SIZE DISTRIBUTION OF POLYMERS IN SOL-GEL CONDENSATION

Silicon tetraethoxide (TEOS) monomers have been dissolved in ethanolic solutions and hydrolyzed to form reactive, multifunctional monomers. These monomers polymerize spontaneously; they form a collection of branched polymers which overlap each other and entrap the solvent into a macroscopic gel. At various times during the reaction, individual polymers have been extracted from the reaction bath and their size distribution has been examined through size-exclusion chromatography coupled with low-angle light scattering. We report results from the early times in the reaction, before the polymers have overlapped. The number distribution of polymers with mass M can be approximated by a power law n(M) almost-equal-to M-tau with tau = 2 and cutoffs at the lowest and highest mass. As the reaction time increases, the distribution shifts as a whole to higher mass, and both cutoffs remain proportional to each other. The growth of the average mass with reaction time is exponential. These features are in general agreement with predictions from kinetic models where the polymers diffuse through the solvent and recombine with a low reaction probability.