF solution. THF and chloroform have almost the same surface tensions,24,25 heats of vaporization, and vapor pressures.27 The interfacial tension between water and solvent could be gradually varied from 0 to that of chloroform (28 mN/ m) by adding chloroform into THF. In this situation, the value of zo in eq 1 is varied from 2.76 to 1.63, which implies that the multilayer stack of condensed water droplets could be formed when the commonly used drop and evaporation under humid conditions method is used. However, polymer concentration and (23) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; WileyInterscience Publication: New York, 1982; Chapter 9. (24) http://www.surface-tension.de (accessed October 2005) (25) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press Inc.: San Diego, 1991; Chapter 15. (26) Knober, C. M.; Beysens, D. Europhys. Lett. 1988, 6, 707-712. (27) CHEMnetBASE. Dictionary of Organic Compounds, Web Version 2005 (2); Chapman Hall/CRC Press (http://www.chemnetbase.com).

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rotating speed are also important in determining the final porous structures when spin-coating is employed. By changing the polymer concentration and interfacial energy, we could generate various porous structures: (1) an asymmetric porous structure where the top layer has many pores while a few pores with small size are observed inside the film, (2) a porous structure in which pores can be seen in entire film thickness, (3) a symmetric two layered porous structure, and (4) a one layer porous structure. These structure developments were discussed in terms of the interfacial tensions of solvents, polymer concentration, and solvent evaporation speed. The polymer films with various porous structure could be used as dielectric layer and thermal insulator with various capacitances.5,6 2. Experimental Section Cellulose acetate butyrate (CAB) was purchased from Acros Co. and the molecular characteristics are given in ref 18. CAB solutions in THF, chloroform, and mixtures of THF/chloroform with various concentrations under relative humidity of 95% were spin-coated on silicon wafer substrate at 1000 rpm. A cylindrical chamber (25 cm in diameter and 10 cm in height) with a removable cover was placed on a spin coater with the spinning chuck located at the center of the chamber. Silicon wafer as a substrate (25 mm × 25 mm) was heated by a hair drier to prevent water condensation before the solution was dropped on the silicon wafer. To increase humidity, 30 cm3 of hot water (85 °C) was poured into the cylindrical chamber. The removable cover was placed on the chamber. Then, the chamber could completely enclose the silicon wafer and the spinning chuck. After the substrate begun to rotate for 1 min without dropping the solution, the substrate was cooled. The relative humidity (RH) and temperature in the cylindrical chamber increased to 95% and 32 °C, respectively. Without opening the chamber, CAB solutions were dropped on the substrate through a small hole in the cover. The substrate was rotated for 90 s. Although a small amount of water droplets can condense onto the solution right after dropping of the solution on the substrate, an excess amount of the solution was removed in the beginning of the rotation and thin layer of clear solution formed on silicon wafer without water condensation. Within 20 s, CAB solutions turned hazy due to the light scattering by water condensates. To ensure complete drying, spin coating was performed for 90s. Surface and cross-sectional morphologies of spin-coated films were investigated by field emission scanning electron microscopy (FE-SEM; Philips XL30SFEG). Since the edge parts of the spin-coated film were not used, porous morphologies did not change with the radial direction of the spin-coater.

3. Results and Discussion Figure 1 shows SEM images of top surfaces and cross sections of CAB films prepared by spin-coating of a high polymer concentration (0.15 g/cm3) in mixed solvents of THF/chloroform at 1000 rpm under RH ) 95%. With the addition of chloroform to THF, the interfacial tensions of water and mixed solvents varied gradually from 0 to 28 mN/m.25 When pure THF was used, the top surface was covered with closely packed pores resulting from condensed water droplets, as shown in Figure 1A. The inner part of the film contains pores of smaller size, as shown in Figure 1a. Formation of these pores could be explained by two reasons. First, pores can be naturally formed during the evaporation of the solvent. Second, condensed water droplets sink into the solution, and they are depleted by the mixing with the water-miscible solution. In this situation, the droplet size in the solution (thus final pore sizes) reduces. When CAB porous film is prepared with “drop and evaporation” from THF solution under humid condition, larger pores than those on the top surface are coexistent with smaller pores in the inner film.18 Larger pores originate from the coalescence of water droplets and small pores from the depletion by mixing of water droplets with the solution.

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Figure 1. SEM images of top surfaces (A-D) and cross sections (a-d) of spin-coated CAB film from 0.15 g/cm3 in mixed solvent of THF and chloroform at 1000 rpm and RH of 95%. The amounts of THF and chloroform (v/v) are (A, a) 10/0, (B, b) 7/3, (C, c) 3/7, and (D, d) 0/10.

Figure 2. SEM images of top surfaces (A-D) and cross sections (a-d) of spin-coated CAB film from 0.08 g/cm3 in mixed solvent of THF and chloroform at 1000 rpm and RH of 95%. The amounts of THF and chloroform (v/v) are (A, a) 10/0, (B, b) 7/3, (C, c) 3/7, and (D, d) 0/10.

On the other hand, for the spin coating under humid conditions method, coalescence between water droplets hardly occurs because solvent evaporates much faster compared to the drop and evaporation method. Thus, the mixing of droplets with the solution is dominant over the coalescence. This suggests that the film prepared from THF solution has higher thickness and less compact structure in the inner part compared with another film prepared from water immiscible solution (compare parts a and d of Figure 1). Porous structures for all the cases of mixed solvents were only limited at the top surfaces, as shown in Figure 1. This is of interest because more condensed water droplets would be generated to form stacking of water droplets due to the slow solvent evaporation resulting from higher polymer concentration. However, higher solution viscosity prevents water droplets from being embedded in polymer solution; thus, condensed droplets could not easily sink into solution. Peng et al.21 showed that a few pores were observed due to very high viscosity of polymer solution when polystyrene film with high molecular weight was prepared from toluene solution with the drop and evaporation method under humid air flow. According to eq 1, THF and chloroform could allow multilayer stacking of water droplets (thus a three-dimensional porous structure). Therefore, we can conclude that the inability to stack water droplets (thus threedimensional porous structure) in the above situation is mainly due to the high viscosity of polymer solution preventing sinking of water droplets inside the film. Figure 2 shows SEM images of top surfaces and cross sections of CAB film prepared by spin-coating of a medium polymer concentration (0.08 g/cm3) in mixed solvents of THF/chloroform at 1000 rpm under RH ) 95%. Film surfaces prepared from pure

THF (Figure 2A) were covered with pores generated from the traces of condensed water droplets. From cross-sectional morphology (Figure 2a), porous film consists of two layers: (1) a top surface with closely packed pores and (2) an inner part where smaller sized pores than those observed at top layers were distributed randomly. Top porous layer thickness (longitudinal pore size) and total film thickness were 500 and 1500 nm, respectively; thus, the ratio of film thickness to pore size is ∼3. Smaller pore sizes inside the film are generated because condensed water droplets were depleted by mixing them into THF solution even if they could sink into polymer solution. Film morphology prepared by 7/3 (v/v) THF/chloroform mixture showed that the top surface (Figure 2B) was covered with pores that originated from traces of condensed water droplets similar to that prepared from pure THF solution. Interestingly, the inner part of the film (Figure 2b) is also filled with pores, which is distinctly different from that prepared from pure THF solution. This is because of the increased interfacial energy between the solution and water, which reduces the mixing of water droplets with the solvent. Upon further increasing the amount of chloroform, a porous structure was observed only at the top surface (Figure 2C), but there are almost no pores inside the film (Figure 2c). This is attributed to the fact that the nucleation rate of water condensation was greatly reduced. Finally, when pure chloroform was used (Figure 2D,d), the porous structure was not well-developed. These results indicate that an optimum amount of chloroformsso that the mixing of condensed water and solvent inside the film is effectively prevented, but large numbers of water droplets should be nucleated at the film surfaces must exist to have porous structures spanning the entire film thickness.

Porous Structures of Polymer Films Prepared by Spin Coating

Figure 3. SEM images of top surfaces (A-D) and cross sections (a-d) of spin-coated CAB film from 0.05 g/cm3 in mixed solvent of THF and chloroform at 1000 rpm and RH of 95%. The amounts of THF and chloroform (v/v) are (A, a) 10/0, (B, b) 7/3, (C, c) 3/7, and (D, d) 0/10.

Figure 3 shows SEM images of top surfaces and cross sections of CAB film prepared by spin-coating of a small polymer concentration (0.05 g/cm3) in mixed solvents of THF/chloroform at 1000 rpm under RH ) 95%. Film surface prepared by pure THF (Figure 3A) was covered with pores generated from the traces of condensed water droplets. From cross-sectional morphology (Figure 3a), two layers of porous structures were formed, and the sizes of pores at the two layers are almost the same. To prepare this kind of porous structure by spin coating from a water-miscible solvent under high humidity, the polymer concentration is carefully controlled. This is because, at a higher polymer concentration, inlets of consecutive water condensations to form stacking of water droplets are favorable due to slow evaporation, whereas the ability to stabilize droplets in the polymer solution is greatly reduced. On the other hand, at a lower concentration, a fast evaporation (and solidification) could prevent water droplets from mixing into the solution, whereas the inlets of condensed water become smaller. We found that when a faster rotating speed (7000 rpm) was employed for the same concentration (0.05 g/cm3) in pure THF, the inner film had a few pores compared with the top layer due to limited inlets of water droplets.19 Even at 7/3 (v/v) THF/chloroform, the inner film had many pores, though the top surface had fewer pores than that prepared with pure THF. With further increase in chloroform [3/7 (v/v) of THF/chloroform], the condensation of water droplets was limited; thus, pores were limited to only the top layer. We found that porous structure was not well developed (Figure 3D,d) when the film was prepared from pure chloroform. We could not obtain porous structures with three or more layers even when higher concentrated solution (0.06 and 0.07 g/cm3) was used.

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Figure 4. SEM images of top surfaces (A-D) and cross sections (a-d) of spin-coated CAB film from 0.03 g/cm3 in mixed solvent of THF and chloroform at 1000 rpm and RH of 95%. The amounts of THF and chloroform (v/v) are (A, a) 10/0, (B, b) 7/3, (C, c) 3/7, and (D, d) 0/10.

This is because water droplets could be depleted by mixing with THF, as shown in Figure 2a. Figure 4 shows SEM images of top surfaces and cross sections of CAB film prepared by spin-coating of a low polymer concentration (0.03 g/cm3) in mixed solvents of THF/chloroform at 1000 rpm under RH ) 95%. Closely packed pores were observed at the top surface and only one layer of pores was formed due to thin thickness, comparable with pore size. With increasing chloroform content, the regularity of pore size and packing deteriorated and small pores were placed between large pores, resulting in one layer of porous structure. CAB film prepared by spin-coating from less than 0.03 g/cm3 solution did not give well-developed porous structures due to very thin thickness, regardless of solvent compositions. Finally, we prepared porous CAB films by drop and evaporation under humid conditions. Figure 5 shows top and cross-sectional images prepared by direct evaporation of CAB in 7/3 (v/v) THF/ chloroform solution (0.05 g/cm3) under RH ) 95%. We showed previously that the top surface of the CAB film prepared from both pure THF and chloroform exhibited hexagonally ordered pores.18 But, the inner part of the CAB film prepared from pure THF solution exhibited a small number of larger sized pores due to the mixing of water droplets with THF solution and coalescence with droplets, whereas the inner part of the CAB film prepared from pure chloroform solution exhibited closely packed pores without the coalescence.18 It is seen in Figure 5 that, with the addition of 30 vol % of chloroform to THF, the inner part of the

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4. Conclusion

Figure 5. SEM images of top surface (a) and cross section (b) of CAB film prepared by direct evaporation of CAB in 7/3 (v/v) THF/ chloroform solution (0.05 g/cm3) under RH ) 95%.

CAB film had many pores, which is quite different from that prepared with pure THF (see Figure 1f in ref 18). This is because the coalescence of water droplets and mixing of water droplets with THF decreased upon increasing the interfacial tension.

We have shown that the interfacial energy between water and solvent as well as polymer concentration profoundly affected the porous structure of CAB films prepared by spin coating from a water-miscible solvent under humid conditions. From high to low polymer concentrations, various porous morphologies are generated: (i) a porous top surface and inner part with a few small pores; (ii) pores distributed throughout the film; (iii) two layers of porous structure with almost the same size of pores at each layer, and (iv) a single layer of porous structure. These results could be explained by the combination of effects of solvent evaporation speed (thus film thickness) and the interfacial energy between water droplets and the solvent. This kind of strategy could be applied to prepare polymer film showing various physical properties due to various porous structures, such as dielectric layer and heat insulator with various capacitances. Acknowledgment. This work was supported by the Creative Research Initiative Program. LA053009T