THE INFLUENCE OF CROSS-LINKING ON GEL FORMATION

Besides the rate of the hydrolysis and condensation reactions, cross-linking also has a strong influence on gel formation of tetraethoxysilane (TEOS). Parameters which have an effect on cross-linking, as the water concentration, the functionality of the precursor and the addition of tetraethoxygermane (TEOG) are investigated. Gelation time versus hydrolysis time curves were recorded of different TEOS, ethanol, water systems with different water concentrations and systems to which different alkyl-substituted silanes and TEOG were added. It is found that the polymerization reaction of TEOS is dominated by silanol-silanol condensation whereas for condensation of TEOG an ethoxy group reacts with silanol under ethanol formation. It is also found that the gelation process of mixtures to which an alkyl-substituted silane is added, is slower than that of TEOS itself although both the hydrolysis rate and the condensation rate of each of these compounds are higher.