Influence of unlimited 3-membered ring cyclization on a multiscale dynamic Monte Carlo/continuum model of drying and curing in sol-gel silica films

The coating process of sol-gel silica films involves multiple length and time scales ranging from molecular to macroscopic. At the molecular scale, cyclization during polymerization is so extensive that it cannot be neglected or treated statistically. Here we present a multiscale model of a 1D sol-gel drying process coupled to a Monte Carlo polymerization model with unlimited cyclization. Because our model allows cyclic and cage-like siloxanes to form, it is better able to predict the silica gelation conversion than any other reported kinetic model. By studying the competition between molecular growth and cyclization, and the competition between mass transfer (drying) and reaction (gelation) on the drying process of the sol-gel silica film, we observe that cyclization delays the gelation, shrinks the molecular size, increases the likelihood of literal skinning, and leads to a molecular structure gradient inside the film. Although for simplicity our model just considers the possibility of forming 3-membered ring, it is the first multiscale model to couple unlimited cyclization in polycondensation with a continuum mass transfer process. Also it is the first model that can predict structure gradients caused by drying in sol-gel silica films.