Langmuir 2006, 22, 5233-5236
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Formation of a Highly Ordered Dot Array of Surface Micelles of a Block Copolymer via Liquid Crystal-Hybridized Self-Assembly Shusaku Nagano,*,†,‡ Yu Matsushita,† Yuki Ohnuma,† Satoshi Shinma,§ and Takahiro Seki*,†,‡ Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, CREST-JST, Japan, and Nagoya Research & DeVelopment Center, Mitsubishi HeaVy Industries, Ltd., 1, Aza-Takamichi, Iwatsuka-cho, Nakamura-ku, Nagoya 453-8515, Japan ReceiVed February 6, 2006. In Final Form: April 11, 2006 A highly ordered dot array pattern of surface micelles on water is formed by a spread monolayer of an amphiphilic block copolymer, polystyrene-block-poly(4-vinylpyridine) (PS-4VP), via hybridization with a liquid crystal molecule, 4′-pentyl-4-cyanobiphenyl (5CB), on water. Simple co-spreading of PS-4VP with 5CB provides a flat homogeneous monolayer of PS-4VP on water without the aggregation of PS blocks. With increasing surface pressure, well-defined dots of the PS blocks start to grow and are arrayed in a highly ordered hexagonal structure. The exact coincidence of the surface pressure-area curves for the hybrid monolayer in the compression and expansion processes confirms that the flat spread monolayer and the dot array are formed on water in the equilibrium state by a self-assembly process.
Introduction The fabrication of highly ordered nanostructures over macroscopic scales has been a subject of increasing attention with respect to advanced nanoscience and nanotechnology in the next generation of electronic and optical devices. Despite great progress in photolithography, nanoscale patterning encounters many difficulties because of the optical limitation and the requirement of high-cost processes. Block copolymer systems are promising candidates for low-cost, simple production of periodic nanostructures by self-assembly and have great potential for applications in high-density data storage devices, high-capacity optical storage media, fine nanostructured membranes, and surface nanopatterns and nanotemplates for various functional materials.1-3 Surface microphase separation patterns of block copolymers via a self-assembly process have been studied in bulk films,1-4 ultrathin films,1-3,5-7 micelles in solution,8 and Langmuir monolayers formed at the air/water interface.9-12 The air/water interface provides an ideal planar field to arrange various types of microphase separation patterns in one plane.9-12 Furthermore, * Corresponding authors. (S.N.) E-mail:
[email protected]. Tel: +81-52-789-3199. Fax: +81-52-789-4669. (T.S.) E-mail:
[email protected]. Tel: +81-52-789-4668. Fax: +81-52-7894669. † Nagoya University. ‡ CREST-JST. § Mitsubishi Heavy Industries, Ltd. (1) Hamley, I. W., Ed. DeVelopments in Block Copolymer Science and Technology; John Wiley & Sons: West Sussex, England, 2004. (2) Nikos Hadjichristidis, N.; Pispas, S.; Floudas, G. A. Block Copolymers: Synthetic Strategies, Physical Properties, and Applications; John Wiley & Sons: Hoboken, NJ, 2003. (3) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. (4) (a) Hashimoto, T.; Nagatoshi, K.; Todo, A.; Hasegawa, H.; Kawai, H. Macromolecules 1974, 7, 364. (b) Thomas, H. R.; O’Malley, J. J. Macromolecules 1979, 12, 323. (c) Chen, X.; Gardella, J. A., Jr.; Kumler, P. L. Macromolecules 1992, 25, 6631. (d) Watanabe, K.; Tian, Y.; Yoshida, H,; Iyoda, T. Trans. Mater. Res. Soc. Jpn. 2003, 28, 553. (5) (a) Russell, T. P.; Menelle, A.; Anastasiadis, S. H.; Satija, S. K.; Majkrzak, C. F. Macromolecules 1991, 24, 6263. (b) Mansky, P.; Chaikin, P.; Thomas, E. L. J. Mater. Sci. 1995, 30, 1987. (6) (a) Heier, J.; Kramer, E. J.; Walheim, S.; Krausch, G. Macromolecules 1997, 30, 6610. (b) Yang, X. M.; Peters, R. D.; Nealey, P. F.; Solak, H. H.; Cerrina, F. Macromolecules 2000, 33, 9575. (c) Sundrani, D.; Sibener, S. J. Macromolecules 2002, 35, 8531. (7) Knoll, A.; Horvat, A.; Lyakhova, K. S.; Krausch, G.; Sevink, G. J. A.; Zvelindovsky, A. V.; Magerle, R. Phys. ReV. Lett. 2002, 89, 035501.
the film balance technique offers precise, facile tuning of the surface density of microdomains by compression on water, which does not require tedious synthetic control of block length.11a,c Thus, the LB method has a great advantage in constructing the surface nanostructure of block copolymers on the macroscopic scale that are a few nanometers flatness. When the amphiphilic block copolymer is spread on water with a highly volatile solvent, the hydrophobic blocks (e.g. polystyrene block in many cases) immediately aggregate to form surface micelles. The aggregates formed from hydrophobic block chains are rigid enough on water and do not change their shape because of the stronger cohesive force among the chains on water and the relatively high glasstransition temperature. Therefore, the surface micelles exhibit various surface nanostructures governed by various parameters such as physical and chemical characteristics of blocks (amphiphilic nature, solubility, molecular weight, block size, etc.) and processing factors (solvent used, concentration of spreading solution, temperature, compression speed, etc).10-12 The resulting surface nanostructures of such conventional spreading method contain dissipative factors in large parts. To obtain ordered nanostructures of surface micelles in macroscopic regions, one requires highly severe customizations of spreading conditions. Fully hydrophobic polymers consisting of only hydrocarbon side and main chains cannot be spread as a monolayer on water13 because of the lack of affinity for water.14 In this respect, we (8) (a) Potemkin, I. I.; Kramarenko, E. Yu.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P.; Eibeck, P.; Spatz, J. P.; Mo¨ller, M. Langmuir 1999, 15, 7290. (b) Spatz, J. P.; Eibeck, P.; Mossmer, S.; Mo¨ller, M.; Kramarenko, E. Yu.; Khalatur, P. G.; Potemkin, I. I.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P.; Macromolecules 2000, 33, 150. (c) Soo, P. L.; Eisenberg, A. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 923. (9) Cox, J. K.; Eisenberg, A.; Lennox, R. B. Curr. Opin. Colloid Interface Sci. 1999, 4, 52. (10) (a) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583. (b) Zhu, J.; Eisenberg, A.; Lennox, R. B. Langmuir 1991, 7, 1579. (c) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6547. (d) Meli, M.-V.; Badia, A.; Gru¨tter, P.; Lennox, R. B. Nano Lett. 2002, 2, 131. (11) (a) Baker, S. M.; Leach, K. A.; Devereaux, C. E.; Gragson, D. E. Macromolecules 2000, 33, 5432. (b) Devereaux, C. E.; Baker, S. M. Macromolecules 2002, 35, 1921. (c) Seo, Y.; Paeng, K.; Park, S. Macromolecules 2001, 34, 8735. (12) Lu, Q.; Bazuin, C. G. Nano Lett. 2005, 5, 1309. (13) (a) Crisp D. J. J. Colloid Sci. 1946, 1, 191. (b) Kumaki, J. Macromolecules 1988, 19, 2258. (c) Kajiyama, T.; Oishi, Y. Trends Polym. Sci. 1995, 3, 30. (14) Gaines, G. L., Jr. Langmuir 1991, 7, 834.
10.1021/la060350k CCC: $33.50 © 2006 American Chemical Society Published on Web 05/03/2006
5234 Langmuir, Vol. 22, No. 12, 2006
Letters
Scheme 1. Chemical Structures of PS-P4VP and 5CB
have recently proposed a method to prepare ideally spread monolayers of fully hydrophobic polysilanes on water with the assistance of a polar liquid crystal (LC) molecule, 4′-pentyl-4cyanobiphenyl (5CB).15a,b This procedure allows for precise evaluations of the conformational state of polymer chains near a solid surface by the stepwise LB depositions.15c In this method, the hydrophobic polymer is placed in a highly fluid and equilibrating environment on the floating two-dimensional 5CB monolayer. In this letter, this liquid crystal-assisted spreading is applied for a block copolymer of polystyrene-block-poly(4-vinylpyridine) (PS-P4VP) (Scheme 1). We demonstrate here that a highly ordered dot array of surface micelles is formed via a 5CB-induced equilibrium state of a self-assembled spread monolayer of the amphiphilic block copolymer. Simple co-spreading of PS-P4VP with 5CB first provides a homogeneously spread, molecularly flat monolayer of PS-4VP on water without the aggregation of the PS segment. With increasing surface pressure, a regular hexagonal dot array of the PS block grows. However, the selforganization process without 5CB provides irregularly sized and arranged polystyrene dots involving dissipative factors. Experimental Section The diblock copolymer of PS-P4VP (PS191-P4VP153, Mw ) 49 000, Mw/Mn ) 1.22) was synthesized by anionic polymerization. We prepared two mother chloroform solutions of the pure PS-P4VP at ca. 1 mmol (pyridine unit) dm-3 and 5CB at 1 mmol dm-3. The co-spreading solution was prepared by mixing the two mother solutions at R ([pyridine unit]/[5CB]) ) 1.0. The spreading behavior of the pure and mixed monolayers was characterized with a Lauda FW-1 film balance filled with pure water (Milli-Q grade, 18 MΩ cm) at 10 °C. The sliding barrier was compressed at a speed of 20 cm2 min-1. The morphologies of the Langmuir monolayer were observed by Brewster angle microscopy (BAM) using an NLE EMM633 (Filgen, Inc.). Monolayers were transferred onto a freshly cleaved mica substrate by the conventional vertical dipping method for atomic force microscopy (AFM) studies. The topographical AFM observation was carried out on a SPA300/SPI3700 system (Seiko Instruments) in noncontact mode. UV absorption spectra of deposited LB monolayers on a fused silica substrate were taken using a Jasco MAC-1 spectrometer system.
Results and Discussion Figure 1a shows the surface pressure (π)-area (A) isotherms for pure 5CB, pure PS-P4VP, and a PS-P4VP/5CB mixture (R ) 1.0) obtained in the compression process at 10 °C. The pure 5CB monolayer exhibited a lift-off area at 0.45 nm2 and a plateau at 5 mN m-1, as commonly observed for the 4′-n-alkyl-4cyanobiphenyl monolayers.16 Pure PS-P4VP showed a gradual pressure increase from 0.45 nm2 with a quasi-plateau at 7 mN m-1 in the range from 0.2 to 0.15 nm2. For larger areas above (15) (a) Nagano, S.; Seki, T.; Ichimura, K. Chem. Lett. 2000, 613. (b) Nagano, S.; Seki, T.; Ichimura, K. Langmuir 2001, 17, 2199. (c) Nagano, S.; Seki, T. J. Am. Chem. Soc. 2002, 124, 2074.
Figure 1. (a) Surface pressure-area isotherms of pure PS-P4VP surface micelles, the pure 5CB monolayer, and the PS-P4VP/5CB hybrid film at 10 °C. (b) Compression-expansion cycles of pure PS-P4VP and the PS-P4VP/5CB hybrid film on water.
0.2 nm2, the P4VP block should be monomolecularly extended on water with an aggregated PS core to form surface micelles as actually indicated in Figure 2a. As the surface micelle was compressed, the increase in the two-dimensional density of the P4VP unit raised the surface pressure. Around an area of 0.2 nm2 where the P4VP density on a water surface exceeded a critical concentration, we observed the quasi-plateau region associated with the phase transition from the two-dimensional state of P4VP chains to the three-dimensional one on water. Further compression gave a sharp pressure lift above ca. 20 mN m-1 around an area of 0.15 nm2, originating from the overlapping of the PS cores at the water surface.11 However, the PS-P4VP/5CB mixed film gave the first uprise at 0.45 nm2, which is almost the same area of the pure block copolymer and 5CB despite the fact that the two components are coexisting. The shape of the π-A curve, furthermore, illustrated no characteristic plateaus corresponding to 5CB or pure PS-P4VP, strongly suggesting that the two components are hybridized at molecular levels without phase separations. In the case of polymer/fatty acid mixed films, in contrast, the π-A curves retain the features derived from the fatty acid monolayer.17 We also performed BAM observation of the PS-P4VP/5CB hybrid monolayer. Nevertheless, the floating monolayer was hardly observed because of low reflectivity in the range from 0.6 to 0.2 nm2, and features of phase separation and aggregated structures were not admitted (data not shown). It is thus confirmed that molecular mixing was attained and the homogeneous hybrid monolayer could be formed on a macroscopic scale of more than several hundred micrometers of the BAM observation field on the water surface. A compression-expansion cycling experiment on water provides further information on the differences between the pure PS-P4VP surface micelle and the PS-P4VP/5CB hybrid film (Figure 1b).14 The surface micelles of the pure block copolymer (16) (a) Daniel, M. F.; Lettington, O. C.; Small, S. M. Thin Solid Films 1983, 99, 61. (b) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251. (c) Ubukata, T.; Seki, T.; Ichimura, K. J. Phys. Chem. B 2000, 104, 4141. (17) (a) Murakata, T.; Miyashita, T.; Matsuda, M. Macromolecules 1988, 21, 2730. (b) Murakata, T.; Miyashita, T.; Matsuda, M. Macromolecules 1989, 22, 2706. (c) Watanabe, I.; Hong, K.; Rubner, M. F. Langmuir 1990, 6, 1164. (d) Seki, T.; Ichimura, K. Langmuir 1997, 13, 1361. (e) Seki, T.; Tohnai, A.; Tanigaki, N.; Ichimura, K. Macromol. Chem. Phys. 1999, 200, 1446.
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Figure 2. Topographical AFM images (3 × 3 µm2) of the PS-P4VP surface micelle (a and b) and PS-P4VP/5CB hybrid films (c-f) transferred onto mica at surface pressures of 0 mN m-1 (corresponding to a surface area of 0.6 nm2) (a and c), 1 mN m-1 (b and d), 4 mN m-1 (e), and 10 (f) mN m-1. The brightness contrast corresponding to height is shown in the top right legend, and the lateral scale is indicated at the bottom right of panel a.
exhibited clear hysteresis in the compression-expansion cycles. An expansion from a point at ca. 15 mN m-1 indicated a rapid pressure reduction due to the overlapping of PS cores. In contrast and interestingly, the expansion curve of the hybridized film exactly reproduced the initial compression curve even starting near 20 mN m-1, indicating that the π-A curves represent the equilibrium state at each stage and that the aggregated PS cores should be respread in the expansion process. π-A isotherms for the PS-P4VP/5CB hybrid films were further obtained by spreading from other solvents such as tetrahydrofuran, benzene, and cyclohexane. These hybrid films on water exhibited exactly the same features independent of spreading solvents. This provides further evidence that the present process does not have a dissipative nature. Topographical AFM images of the pure block copolymer and the hybrid films transferred onto mica are shown in Figure 2. The pure amphiphilic block copolymer film at 0.6 nm2 (before the pressure increases) exhibited dotlike particles and ribbons (a). The particles corresponded to the core aggregates of the hydrophobic PS parts, and the surface micelles were formed concurrently with spreading on water. However, no aggregate feature of the PS parts was recognized in the PS-P4VP/5CB hybrid film under the same area condition (c). Instead, we observed a smooth surface having monomolecular-level undulation (
