SI-29 NMR MONITORING AND KINETIC MODELING OF AN ACID-CATALYZED TMOS SOL-GEL SYSTEM WITH MOLAR H2O/SI = 8

Simplified kinetics are used to describe the rates of hydrolysis and condensation in a Si(OCH3)4/CD3OD/H2O/HCl sol-gel system with H2O/Si = 8. The kinetics of the system can be expressed by three principal reactions: hydrolysis, esterification and water-producing condensation. The statistical model of Kay and Assink [J. Non-Cryst. Solids 104 (1988) 112] extended with a multiplicative factor, R, representing the increasing steric and inductive effects with progressing condensation [Brinker and Assink, J. Non-Cryst. Solids 11 1 (1989) 48]. is additionally corrected for the esterification, which plays an important role in sol-gel systems with H2O/Si greater-than-or-equal-to 4. Numerically simulated concentration curves of species calculated with this model are compared with experimental data. In contradiction with the linear statistical model, good fittings are obtained with the extended Kay-Assink-Brinker (KABe) model when the experimental data are used without Correction for intramolecular condensation. By simple reduction of the cyclic oligomers/species to their linear equivalents assuming (1) an additional or (2) a competitive intramolecular condensation, the model calculations do not fit. These results point to a more complex intramolecular condensation or to the failure of the model to simulate sol-gel processes attendent with intramolecular condensation.