Structure and characterization of sol-gel and aerogel materials and oxidation products from the reaction of (CH3O)(4)Si and C16H33Si(OCH3)(3)

The hexadecyl substituted siloxane, C16H33Si(OCH3)(3), was reacted with (CH3O)(4)Si at the mole ratio of 1-3 in methanol using HBF4 catalysis. Sol-gel materials were formed that have low surface areas (similar to10 m(2)/g). Subsequent supercritical drying using CO2 at 40degreesC produced materials that are very different than traditional aerogels, with surface areas around the same values as the corresponding sol-gels, as well as no detectable meso-pore features. In some cases, the aerogels even melted upon heating. Spectroscopic characterization, using IR, Si-29 and C-13 NMR revealed normal Si-O substitution as well as incorporation of the carbon substituent into the Si framework. Heating of the stable forms of the materials in air at different temperatures yielded, depending upon oxidation conditions, several materials with much higher surface areas, typical of aerogels. Pore size distribution measurements revealed meso-pore features with a narrow distribution of 37-39 Angstrom. Spectroscopic characterization revealed the disappearance of the R-Si substitution and the appearance of an oxidized intermediate. This paper describes the chemistry and characterization of these unusual sol-gels, aerogels, and oxidation products. (C) 2003 Elsevier B.V. All rights reserved.