Journal of Physical Chemistry, Vol.100, No.30, 12517-12531, 1996
Acidic Sol-Gel Polymerization of Teos - Effect of Solution Composition on Cyclization and Bimolecular Condensation Rates
For batch sol-gel polymerization of TEOS in acidic ethanol solutions, we report kinetic trends over a wide range of initial solution compositions that yield homogeneous gels. We find in these systems the prevalence of extensive nonrandom cyclization over a wide composition range. We unambiguously monitor the formation of various well-defined silicate oligomers early in the reaction to quantify the competing processes of cyclization and bimolecular condensation. We introduce a new kinetic model that is substantially modified from previously used random branching models in order to account for cyclization reactions, As do previous models, this model allows first-shell substitution effects on condensation rate constants, but it does not rely on mean field site kinetics. The dimerization rate constant varies strongly with pH in a manner consistent with a reaction mechanism involving ionized intermediates. it also decreases mildly with the water concentration experienced during reaction. Higher condensation rate constants vary among each other and with pH in a manner consistent with the expected ionization behavior. The cyclization rate constants are comparable in magnitude to the dimerization rate constant. Near the expected isoelectric point of the end-group, cyclization is favored over end-group/end-group bimolecular condensations. At higher pH though, bimolecular reactions are favored and there is no longer a negative first-shell-substitution effect. Dilution with ethanol increases the cyclization rate constants and tends to lower bimolecular rate constants, These results suggest a correlation between the compositions that favor homogeneous gelation and those that exhibit selective cyclization early in the reaction.
Keywords:NUCLEAR MAGNETIC-RESONANCE;AQUEOUS SILICATE SOLUTIONS;SI-29 NMR;HYDROLYTIC POLYCONDENSATION;KINETICS;TETRAETHOXYSILANE;MECHANISMS;RELAXATION;ALKOXIDES;GELATION