Reactive modeling of the initial stages of alkoxysilane polycondensation: effects of precursor molecule structure and solution composition.

Reactive molecular dynamics simulations were performed to study the polycondensation of alkoxysilane in solution with alcohol and water. The dynamic formation of siloxane clusters and rings was observed with simulation time. Two mechanisms for the growth of siloxanes were observed: monomer addition and cluster-cluster aggregation. The impacts of the alkoxysilane monomer chemical structure and solution composition on the rates of hydrolysis and condensation were explored. The polycondensation of different precursor alkoxysilane monomers (tetramethoxysilane, trimethoxysilane, methyltrimethoxysilane, or tetraethoxysilane) was modeled. The steric bulk of chemical groups attached to the monomer, such as silyl or alkoxy groups, were found to impact reaction rates. The influence of solution composition was investigated by simulating multiple systems with different concentrations of tetramethoxysilane, methanol, and water. Reactive molecular dynamics is used for the first time to study the polycondensation of alkoxysilanes, creating opportunities for future theoretical studies of the sol-gel process.