Synthesis of New Flexible Aerogels from Di- and Trifunctional Organosilanes
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Synthesis of New Flexible Aerogels from Di- and Trifunctional Organosilanes 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 aerogels are expected to be the promising material for energy savings, the lack of mechanical strength prevents from commercial applications such as to low-density thermal insulators. To improve mechanical properties, methyltrimethoxysilane (MTMS) and dimethyldimethoxysilane (DMDMS) are used in this study as the co-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/DMDMS-derived condensates, phase separation occurs in aqueous sol and must be suppressed to obtain uniform and monolithic gel. 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, various microstructures of pores are obtained. In the uniaxial compression test, the aerogel showed high flexibility and spring-back to the original shape after removing the stress. INTRODUCTION Silica aerogels have outstanding properties such as high optical transparency (> 90 %), high surface areas (~ 1000 m2 g−1), low refractive indices (< 1.01), high thermal and acoustic insulation abilities, and low dielectric constants [1]. Owing to these excellent properties, there are many applications on the front of science [2], but only a limited number of commercial products are available because of the serious fragility of this material. For a wide application of aerogels, it is necessary to improve mechanical properties and to develop a new simple process without using supercritical drying. 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. Adding organoalkoxysilanes with small organic groups such as methyl group to obtain organic-inorganic hybrid aerogels is one of the promising ways to improve the mechanical properties. Compared to tetraalkoxysilane- or water glassderived silica aerogels [4-5], the hybrid aerogels improve the mechanical properties and water resistance owing to the hydrophobic substituent. Recent years, some flexible aerogels are reported by using a couple of organoalkoxysilanes as co-precursor [6-7]. We also have reported the hybrid aerogels prepared using methyltrimethoxysilane (MTMS) as the single precursor and surfactant n-hexadecyltrimethylammonium salts (bromide denoted as CTAB and chloride as CTAC) which are added to control phase separation, by a modified acid/base two-step sol-gel reaction utilizing acetic acid and urea as catalysts. These aerogels are transparent and possess improved mechanical properties (such
as reversible shrin
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