2614
Langmuir 2005, 21, 2614-2618
Effects of Film-Forming Conditions on Surface Properties and Structures of Diblock Copolymer with Perfluoroalkyl Side Chains Yoshimasa Urushihara and Takashi Nishino* Department of Chemical Science and Engineering, Faculty of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan Received November 2, 2004. In Final Form: December 7, 2004 Film-forming conditions (cast solvents and film-forming technique: casting or spin-coating) are found to regulate the surface free energy of a diblock copolymer composed of methyl methacrylate and 2-perfluorooctylethyl methacrylate (PMMA-b-PFEMA). The surface properties and structures both in the solid state and in the solution of this polymer were investigated using dynamic contact angle, X-ray photoelectron spectroscopy, X-ray diffraction, reflection-adsorption Fourier transform infrared spectroscopy, dynamic light scattering, and surface pressure measurements. The surface free energy of the PMMA-bPFEMA film drastically changed from 7.8 to 28.4 mJ/m2, depending on the film-forming conditions. For the film cast from the micellar solution, the surface free energy is governed by the unimers, localized at the air-solution interface. On the other hand, higher amounts of micelles with the laid perfluoroalkyl side chains were exposed on the surface of the spin-coated film, which yielded a relatively high surface free energy. These findings demonstrated a significant effect of the in-solution molecular association on the surface structures and properties of the solid. In particular, the in-solution unimers played the most important role in lowering the surface free energy of the PMMA-b-PFEMA films.
Introduction Materials with low surface free energy have recently been attracting a lot of interest for water- and oil-repellent nonadhesive applications and microelectronics. The surface free energy, γs, of materials is related to the species and the concentration of the functional surface groups. Fluorinated groups are known to be effective for lowering the surface free energy.1 Zisman et al. reported that the surface free energy decreased in the order -CH2- > -CH3 > -CF2- > -CF2H > -CF3 and predicted the lowest critical surface tension (6 mJ/m2) using oriented monolayers of carboxylated perfluoro compounds.2 Recently, we succeeded in directly measuring a value of 6.7 mJ/m2 as the lowest surface free energy for the hexagonal closest packed CF3 group surface made of vapor-deposited C20F42.3 Highly fluorinated polymers, typically poly(tetrafluoroethylene) (PTFE), are widely used as biomaterials, in coating engineering, and as soil-resistant and breathable textiles in many fields. However, the γs value of PTFE is 22 mJ/m2,4 which is far higher than the lowest one mentioned above. Further, these fully fluorinated polymers have disadvantages in their mechanical properties, processing, and costs.5-7 To overcome these disadvantages, a successful approach is to attach perfluoroalkyl groups as side chains to copolymers with acrylate (methacryl* To whom correspondence should be addressed. Telephone: +8178-803-6164. Fax: +81-78-803-6205. E-mail:
[email protected]. (1) Scherirs, J. Modern Fluoropolymers; John Wiley & Sons: Chichester, 1997. (2) Schulman, F.; Zisman, W. A. J. Colloid Sci. 1952, 7, 465. (3) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y. Langmuir 1999, 15, 4321. (4) Sumiya, K.; Taii, T.; Nakamae, K.; Matsumoto, T. J. Adhes. Soc. Jpn. 1982, 18, 345. (5) Re´ve´sz, K.; Hopp, B.; Bor, Z. Langmuir 1982, 13, 5593. (6) Tervoort, T.; Vigjager, J.; Graf, B.; Smith, P. Macromolecules 2000, 33, 6460. (7) Elias, H. G. Macromolecules, 2nd ed.; Plenum: New York, 1984; p 918.
ate),8-15 phenyl ester,16 siloxane,17-19 styrene,20-22 and isocyanate.23 Those polymers reportedly show high surface segregation of the perfluoroalkyl side chains, which causes the very low free energy on their surfaces, while maintaining the processability achieved by the other block components. These very low surface free energies are believed to arise from the uniformly arranged CF3 groups on their surface. X-ray diffraction and near-edge X-ray absorption fine structure (NEXAFS) studies have revealed that the perfluoroalkyl side chains in these polymers are self-assembled, are slightly tilted from the normal surface, and cover the surface with their CF3 end groups.12,21,24,25 On the other hand, the in-solution structures of these (8) Schmidt, D. L.; Coburn, C. E.; DeKoven, B. M.; Potter, G. E.; Meyers, G. F.; Fisher, D. A. Nature 1994, 368, 39. (9) Park, I. J.; Lee, S. B.; Choi, C. K.; Kim, K. J. J. Colloid Interface Sci. 1996, 181, 284. (10) Morita, M.; Ogisu, H.; Kubo, M. J. Appl. Polym. Sci. 1999, 73, 1741. (11) Tsibouklis, J.; Graham, P.; Eaton, P. J.; Smith, J. R.; Nevell, T. G.; Smart, J. D.; Even, R. J. Macromolecules 2000 33, 8460. (12) Nishino, T.; Urushihara, Y.; Meguro, M.; Nakamae, K. J. Colloid Interface Sci. 2005, 283, 533. (13) Park, I. J.; Lee, S. B.; Choi, C. K. Polymer 1997, 38, 2523. (14) Krupers, M. J.; Sheiko, S. S.; Mo¨ller, M. Polym. Bull. 1998, 40, 211. (15) Imae, T.; Tabuchi, H.; Funayama, K.; Sato, A.; Nakamura, T.; Amaya, N. Colloids Surf., A: Physicochem. Eng. Asp. 2000, 167, 73. (16) Pospiech, D.; Jehnichen, D.; Gottwald, A.; Ha¨ussler, L.; Kollig, W.; Grundke, K.; Janke, A.; Schmit, S.; Werner, C. Surf. Coat. Int. Part B: Coat. Trans. 2003, 86, 43. (17) Perutz, S.; Wang, J.; Kramer, E. J.; Ober, C. K.; Ellis, K. Macromolecules 1998, 31, 4272. (18) Tsibouklis, J.; Nevell, T. G. Adv. Mater. 2003, 15, 647. (19) Borkar, S.; Jankova, K.; Siesler, H. W.; Hvilsted, S. Macromolecules 2004, 37, 788. (20) Bertolucci, M.; Galli, G.; Chiellini, E.; Wynne, K. J. Macromolecules 2004, 37, 3666. (21) Wang, J.; Mao, G.; Ober, C. K.; Kramer, E. J. Macromolecules 1997, 30, 1906. (22) Gopalan, P.; Andruzzi, L.; Li, X.; Ober, C. K. Macromol. Chem. Phys. 2002, 203, 1573. (23) van Ravenstein, L.; Ming, W.; van de Grampel, R. D.; van der Linde, R.; de With, G.; Loontjens, T.; Thu¨ne, P. C.; Niemantsverdriet, J. W. Macromolecules 2004, 37, 408. (24) Genzer, J.; Efimenko, K. Science 2000, 290, 2130.
10.1021/la047317n CCC: $30.25 © 2005 American Chemical Society Published on Web 02/10/2005
Film-Forming Conditions of Block Copolymer
Langmuir, Vol. 21, No. 6, 2005 2615
polymers have been reported to depend on the dissolving solvent,13-15 and they were reflected in the surface morphology of the solid.14,15 However, so far as we know, there has been no report discussing the influence of the in-solution structures on the surface properties of the solid in detail. In a previous paper, we investigated the surface properties and structure of diblock copolymers composed of methyl methacrylate and 2-perfluorooctylethyl methacrylate (PMMA-b-PFEMA).12 The PMMA-b-PFEMA film cast from the chloroform solution showed a lower surface free energy (7.8 mJ/m2) than that from the CF3CF2CHCl2 solution. The lower γs value was caused by the higher self-assembly of perfluoroalkyl side chains and more densely packed CF3 surface groups, due to the collapse of the chloroform-phobic PFEMA segments. This suggests that the surface structure and properties of the PMMAb-PFEMA film depend on the film-forming conditions. In this study, the effects of the cast solvents and filmforming techniques on the surface properties of PMMAb-PFEMA were investigated, in correlation with their solid-state and solution structures. Experimental Section Sample Preparation. MMA and PFEMA were purchased from Nacalai Tesque, Inc., and Daikin Industries, Ltd., respectively. PMMA-b-PFEMA was synthesized by a living anionic copolymerization at -78 °C in tetrahydrofuran (THF) using 1,1diphentylethyllithium as an initiator. The details of block copolymerization were described in the previous paper.12 The number-average molecular weight (Mn), the polydispersity index Mw/Mn (Mw is the weight-average molecular weight), and the PFEMA content of PMMA-b-PFEMA were 12 000, 1.08, and 15.2 mol %, respectively. PMMA-b-PFEMA was purified by reprecipitation in methanol from THF solution twice. Two film-forming techniques were used in this study. The film was prepared by casting 0.2 mL of 0.01 g/mL solutions, or spin-coating at 1000 rpm on cleaned Si wafers (25 × 20 mm) with a native oxide layer. The cast film was dried in air for 24 h and then in a vacuum at room temperature for another 12 h. Chloroform, trichloroethylene, ethyl acetate, THF, methyl ethyl ketone (MEK), tolune, benzene, CF3CF2CHCl2 (HCFC-225), and 1,3-bis(trifluoromethyl)benzene (BFB) were used as solvents, and their corresponding films will be expressed as “cast” for the cast films and “spin” for the spin-coated films, respectively. Measurements. The dynamic contact angles of distilled water and diiodomethane were measured at room temperature. The advancing contact angle (θa) was measured when the contact area between the liquid droplet and the film surface was enlarged (
