INFLUENCE OF SUPERCRITICAL DRYING FLUID ON STRUCTURE AND PROPERTIES OF ORGANICALLY MODIFIED SILICA AEROGELS

Organically modified silica aerogels were prepared by NH4OH-catalyzed hydrolysis and condensation of RSi(OMe)(3)/Si(OMe)(4) (R is methyl, methacryloxypropyl, glycidoxypropyl) mixtures, followed by supercritical drying with methanol or CO2. The influence of the supercritical drying fluid on the structure and properties of the resulting aerogels was investigated. Supercritical drying carried out in CO2 results in a larger shrinkage of the gel network compared with the corresponding methanol-dried samples. In methanol-dried aerogels modified by 20 mol% methyl groups, the concentration of residual Si-OH groups is negligible and therefore the aerogels are hydrophobic, unlike the corresponding CO2-dried aerogels which have a higher residual OH concentration. Incorporation of 10 mol% of glycidoxypropyl or methacryloxypropyl groups is possible under mild supercritical drying conditions with CO2. Methanol as fluid leads to a degradation of the functional organic moiety.