Monte Carlo Simulations of Size and Structure of Gel Precursors in Silica Polycondensation

Abstract

Sol−gel processing provides a useful route to novel metastable materials such as molecular hybrids of silica and either transition metal oxides or organic components. Because the properties of these materials depend critically on how the components are combined, we present calculations of the size and structure of silica building blocks that may be prepared as precursors to nanocomposites. By fitting existing silicon-29 NMR data, we find kinetic parameters applicable for acid catalyzed hydrolytic polycondensation of tetraethoxysilane and tetramethoxysilane precursors. In a wide range of conditions, the local connectivity of silicon sites evolves in approximately the same way with respect to the siloxane bond conversion. Using dynamic Monte Carlo simulations, including nearest-neighbor effects and cyclization, we calculate the molecular weight distribution of silica as a function of conversion, in reasonable agreement with available experiments. At low siloxane bond conversions (α ≤ 0.6) the mass weighted ...