Characterization of the sol-gel process using raman spectroscopy organically modified silica gels prepared via the formic acid alkoxide route

For precursor mixtures containing tetraethoxysilane (TEOS) and phenyltriethoxysilane (PhTREOS), time of gelation can be reduced by up to two orders of magnitude depending on reaction conditions employed when reacting the silicon alkoxide mixture with formic acid instead of water. Results indicate that time of gelation depends on the amount of PhTREOS in the precursor mixture. Within the range of concentrations investigated, an exponential law describes best the dependence of reduced time of gelation on the molar fraction of PhTREOS. Therefore, we conclude that the phenyl ring acts as a steric hindrance to network formation. Raman spectroscopy is used to characterize the reaction between the alkoxide mixture and formic acid. During the acidolysis reaction, ethanol is formed as an intermediate. A preliminary reaction scheme is proposed to account for the time dependence of species involved. Furthermore, Raman spectroscopy is successfully employed to monitor the effects of post-gelation thermal treatment of the gel samples. The effects observed are interpreted with a model of a phenyl ring trapped in a siloxane cage.