Macroporous Silica and Alkylene-Bridged Polysilsesquioxane Gels with Templated Nanopores
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Macroporous Silica and Alkylene-Bridged Polysilsesquioxane Gels with Templated Nanopores Kazuki NAKANISHI Department of Material Chemistry, Graduate School of Engineering, Kyoto University Nishikyo-ku, Kyoto 615-8510, JAPAN PRESTO, JST, JAPAN ABSTRACT Gels with hierarchical well-defined macropores and supramolecularly templated mesopores have been synthesized in the systems of pure silica as well as organic-inorganic hybrids. Cationic surfactants such as alkyltrimethylammonium salts and nonionic surfactants such as poly(ethyleneglycol)-poly(propyleneglycol)- poly(ethyleneglycol) triblock copolymers, EOPOEO, were found to be effective for pure silica system both in inducing the phase separation to give macroporous morphology and in templating the mesopores within a narrow distribution width. The partial introduction of methyl-modified alkoxide into the pure silica system lead to increased phase separation tendency accompanied by the disturbance of the supramolecular templating of mesopores. By contrast, in the 1,2-bis(trimethoxysilyl)ethane systems, poly(ethylene glycol) and EOPOEO were much better additive to induce phase separation in spite of the presence of internal hydrocarbon chain. Combination of well-defined macropores and supramolecularly templated mesopores has been achieved by selecting an appropriate molecular weight and concentration of EOPOEO in the starting composition. INTRODUCTION Mesoporous oxides with highly ordered pore structure via liquid-phase synthesis route have been drawing increasing attention [1-7]. The liquid-phase preparation method assisted by supramolecular templates is highly versatile for a wide range of pore structures with various oxide or inorganic-organic hybrid compositions. For the development of actual applications of their excellent structural properties, however, materials should be made into desired shapes in macroscopic scale such as fiber, film or bulk [8-11]. In addition to the precipitated powders, mesoporous materials in the forms of film and small particles have been successfully prepared by utilizing so-called evaporation-induced order formation process in which highly ordered mesophases are formed due to the preferential evaporation of compatibility-mediating constituents such as alcohols [12-14]. On the other hand, a novel sol-gel process to fabricate well-defined macroporous monoliths has been established based on the polymerization-induced phase separation in various siloxane-based sol-gel systems [15]. The integration of supramolecularly templated mesopores into the well-defined macroporous network has been challenged for these years. Since the supramolecular templating requires strong interaction between surfactant miecelles and polymerizing oligomers of the network component, it is generally difficult to obtain long-range periodic mesopores in a macroscopic scale except for few cases that skillfully controls the liquid crystalline state of the templates or rapid aggregation process of the dispersed oligomers [10]. Even if the material can be built up t
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