Structure and characterization of sol–gel and aerogel materials and oxidation products from the reaction of (CH 3O) 4Si and C 16H 33Si(OCH 3) 3

Abstract

The hexadecyl substituted siloxane, C 16H 33Si(OCH 3) 3, was reacted with (CH 3O) 4Si at the mole ratio of 1–3 in methanol using HBF 4 catalysis. Sol–gel materials were formed that have low surface areas (∼10 m 2/g). Subsequent supercritical drying using CO 2 at 40 °C 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, 29Si and 13C 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 Å. 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.