Controlled Hierarchical Pore Structures in Ethylene-Bridged Polysilsesquioxane Gels
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Controlled Hierarchical Pore Structures in Ethylene-Bridged Polysilsesquioxane Gels Yuki Kobayashi, Kazuki Nakanishi* and Kazuyuki Hirao Department of Material Chemistry, Graduate School of Engineering, Kyoto University Nishikyo-ku, Kyoto 615-8510, JAPAN *PRESTO, JST, JAPAN ABSTRACT Formation of hierarchical macropores and mesopores in ethylene-bridged polysilsesquioxane sol-gel systems via concurrent sol-gel transition and phase separation induced by the incorporation of supramolecular templats has been investigated. A series of poly(ethyleneglycol)-poly(propyleneglycol)- poly(ethyleneglycol) triblock copolymers, EOPOEOs, have been employed as supramolecular templates, while 1,2-bis(trimethoxysilyl)ethane was used as a polysilsesquioxane source. Successful combination of templated mesopores and well-defined macropores due to phase-separation has been found in the system with EOPOEOs having 70 PO units and respective 20 EO units. Although similar macroporous morphology has been obtained for all the EOPOEOs with EO chain longer or shorter than 20 units, the optimum composition for the co-continuous macropores and that for templated mesopores deviated from each other, resulting in the failure of producing hierarchical macropores and mesopores. INTRODUCTION Synthesis of periodic mesoporous oxides using supramolecular templates in inorganic or organic-inorganic hybrid sol-gel systems has been extensively studied in recent years [1]. A series of organic medication of the precursors of mesoporous oxides have been proposed for the purpose of functionalizing their well-defined nanoporosity. Among the materials directly synthesized from organically modified precursors, the bridged polysilsesquioxane has a unique structural feature that its network is composed of hybridized organic-inorganic main chains with small inorganic oxide clusters bridged by organic chains [2]. Supramolecular templating of various bridged polysilsesquioxane has been reported by Inagaki et al. , revealing that ordered mesoporous arrays in 2D hexagonal as well as cubic symmetry can be prepared in ethylene, phyenylene and biphenylene bridged polysilsesquioxanes [3-5]. On the other hand, well-defined macroporous gels can be prepared through concurrent phase separation and sol-gel transition in many alkoxy-derived siloxane-based sol-gel systems [6]. It has been already found that tetramethoxysilane with cationic surfactants such as alkyltrimethylammonium halides or nonionic surfactants such as poly(ethyleneglycol)block-poly(propyleneglycol)-block-poly(ethyleneglycol), EOPOEO, can form monolithic gels with hierarchical pore structure in micrometer and nanometer ranges [7,8]. Although the origin of mesopores in macroporous gels was reasonably attributed to the supramolecular templating effect of the surfactants, no decisive datum indicating the presence of long-range ordering of the mesopores was presented. In the present study, the synthesis of ethylene-bridged polysilsesquioxane gel monoliths which have hierarchical pore structure using EOPOEO tri
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