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Preparation of Honeycomb-Patterned Polyimide Films by Self-Organization Hiroshi Yabu,†,‡ Masaru Tanaka,§,| Kuniharu Ijiro,§,| and Masatsugu Shimomura*,‡,⊥ Graduate School of Science, Hokkaido University, N10W8, Sapporo, 060-0810, Japan, Frontier Research System, Institute of Physical and Chemical Research (RIKEN Institute), 1-12, Hirosawa, Wako, Saitama, 351-0198, Japan, Research Institute for Electronic Science, Hokkaido University, N12W6, Sapporo, 060-0812, Japan, PRESTO, Japan Science and Technology Corporation (JST), N12W6, Sapporo, 060-0812, Japan, and Nanotechnology Research Center, Research Institute for Electronic Science, Hokkaido University, N12W6, Sapporo, 060-0812, Japan Received March 17, 2003. In Final Form: May 12, 2003
Microporous polymer films are attractive materials with potential application in the fields of electronics, photonics, and biotechnology. Chemical and thermal stabilities of the microporous polymer films are required for their materials application. Besides preparation by conventional photolithography, we have reported that honeycomb-patterned porous polymer films are prepared by a method utilizing the condensation of small water droplets on solutions of amphiphilic copolymers. Here, we show preparation of honeycombpatterned microporous films of a thermally and chemically stable material, polyimide. A water-templateassisted honeycomb structure was formed from a polyion complex of polyamic acids and dialkylammonium salt. The pore size of films was controlled by the casting volume of polymer solution. The patterned polyion complex film converted into polyimide by simple chemical treatment, keeping the porous structure. Selfsupporting microporous polyimide films are fabricated. The honeycomb-structured film has high thermal and chemical stability like that of conventional cast films of polyimides.
1. Introduction Microporous polymer films are attractive materials with potential application in the fields of electronics, photonics, and biotechnology.1-2 Microporous polymer films have been fabricated by photolithography,3 soft lithography,4-7 inversed opals,8-10 and phase separation of block copolymers.11-12 It was reported that honeycomb-patterned * To whom correspondence should be addressed. Tel & Fax: +81-11-706-3665. E-mail:
[email protected]. † Graduate School of Science, Hokkaido University. ‡ Frontier Research System, Institute of Physical and Chemical Research (RIKEN Institute). § Research Institute for Electronic Science, Hokkaido University. | PRESTO, Japan Science and Technology Corporation (JST). ⊥ Nanotechnology Research Center, Research Institute for Electronic Science, Hokkaido University. (1) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals: Molding the Flow of Light; Princeton University Press: Princeton, NJ, 1995. (2) Ostuni, E.; Chen, C. S.; Ingber, D. E.; Whitesides, G. M. Langmuir 2001, 17 (9), 2828. (3) Campbell, M.; Sharp, D. N.; Harrison, M. T.; Denning, R. G.; Turberfield, A. J. Nature 2000, 404, 53. (4) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153. (5) Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Langmuir 2002, 18, 5314. (6) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Schueller, O. J. A.; Whitesides, G. M. Electrophoresis 2000, 21, 27. (7) Rogers, J. A.; Paul, K. E.; Jackmann, R. J.; Whitesides, G. M. Appl. Phys. Lett. 1997, 70 (20), 2658. (8) Cassagneau, T.; Caruso, F. Adv. Mater. 2002, 14 (24), 1837. (9) Mı´guez, H.; Yang, S. M.; Te´treault, N.; Ozin, G. A. Adv. Mater. 2002, 14 (24), 1805. (10) Yu, J.-S.; Kang, S.; Yoon, S. B.; Chai, G. J. Am. Chem. Soc. 2002, 124, 9382. (11) Thurn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. P. Adv. Mater. 2000, 12 (11), 787. (12) Templin, M.; Frank, A.; Du Chesne, A.; Leist, A.; Zhang, A.; Ulrich, R.; Scha¨lder, V.; Wiesner, U. Science 1997, 278, 1795.
porous polymer films are prepared by simple casting of polymer solutions of a water-immiscible solvent under high humidity.13-29 Hexagonally packed water microdroplets are formed by evaporation cooling on the solution surface and then transferred to the solution front in the convectional flow or by the capillary force. After solvent evaporation, the honeycomb-patterned polymer film is (13) Widawski, G.; Rawiso, M.; Franc¸ ois, B. Nature 1994, 369, 397. (14) Pitois, O.; Franc¸ ois, B. Eur. Phys. J. B 1999, 8, 225. (15) Pitois, O.; Franc¸ ois, B. Colloid Polym. Sci. 1999, 277, 574. (16) Srinibasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79. (17) Govor, L. V.; Bashmakov, I. A.; Kaputski, F. N.; Pientka, M.; Parisi, J. Macromol. Chem. Phys. 2000, 201, 2721. (18) Govor, L. V.; Bashmakov, I. A.; Kiebooms, R.; Dyakonov, V.; Parisi, J. Adv. Mater. 2001, 13 (8), 588. (19) Nishida, J.; Nishikawa, K.; Nishimura, S.; Wada, S.; Karino, T.; Nishikawa, T.; Ijiro, K.; Shimomura, M. Polym. J. 2002, 34, 166-174. (20) Kurono, N.; Shimada, R.; Ishihara, T.; Shimomura, M. Mol. Cryst. Liq. Cryst. 2002, 377, 285-288. (21) Shimomura, M.; Sawadaishi, T. Curr. Opin. Colloid Interface Sci. 2001, 6 (1), 11-16. (22) Shimomura, M. Hierarchical Structuring of Nanostructured 2-Dimensional Polymer Assemblies. In Organic Mesoscopic Chemistry; Masuhara, H., DeSchryver, F. C., Eds.; IUPAC “Chemistry for the 21st Century” Monograph; Blackwell Science: Malden, MA, 1999; pp 107126. (23) Karthaus, O.; Maruyama, N.; Cieren, X.; Shimomura, M.; Hasegawa, H.; Hashimoto, T. Langmuir 2000, 16 (15), 6071. (24) Karthaus, O.; Cieren, X.; Maruyama, N.; Shimomura, M. Mater. Sci. Eng. 1999, C10, 103-106. (25) Nishikawa, T.; Nishida, J.; Ookura, R.; Nishimura, S.; Wada, S.; Karino, T.; Shimomura, M. Mater. Sci. Eng. 1999, C8-9, 485-500. (26) Maruyama, N.; Karthaus, O.; Ijiro, K.; Shimomura, M.; Koito, T.; Nishimura, S.; Sawadaishi, T.; Nishi, N.; Tokura, S. Supramol. Sci. 1998, 5, 331. (27) Stenzel, M. H. Aust. J. Chem. 2002, 55, 239. (28) de Boer, B.; Stalmach, U.; Nijland, H.; Hadziioannou, G. Adv. Mater. 2000, 12, 1581. (29) de Boer, B.; Stalmach, U.; van Huttern, P. F.; Melzer, C.; Krasnikov, V. V.; Hadziioannou, G. Polymer 2001, 42, 9097-9109.
10.1021/la034454w CCC: $25.00 © 2003 American Chemical Society Published on Web 06/14/2003
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Yabu et al. dispersion of N,N-dimethyldioctadecylammonium bromide 3 (Sogo Chemical Industry, Japan). The white precipitates were extracted in chloroform and then purified by reprecipitation in ethanol and acetonitrile.31 Chloroform solutions of the polyion complex were dropped on a clean glass substrate. Applying humid air (relative humidity, 50-70%) flowed vertically to the solution surface, chloroform was gradually evaporated and water droplets condensed on the solution surface. The interference color that emerged on the solution surface indicates regular packing of uniform-sized water droplets. After complete evaporation of the solvent, the film was soaked in a mixed solution of benzene, pyridine, and acetic anhydride (volume ratio 3:1:1, respectively) for over 12 h to convert polyamic acid to polyimide. Optical micrographs and scanning electron micrographs were taken by a BH-2 (Olympus, Japan) and an S-3500 (Hitachi, Japan), respectively. The pore size was measured by laser light diffraction experiments. A red laser beam (Sigma Koki InGaAlP laser; 3 mW power; wavelength, 670 nm; spot size, 400 µm) was passed vertically through the center of the self-standing polymer film. The diffraction pattern was projected onto a paper screen placed horizontally 93 mm apart from the film center. FT-IR reflection-absorption spectroscopy (RAS) measurement was carried out by an FT-IR200 spectrometer (JASCO, Japan) with a polarizing reflection unit (PR-500). The patterned films were prepared from 20 µL of a 1.0 g/L solution on an Au-coated indium-tin oxide (ITO) substrate for the FT-IR measurement. The polyimide films were heated on a hot stage equipped with an optical microscope, RINKAM RH-600 (Japan Hi-Tech, Japan) to observe the thermal stability. Thermogravimetric analysis (TGA) up to 600 °C with a heating rate of 20 °C/min was carried by a TA-60 (SHIMADZU, Japan).
3. Results and Discussion
Amphiphilic polyion complexes were obtained as precipitates after mixing of an aqueous solution of polyamic acids 1 and 2 (Nissan Chemical Industry, Japan, Chart 1) with an aqueous
It was reported that polyion complexes of protonated long-chain alkylamine and polyamic acid were dissolved in chloroform to prepare Langmuir-Blodgett films.30 The polyion complexes of polyamic acids 1 and 2 and dialkylammonium salts 3 prepared in this experiment were soluble in chloroform, too. A typical scanning electron microscopy (SEM) image of a honeycomb-patterned polyion complex film is shown in Figure 1a. A well-arranged hexagonal lattice was observed. When the SEM sample stage was tilted at 70°, the doublelayer structure of the patterned film is clearly imaged. Two hexagonal lattices are connected vertically by pillars at the vertex of hexagons (Figure 1b). A schematic model of the double-layered structure is shown in Figure 1c. To control the pore sizes of the honeycomb structures, the casting volume of the solutions was changed. When the volume of the casting solution increased from 20 µL to 5 mL, the pore sizes of the honeycomb structures were increased from ca. 500 nm to 9 µm. The solvent evaporation time, which is equal to the water condensation time, is a main controlling factor of pore sizes in any case.32 A larger amount of casting solution requires a longer time for complete solvent evaporation; then the water droplet grows larger. Casting temperature is another controlling factor of the pore size. When the substrate was cooled at 4 °C, the diameters of the pores were drastically increased to 18 µm from 5.2 µm (20 °C). The solution surface was cooled rather than the case of casting at room temperature, and solvent evaporation was suppressed. Therefore, a large amount of condensed water was provided for a longer time. As a result, the pore size grew larger. The precursor films were too fragile to be peeled off from the substrate. However, after soaking in the mixed solution of benzene, pyridine, and acetic anhydride, the film can be peeled off as a self-supported film from a
(30) Kakimoto, M.; Suzuki, M.; Konishi, T.; Imai, Y.; Iwamoto, M.; Hino, T. Chem. Lett. 1986, 823.
(31) Tanaka, K.; Okahata, Y. J. Am. Chem. Soc. 1996, 118 (44), 10679. (32) Beysens, D. Atmos. Res. 1995, 39, 215.
formed with the water droplet array as a template. Finally, microporous polymer films are obtained after water evaporation. Various types of polymers can be fabricated as a honeycomb-patterned film with controlled pore size, ranging from hundreds of nanometers to hundreds of microns. Chemical and thermal stabilities of the microporous polymer films are required for their materials application. Polyimide is one of the well-known engineering plastics with highly chemical and thermal durability. Here, we report preparation of honeycomb-patterned microporous films of polyimide. Due to the poor solubility of polyimide in most solvents, polyamic acid, the precursor of the polyimide, is used for the pattern formation. Most polyamic acid, however, can be dissolved only in water and water-miscible solvents, for example, N-methylpyrrolidone. The solubility of the polyamic acid to organic solvents was reported to be improved by forming the ionic complex with amphiphilic cations, which does not affect the imidation reaction.30 In this paper, we first prepare polyion complexes of polyamic acids and dialkylammonium salts to dissolve them in water-immiscible organic solvents. Then, honeycomb-patterned films of the polyioncomplexed precursor are prepared by simple casting under humid conditions. The precursor films are converted to the corresponding polyimide by chemical treatment. The cyclization reaction of polyamic acid to polyimide with removal of the amphiphiles is confirmed by Fourier transform infrared (FT-IR) measurement. The thermal and chemical stability of the honeycomb-patterned films is discussed. 2. Experimental Section
Honeycomb-Patterned Polyimide Films
Figure 1. (a) SEM image of a honeycomb-patterned film from a polyion complex of 2 and 3 prepared by casting 20 µL of a 1.0 g/L chloroform solution. (b) SEM image obtained from a honeycomb-patterned film tilted 70°. The white arrow indicates the pillar supporting two honeycomb layers. (c) Schematic illustration of the double-layered structure of the honeycomb film. (d) The self-standing honeycomb film peeled off from the glass substrate after imidization reaction. (e) SEM image of the honeycomb-patterned film after chemical treatment. (f) The hexagonally arranged diffraction spots reflected from the polyimide film.
substrate (Figure 1d). The mechanical property of the honeycomb film was improved by imidization reaction. After chemical treatment, the color of the film of 1 and 3 was converted from white into yellow. This color change was ascribable to the imide formation with elongation of the π-electron conjugate length. The infrared absorption attributed to the C-H stretching disappeared in the FTIR RAS spectra of films, and the carbonyl group absorptions appeared at 1780 and 1720 cm-1 (Figure 2a,b). This measurement revealed that polyamic acids 1 and 2 were completely converted into polyimides 4 and 5, respectively. SEM observation clearly shows that the double-layer structure was collapsed after chemical treatment, but the honeycomb structure was still kept after chemical treatment (Figure 1e). The ordered arrangement of micropores was clearly indicated by a light scattering experiment. A laser with a beam diameter of 400 microns was used as a light source. The diffraction pattern was projected on a white paper screen and imaged by a video camera. A diffraction pattern with hexagonally arranged spots was obtained from the honeycomb-patterned film of 5 (Figure 1f). According to Bragg’s law,
nλ ) 2d sin θ Here λ is the wavelength of the incident light, d is the lattice constant of the scattering centers, and θ is the angle
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Figure 2. Change of FT-IR RAS spectra by imidization reaction. (a) Honeycomb-patterned films of the precursor (1, 3) and 4. (b) 2, 3, and 5.
of diffraction; the lattice constant d was calculated as a function of camera length. In the case of the film prepared from 1 mL of the sample solution, the average lattice constant of the micropore was calculated as 3.1 µm. This value was consistent with the pore size estimated from optical microscopy and scanning electron microscopy. The higher order diffractions (up to 7 orders) indicate that the micropore lattice in the film has a highly ordered hexagonal arrangement. To evaluate the thermal stability of the honeycomb film, the precursor and imidized films were annealed on a hot stage up to 300 °C for 1 h, respectively. In situ optical microscopy revealed that the polyimide film kept its honeycomb structure at 300 °C, while the polyion complex precursor film melted at about 150 °C. The TGA measurement shows that the pyrolysis of polyimide films of 4 starts at 400 °C and the decomposition completes at 511 °C (Figure 3b). The pyrolysis of the film of 5 starts at 400 °C, and the decomposition completes at 449 °C (Figure 3c). After the patterned polyimide films were soaked in chloroform, tetrahydrofuran, and concentrated H2SO4, respectively, for 2 weeks, the SEM investigation revealed the films kept their honeycomb structures. These results indicate that the honeycomb-patterned structure does not affect thermal and chemical durability of polyimide films. 4. Conclusion This is the first report of a simple method to produce highly arranged microporous films from polyimides, which have chemical and thermal durability, by using selforganization of water droplets. A water-template-assisted honeycomb structure was formed from polyion complexes of polyamic acids and dialkylammonium salt. The pore size of films was controlled by the casting volume of the
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Figure 3. Thermal stability of the honeycomb-patterned films of the precursor complex and polyimides. (a) Observation of morphological change on a hot stage under an optical microscope. TGA and differential TGA curves of the patterned polyimide films of 4 (b) and 5 (c).
polymer solution. The patterned polyion complex film converted into polyimide by simple chemical treatment, keeping the porous structure. Self-supporting microporous polyimide films are fabricated. The honeycomb-structured film has high thermal and chemical stability like that of conventional cast films of polyimides. Polyimide microporous films can be utilized for thermally stable separation membranes, microreactors, templates for chemical etchings, and 2-D photonic crystals.
Acknowledgment. We thank Nissan Chemical Industry Co. Ltd. for providing polyamic acids. We also thank Dr. Olaf Karthaus, Chitose Institute of Science and Technology, for helping with TGA measurements. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan. LA034454W
