Phase Separation in Alkylene-Bridged Polysilsesquioxane Sol-Gel Systems
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Phase Separation in Alkylene-Bridged Polysilsesquioxane Sol-Gel Systems Kazuki Nakanishi1,2, Takamitsu Yamato1 and Kazuyuki Hirao1 1
Department of Material Chemistry, Graduate School of Engineering, Kyoto University Yoshida, Sakyo-ku, Kyoto 606-8501, Japan 2 PRESTO, Japan Science and Technology Corporation, Japan ABSTRACT Polymerization-induced phase separations and concurrent sol-gel transitions in the hydrolysis and polycondensation of alkylene-bridged polysilsesquioxane system resulted in the formation of co-continuous macroporous gels. Gel morphology with co-continuous macropores and micrometer-sized gel skeletons was obtained in the composition region containing much higher amount of water compared with the case with other alkyltrialkoxysilane sol-gel systems. A narrow pore size distribution and high volume fraction of macropores were obtained in the median size range from few micrometers down to 0.1µm. INTRODUCTION Bis(trialkoxysilyl)alkanes are unique precursor for synthesizing organic-inorganic hybrid materials with controlled porosity [1,2]. It has been reported that when hydrolyzed and polycondensed in basic conditions, they give mesoporous hybrid gels with hydrophobic mesopores. Although the median pore size show some correlation with the alkylene bridge length between silicon atoms, the pore formation mechanism has not yet been fully rationalized. We have been exploring the synthesis route of macro- to mesoporous hybrid materials utilizing the spontaneous order formation due to the phase separation in chemically and irreversibly gelling system [3]. It has been already found that methyl- and vinyl-modified alkoxysilanes, as well as their copolymerization system with tetraalkoxysilanes, give well-defined macroporous morphologies under the conditions where transient co-continuous structures induced by the spinodal decomposition are frozen-in by the sol-gel transition [4]. In the present paper, the phase separation - gelation behavior and the resultant macroporous morphologies have been investigated in the bis(trimethoxysilyl)hexane - water - methanol system in the acidic conditions. The relation between starting composition and resultant gel morphology has been discussed in comparison with other 3-functional alkoxysilane systems. EXPERIMENTAL DETAILS Bis(trimethoxysilyl)hexane, BTMH (GELEST, INC., USA) was used as a source of bridged polysilsesquioxane network without further purification. BTMH was added to the solution of 0.01M nitric acid (0.01M HNO3 aq) and methanol, and stirred at room temperature for 30min. The composition of starting mixture was BTMH : methanol : 0.01M HNO3 = 1 : 1-40 : 50-70 in the molar ratio. The resultant homogeneous solution was poured into a Pyrex glass container, sealed and kept at 40°C or 60°C for gelation. Gelation time was about 30min to 2d. After gelation, the wet gel was aged at the gelation temperature for 1d, and finally evaporation-dried at 60°C. The morphology of the sample gels was observed by a scanning electron microscope (SEM, S-510, Hitachi Ltd., Japan) using fla
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