SOL-GEL POLYMERIZATION IN ALKOXYSILANES - SI-29 NMR-STUDY AND SIMULATION OF CHEMICAL-KINETICS

We report on the results of Si-29 NMR monitoring of alkoxysilane/ethan ol/acid water sol-gels with tetraethoxysilane (TEOS), methyltriethoxys ilane (MTEOS) and vinyltriethoxysilane (VTEOS). The gel time of the TE OS solution was 49 days at the degree of condensation c = 0.81, wherea s MTEOS and VTEOS do not gel in spite of the high c = 0.95 achieved. T he polymerization kinetics progressively slows with the time owing to the growth of sterical restrictions. To account for the effect we deve loped a kinetic model based on the reaction rates which explicitly dec ay as a function of the degree of condensation. The model describes th e complete sol-gel process in all the studied systems including a gel transition in TEOS and aging of the gel.