Reactive Molecular Dynamics Simulations of Siliceous Solids Polycondensed from Tetra- and Trihydroxysilane

The chemical structure of siloxanes polycondensed from tetra- and trihydroxysilane monomers is investigated with reactive molecular dynamics. Solids of varying densities, ranging from porous gels to glasses are obtained through an iterative scheme in which silane is periodically added to the reacting system. It is found that tetrahydroxysilane forms denser solids, with smaller siloxane rings, and a greater concentration of siloxane bridges in comparison to trihydroxysilane, which produces networks of silsesquioxane clusters and sheets. The thermal treatment of solids produced from tetrahydroxysilane is modeled after drying, which leads to densification, and a reduction of porosity and silanol content. Dense solids with low silanol concentration are found to be less susceptible to shrinkage during thermal treatment.