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

Abstract

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