Effects of Starting Compositions on the Properties of Methylsilsesquioxane Aerogels
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Effects of Starting Compositions on the Properties of Methylsilsesquioxane Aerogels Gen Hayase, Kazuyoshi Kanamori, Kazuki Nakanishi, Teiichi Hanada Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto, 606-8502, Japan. ABSTRACT Recent years, although silica aerogel is expected to be the material for energy savings, the lack of the strength prevents from commercial usages such as heat-insulating windows. To improve mechanical properties, methyltrimethoxysilane is used as a precursor of aerogels because the network becomes flexible due to the relatively low cross-linking density and to the unreacted methyl groups. Because of the strong hydrophobicity of MTMS-derived condensates, uniform and homogeneous gel networks are hardly attained. In this study, we employed surfactant n-hexadecyltrimethylammonium chloride (CTAC) in starting compositions to control phase separation during a 2-step acid/base sol-gel reaction. By changing the starting composition, properties of aerogels such as bulk density and light transmittance are affected. With increasing amount of CTAC, the gel networks became denser and less transparent. Highly transparent aerogels were obtained when the amount of urea was increased. INTRODUCTION Since aerogels have been invented in 1931 by Kistler [1], a lot of studies of this unique material have been done. Typical silica aerogels have outstanding properties such as high optical transparency (> 90 %), high surface areas (~ 1000 m2/g), low refractive indices (< 1.01), high thermal and acoustic insulation abilities, and low dielectric constants [2]. Owing to these excellent properties, there are many applications on the front of science [3]. For example, NASA has used this material for the thermal insulator of the Mars explorer and the sample collector [4], and KEK for the detector of the Cherenkov counter [5]. Recently, applications to high thermal insulating windows are awaited as one of the solutions of the energy issues and the global warming. However, aerogels are still far from such applications due to their fragility and to a necessity of the supercritical drying process at high pressure to dry precursory wet gels without collapsing delicate porous structures. There has been a lot of research to improve the mechanical properties of silica aerogels and to obtain aerogels without supercritical drying while maintaining the abovementioned excellent properties. Crosslinking of gel networks with organic components such as polymers is one of the most promising ways to improve the mechanical properties. However, hybridization with polymers reduces porosity and that with organoalkoxysilanes reduces transparency in general [6]. Another way is adding organoalkoxysilanes which have small organic groups such as methyl group. Organic substituent groups make the internal surface hydrophobic, which improves water resistance and decreases shrinkage during drying under ambient conditions comparable to the tetramethoxysilane (TEOS)- or water glassderived comm
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