Fabrication of Self-Organized Chemically and Topologically

films over an area of more than 1 square centimeter in a single step was demonstrated by using the self-organizing character of polystyrene-b-oligothi...
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Fabrication of Self-Organized Chemically and Topologically Heterogeneous Patterns on the Surface of Polystyrene-b-Oligothiophene Block Copolymer Films Teruaki Hayakawa* Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-S8-26 O-okayama, Meguro-ku, Tokyo 152-8552, Japan

Hideaki Yokoyama* Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received July 15, 2005. In Final Form: September 9, 2005 A novel fabrication of the chemically and topologically heterogeneous patterns on the surface of polymeric films over an area of more than 1 square centimeter in a single step was demonstrated by using the self-organizing character of polystyrene-b-oligothiophene block copolymers. Hexagonally arranged open pores of a size of approximately 2 µm are spontaneously formed by casting the polymer solution under a moist air flow. The amphiphilic character of the polystyrene-b-oligothiophene block copolymers played the crucial role as a surfactant to stabilize the inverse emulsion of water droplets in the organic solvent, and subsequently the structure of the arranged hydrophilic oligothiophene segments remained on the interiors of the micropores. The chemical composition of the surface of the microporous films was characterized by time-of-flight secondary ion mass spectrometry (ToF-SIMS) to prove the chemical heterogeneity. The ToF-SIMS imaging clearly indicated that the oligothiophene forms the aggregated structure on the interior of the open micropores on the surface while the flat area on the surface was covered with the polystyrene.

Introduction Nano- or micropattering of self-assembled monolayers or polymer films has become increasingly important in a variety of technical applications such as electronic and optical devices1-7 or biosensors.8-11 The fabrication of the surfaces that are nano- or micropatterned with complex organic functional groups is also an essential part of these applications especially for the signal transduction or the molecular recognition.10,12 The surface covered with two or more different functional groups can be termed a “chemically heterogeneous surface”. The fabrication of chemically heterogeneous patterned surfaces has been achieved by the micro contact printing of self-assembled monolayers with a single step,11,13 but none of the other * Corresponding authors. Dr. T. Hayakawa (Tel: +81-3-57342429. Fax: +81-3-5734-2875. E-mail: [email protected]). Dr. H. Yokoyama (Tel: +81-29-861-6397. Fax: +81-29-861-4432. E-mail: [email protected]). (1) Rozsnyai, L. F.; Wrighton, M. S. Langmuir 1995, 11, 3913-3920. (2) Huang, Z.; Wang, P.-C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480-6484. (3) Lidzey, D. G.; Pate, M. A.; Weaver, M. S.; Fisher, T. A.; Bradley, D. D. C. Synth. Met. 1996, 82, 141-148. (4) Noach, S.; Faraggi, E. Z.; Cohen, G.; Avny, Y.; Neumann, R.; Davidov, D.; Lewis, A. Appl. Phys. Lett. 1996, 69, 3650-3652. (5) Fichet, G.; Corcoran, N.; Ho, P. K. H.; Arias, A. C.; MacKenzie, J. D.; Huck, W. T. S.; Friend, R. H. Adv. Mater. 2004, 16, 1908-1912. (6) Allard, D.; Allard, S.; Brehmer, M.; Conrad, L.; Zentel, R.; Stromberg, C.; Schultze, J. W. Electrochim. Acta. 2003, 48, 3137-3146. (7) Mu¨ller, M.; Zentel, R.; Maka, T.; Romanov, S. G.; Torres, C. M. S. Adv. Mater. 2000, 12, 1499-1503. (8) Kasemo, B. Surf. Sci. 2002, 500, 656-677. (9) Blawas, A. S.; Reichert, W. M. Biomaterials 1998, 19, 595-609. (10) Mrksich, M.; Whitesides, G. M. Trends Biotechnol. 1995, 13, 228-235. (11) Kumar, A.; Abbott, N.; Kim, E.; Biebuyck, H.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219-226. (12) Dai, L.; Mau, A. W. H. J. Phys. Chem. B 2000, 104, 1891-1915. (13) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 551-575.

techniques goes beyond the micro contact printing for the chemical patterning on a micron scale. A possible strategy for creating the patterned surfaces uses the self-organizing character of polymeric materials.14 Block copolymers are an important class of the selforganizing materials that offer the intriguing tunable built-in structures and tailored properties.15-29 We recently reported the formation of the self-organized hierarchical structure by casting a solution of a semi-rod-coil type block copolymer, polystyrene-b-isoprene with oligothiophene(14) Muthukumar, M.; Ober, C. K.; Thomas, E. L. Science 1997, 277, 1225-1232. (15) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525-557. (16) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725-6760. (17) Li, Z.; Zhao, W.; Liu, Y.; Rafailovich, M. H.; Sokolov, J. J. Am. Chem. Soc. 1996, 118, 10892-10893. (18) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz, A. Science 1997, 276, 384-389. (19) Messmore, B. W.; Hulvat, J. F.; Sone, E. D.; Stupp, S. I. J. Am. Chem. Soc. 2004, 126, 14452-14458. (20) Ruokolainen, J.; Ma¨kinen, R.; Torkkeli, M.; Ma¨kela¨, T.; Serimaa, R.; ten Brinke, G.; Ikkala, O. Science 1998, 280, 557-560. (21) Wang, H.; Wang, H. H.; Urban, V. S.; Littrell, K. C.; Thiyagarajan, P.; Yu, L. J. Am. Chem. Soc. 2000, 122, 6855-6861. (22) Hempenius, M. A.; Langeveld-Voss, B. M. W.; van Haare, J. A. E. H.; Janssen, R. A. J.; Sheiko, S. S.; Spatz, J. P.; Mo¨ller, M.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 2798-2804. (23) Illala, O.; ten Brinke, G. Science 2002, 295, 2407-2409. (24) Fo¨rster, S.; Plantenberg, T. Angew. Chem., Int. Ed. 2002, 41, 688-714. (25) Kim, S. H.; Misner, M. J.; Xu, T.; Kimura, M.; Russell, T. P. Adv. Mater. 2004, 16, 226-231. (26) Li, L.; Yokoyama, H.; Nemoto, T.; Sugiyama, K. Adv. Mater. 2004, 16, 1226-1229. (27) Yokoyama, H.; Li, L.; Nemoto, T.; Sugiyama, K. Adv. Mater. 2004, 16, 1542-1546. (28) Hayakawa, T.; Wang, J.; Xiang, M.; Li, X.; Ueda, M.; Ober, C. K.; Genzer, J.; Sivaniah, E.; Kramer, E. J.; Fisher, D. A. Macromolecules 2000, 33, 8012-8019. (29) Hayakawa, T.; Wang, J.; Sundararajan, N.; Xiang, M.; Li, X.; Glu¨sen, B.; Leung, G. C.; Ueda, M.; Ober, C. K. J. Phys. Org. Chem. 2000, 13, 787-795.

10.1021/la0519195 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/29/2005

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modified side chains (PS-b-POTI), under a moist air flow.30 Three tiers of the ordered empty micropores, the walls of which consist of the ordered hierarchical structure of PSb-POTI, are formed in the polymer thin film. We utilized the self-assembling process of water droplets as a template of the micrometer-sized hexagonally ordered porous structure. The moist air flow provides the microdroplets of water that work as a template for the micropores in the condensing and self-assembling block copolymer solution.31-35 Although very little is known about the role of polymers, we speculate that the amphiphilic character of the rod-coil type block copolymer plays the crucial role as a surfactant to stabilize the inverse emulsion of water in the organic solvent. After evaporation of the solvent and water, the rod segments may still remain at the interiors of the micropores, which had been the interface with the water droplets until the evaporation completed. Given that this speculation is correct, the same methodology can be easily developed for creation of a chemically and topologically heterogeneous surface simultaneously via the self-organization. In this paper, we report a novel fabrication of the chemically and topologically heterogeneous patterns on the surface of polymeric films over an area of more than 1 cm2 using simple polystyrene-b-oligothiophene block copolymers (PS-b-OT). Hexagonally arranged open pores of a size of approximately 2 µm are spontaneously formed by casting the solution of PS-b-OT under a moist air flow. The topological porous structure in micrometer scale and the chemical composition on the top of the surface of the microporous film were characterized by a scanning electron microscope (SEM) and a time-of-flight secondary ion mass spectrometer (ToF-SIMS), respectively. Whereas the surface was covered with PS as expected from its lower surface energy, the interior of the open pores on the surface was covered with oligothiophene. Furthermore, this experiment is also a test of the proposed mechanism of microporous formation under a moist air flow. Experimental Section A series of polystyrene-b-oligothiophenes block copolymers (PSb-OT) was synthesized as follows. The PS as coil segment was prepared by anionic polymerization of styrene monomers and end-capped with 1-bromo-[6-(tert-butyldimethylsilyl)oxy]hexane followed by deprotection with tetra-n-butylammonium fluoride in THF to obtain hydroxyl-terminated polystyrene. The resulting hydroxyl-terminated PS had a small polydispersity less than 1.1 (size exclusion chromatography with polystyrene standards in THF at 40 °C). Attachment of bithiophene to the PS end groups was performed by the reaction of the hydroxyl-terminated PS with 5-bromo-2,2′-bithiophene-5′-carbonyl chloride in the presence of N,N-dimethylamino pyridine in dichloromethane to give the polymer named as PS-2T. Extension to tetramer was carried out by the Stille coupling reaction36,37 of the PS-2T with 5-(trimethylstannyl)-2,2′-bithiophene in the presence of Pd(PPh3)2Cl2 in DMF to yield the PS-4T. The PS-6T-PS was obtained by the reaction of PS-2T with 5,5′-bis(trimethylstannyl)-2,2′(30) Hayakawa, T.; Horiuchi, S. Angew. Chem., Int. Ed. 2003, 42, 2285-2289. (31) Widawski, G.; Rawiso, M.; Franc¸ ois, B. Nature 1994, 369, 387389. (32) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372-375. (33) Nishikawa, T.; Nishida, J.; Ookura, R.; Nishimura, S.; Wada, S.; Karino, T.; Shimomura, M. Mater. Sci. Eng. C 1999, 10, 141-146. (34) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79-83. (35) Song, L.; Bly, R. K.; Wilson, J. N.; Bakbak, S.; Park, J. O.; Srinivasarao, M.; Bunz, U. H. F. Adv. Mater. 2004, 16, 115-118. (36) Groenendaal, L.; Bruining, M. J.; Hendrickx, E. H. J.; Persoons, A.; Vekemans, J. A. J. M.; Havinga, E. E.; Meijer, E. W. Chem. Mater. 1998, 10, 226-234. (37) Malenfant, P. R. L.; Jayaraman, M.; Frechet, J. M. J. Chem. Mater. 1999, 11, 3420-3422.

bithiophene. The synthetic procedure of 4T-PS-4T was the same as that of PS-4T except the use of a R,ω-hydroxylated PS as a starting polymer. The molecular weights of the block copolymers were determined by gel permeation chromatography (GPC) (polystyrene standard, THF, 40 °C) and matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-ToF-MS). PS-4T (GPC: Mw ) 3330 and Mw/Mn ) 1.08, MALDI-ToF-MS: Mw ) 2710 and Mw/Mn ) 1.05), PS-6T-PS (GPC: Mw ) 7760 and Mw/Mn ) 1.12, MALDI-ToF-MS: Mn ) 5140 and Mw/Mn ) 1.08), 4T-PS-4T (GPC: Mn ) 4070 and Mw/Mn ) 1.08, MALDI-ToF-MS: Mn ) 4820 and Mw/Mn ) 1.10). Time of Flight Secondary Ion Mass Spectrometry (ToFSIMS). ToF-SIMS analysis was performed using a TRIFT III (ULVAC-PHI) spectrometer. Negative ion spectra and images were recorded using a 15 keV primary beam of 69Ga+ liquid metal ion at a current of 2 nA with a pulse width of 18.0 ns. The spectra and images were acquired for 10 min from a 100 µm2 surface area with a primary ion dose of about 1 × 1012 ions/cm2. A lowenergy pulsed electron gun (28.0 eV) was used for surface charge compensation. Scanning electron microscopy (SEM). An environmental scanning electron microscope (ESEM) (Philips XL20 ESEM-FEG) equipped with a field emission gun was used. To observe the structure without conductive coating, ca. 0.8 Pa of water vapor was introduced in the sample chamber of the ESEM to prevent charge build-up. An electron beam (5-6 keV) was used for the observation to obtain secondary electron images.

Results and Discussion Before showing the strategy of the fabrication of the chemically and topologically heterogeneous surfaces, we need to review the self-organization process of the microporous block copolymer film.34 When the moist air flow starts to the surface of the polymer solution onto the substrate such as silicon wafer and glass slides, the water vapor condenses into droplets in the solution owing to the latent heat of evaporation of the solvent. The droplets of water grow with time and form a hexagonally packed array in the condensing solution apparently due to the repulsion between the droplets. Further evaporation of the solvent and the water solidifies the ordered porous structure. However, very little is known about the role of polymers during the process. Apparently stabilizing the water droplets in an organic solvent is essential. In our previous study, the amphiphilic character of the rod-coil type block copolymer of PS-b-POTI plays a crucial role as a surfactant to stabilize the inverse emulsion of water in the organic solvent.30 After evaporation of the solvent and water, the rod segments of POTI blocks may still remain at the interiors of the micropores, which had been the interface with the water droplets until the evaporation completed. To examine the possibility, in this study, we initially attempt to synthesize simple rod-coil type polymers of polystyrene-b-oligothiophene to clarify the effect of the primary polymer structures for the self-organization, and named as polystyrene-b-quaterthiophene (PS-4T), polystyrene-b-sexithiophene-b-polystyrene (PS-6T-PS), and quaterthiophene-b-polystyrene-b-quaterthiophene (4TPS-4T). The PS and oligothiophene blocks represent the coil and rod segments, respectively. PS-4T and 4T-PS-4T, thus, are coil-rod and rod-coil-rod-type structures, respectively. On the other hand, the rod segment of PS6T-PS is in the center of the molecule. The chemical structures of the resulting polymers were characterized by IR and 1H and 13C NMR spectroscopy. Furthermore, the most conclusive spectroscopic evidence for successful preparation of the target block copolymers was provided by MALDI-ToF-MS. The MALDI-ToF-MS spectrum of PS-4T in Figure 1 exhibits a single series of ion peaks at m/z 104.15 n + 514.79 where 104.15 is the mass of repeat unit, n is the number of the repeat units of PS, and 514.79

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Figure 1. Chemical structures and MALDI-TOF Mass spectra of PS-4T, PS-6T-PS, and 4T-PS-4T.

is the sum of the masses of quarterthiophene group, isobutyl end group of PS, and Ag cation. An average molar mass (Mw) and Mw/Mn of PS-4T were 2705.7 Da and 1.05, respectively. PS-6T-PS and 4T-PS-4T in Figure 1b,c also clearly showed the spectra indicating desired chemical structures with estimated Mw (Mw/Mn) of 5137.2 (1.08) and 4816.0 (1.10), respectively. The microporous films were prepared by casting a 0.25 wt % polymer solution (0.2 mL) in carbon disulfide at room temperature (23 °C) under a moist air flow on a variety of substrates such as glass slides, silicon wafers, and polyimide films. The humidity and flow rate were kept at 85% and 3.0 L min-1, respectively. The solvent and water completely evaporated within 30 s, and the films formed over an area of 2∼3 cm2. Interference color appeared on the solution surface immediately after the moist air flow started, indicating the formation of an ordered structure of water droplets on the order of microns with a narrow size distribution. The formation of hexagonally packed micropores was clearly observed in the SEM images of the films of PS-4T and 4T-PS-4T in Figure 2. It should be noted that, using the dilute solutions, i.e., 0.25%, the films allow only a single layer of open micrpores. The open pores on the surfaces showed the extremely narrow size distributions, and the diameters of PS-4T and 4T-PS-4T

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Figure 2. SEM images of microporous structures and mechanism of the film formation. The images of (a) PS-4T and (c) 4T-PS-4T show hexagonally packed microporous structures on silicon wafers. In contrast, the image of (b) PS-6T-PS shows an uneven dewetted structure. (d) Water vapor condenses into droplets in the solvent owing to the latent heat of evaporation of the solvent, grow with time and form a hexagonally packed array. Further evaporation of the solvent and the water solidifies the ordered porous structure. PS-4T and 4T-PS-4T work as emulsifier in the process, and the rod segments of 4Ts may remain at the interiors of the micropores after completely evaporation of the solvent and the water.

were approximately 1.7 and 2.1 µm, respectively. In contrast, PS-6T-PS failed to show a periodic microporous structure in the film as seen in Figure 2b regardless of our exploring the optimum film preparation condition such as humidity, solution concentration, and solvents. The interference color did not appear clearly on the way of this film formation. These findings indicate that the block copolymers with rod segment termini, which are more hydrophilic than PS, effectively stabilize the water droplets with the narrow size distribution, whereas the mid rod segment does not work as an emulsifier. This could be due to the aggregation of oligothiophene termini at the interface between the solutions and water droplets. It seems to be difficult for PS-6T-PS to form the stable interface layer compared with PS-4T and 4T-PS-4T because of the rod segment at the center of the polymer molecule, the rigidity of which hinders the loop conformation. We also consider that the molecular weights and fractions of the rod segment may also be the important factors for forming the stable interface layer, this experiment is now under investigation. A ToF-SIMS was employed to measure the chemical composition on the top of the surface of the microporous PS-4T film. ToF-SIMS has a high surface sensitivity of

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10-20 Å for organic molecules with a submicrometer lateral resolution, which enables us to map the surface chemical composition of our microporous structure.38 The surface sensitivity is essential for this study because the surface 4T layer is expected to be only approximately 15 Å, even if 4T molecules align perpendicular to the surface. A film with the nonmicroporous structure casted from the same polymer solution of PS-4T without a moist air flow was used for the reference sample. The secondary ions from the surface were identical to those of homo polystyrene (not shown here). The oligothiophenes are not present in the top 10-20 Å of the surface of the nonmicroporous film. Hence, this surface is covered with PS segments. However, from the microporous films, the remarkable characteristic peak in negative ion in Figure 3a was found at m/z ) 32, which is assigned to S-. Obviously, the source for S- must be oligothiophenes. Since the intensity of S- is strong enough to be mapped laterally within the static SIMS limit, we are able to resolve the in-plane distribution of the oligothiophenes at the surface. The characteristic secondary ions of m/z ) 32 (S-) are mapped in Figure 3b, where brighter areas have stronger secondary ion intensity of 32. The image clearly indicates hexagonally packed brighter circles, which correspond to oligothiophenes of 4T. The size and shape of all of the circles are almost uniform and the diameter is approximately 2 µm, which is in good agreement with that of the SEM image. This image clearly indicates that the oligothiophene of PS-4T forms the aggregated structure on the interiors of the micropores, whereas the flat area on the surface is covered with polystyrene as expected. It should be emphasized that such a highly ordered chemically and topologically heterogeneous surface structure is fabricated in a single step via self-organization of the block copolymers under a moist air flow. No other high-tech pieces of apparatus are required. The ToF-SIMS results also support the proposed mechanism, in which the block copolymer partitions itself at the interface between the polymer solution and water droplets during the microporous film formation. Conclusions In a summary, a novel method to make a highly ordered chemically and topologically heterogeneous surface on the polymeric materials has been developed using a series of the rod-coil type polystyrene-b-oligothiophene block copolymers. The block copolymers having oligothiophene (38) Belu, A. M.; Yang, Z.; Aslami, R.; Chilkoti, A. Anal. Chem. 2001, 73, 143-150.

Figure 3. ToF-SIMS spectrum and image of PS-4T film with microporous structure. (a) Negative ion spectrum; (b) image of m/z ) 32 (S-). The bright color section shows stronger secondary ion intensity of 32. The chemically heterogeneous surface consists of oligothiophene and polystyrene is formed in the film.

termini formed hexagonally packed micropores in the films via a self-assembly templating process with water droplets. We observed using the ToF-SIMS imaging the periodic oligothiophene chemical pattern, in which only the interiors of the micropores are covered with oligothiophenes, in the top 10-20 Å layer of the surface. Acknowledgment. This work was partially supported by the Project on Nanostructured Polymeric Materials of the New Energy and Industrial Technology Development Organization (NEDO). The ToF-SIMS measurement was assisted by Ulvac-Phi Co. LA0519195