Multiscale modeling of the effects of unlimited four-membered ring formation on sol-gel silica film molecular structure

A modified multiscale model of a sol-gel silica drying process is presented that treats first-shell substitution effects and unlimited four-membered ring cyclization. The inclusion of four-membered (8-atom) rings allows the model to simulate the formation of the tetrasiloxane rings and cubic silsesquioxane cages known to be prevalent components of sol-gel silica systems, as well as a full range of other ring-containing structures. The polymerization process is treated using a dynamic Monte Carlo method where a discrete population of one million monomers evolves according to kinetic rules, including unimolecular-like closure of three-bond blocks. Compared with prior simulations with extensive cyclization that allowed only three-membered rings, the molecular structures formed with unlimited four-membered ring are more complex, and the occurrence of “skinning” (the rapid formation of a gel only at the surface of the film) is more pronounced and leads to more severe structure gradients. © 2012 American Institute of Chemical Engineers AIChE J, 59: 707–718, 2013