Influence of the nature of organic groups on the properties of organically modified silica aerogels

Organically modified aerogels were prepared by NH₄OH-catalyzed hydrolysis and condensation of RSi(OMe)₃ (R = Me, Pr ⁿ , Ph, Bu ⁱ )/Si(OMe)₄ or (MeO)₃Si-Y-Si(OMe)₃/Si(OMe)₄ (Y = C₂H₄, p-C₆H₄, C₆H₁₂) mixtures, followed by supercritical drying of the alcogels with methanol. Starting from 1:4 mixtures of RSi(OMe)₃ and Si(OMe)₄, hydrophobic aerogels with nearly no residual Si-OH or Si-OMe groups were obtained. These aerogels were therefore insensitive towards moisture. Their elastic constant was distinctly lower than that of unmodified silica aerogels. Aerogels similarly prepared from 1:8 mixtures of (MeO)₃Si-Y-Si(OMe)₃ and Si(OMe)₄ had a rather high concentration of residual Si-OMe groups, and therefore they were hydrophilic. Their elasticity was about the same as that of unmodified silica aerogels. The difference between the two types of aerogels suggests different microstructures, depending on the nature of the organic groups.