Physical properties of silica gels and aerogels prepared with new polymeric precursors

Abstract

Silica aerogels were made by sol-gel techniques using industrial silicon derivatives (polyethoxydisiloxanes, PEDS-P x ) obtained by the reaction of tetraethoxysilane (TEOS) and water (water to TEOS molar fraction, n, ranging between 0.8 and 1.8) in the presence of sulfuric acid. For each type of precursor, sol-gel reactions were performed under acidic, neutral or basic conditions, in alcohols (methanol or ethanol) or acetone. The gels were dried under hypercritical conditions with respect to alcohol or CO 2 leading to monolithic organic-dried aerogels and CO 2-dried aerogels. Gels were characterized by the gelling time, t g, and the shrinkage during aging, τ s, and the aerogels by their apparent density, d a, their global shrinkage, τ g, and their specific surface area, S BET. It was shown that these values are strongly correlated. In most cases, t g decreases whereas τ s increases when n varies between 0.8 and 1.8. This study shows that the use of these new precursors allows synthesis of a very large variety of monolithic transparent aerogels.