(485a) Multiscale Modeling of Cyclization Effects in Drying Sol-Gel Silica Films

Sol-gel silica films are of growing interest in industry practice and as sources of novel materials. The process of forming sol-gel silica films by dip coating couples polymerization and drying, i.e., it involves multiple length and time scales ranging from molecular to macroscopic. Therefore, a multiscale model is necessary to better understand and predict the outcome of this process. Here, we present a multiscale model which links film microstructure (which dictates the properties of the films) to macroscopic flow and drying (controlled by process parameters). Because silica polymerization is highly nonideal relative to the random branching theory of Flory and Stockmayer (e.g. extensive cyclization reactions are known to occur), dynamic Monte Carlo (DMC) simulation is the best choice to model this process. The simulation tracks the populations of site pairs and bond blocks to derive the rates of bimolecular reactions and cyclization reactions, respectively. Unlike statistical methods, DMC simulations track the entire molecular structure distribution to allow the calculation not only of molecular weight but also of cycle ranks and topological indices related to molecular size and shape. These topological indices can be used for improved correlations of transport coefficients in polymers with different degrees of branching and cyclization. The entire DMC simulation (containing 10⁶ monomers) is treated as a particle of sol whose position and composition are tracked in the continuum mass transport model of drying. 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 is able. By simulating a swarm of particles starting from different positions in the film and using variable parameters, we observe the effect of drying parameters on the gelation regime, predict different drying/gelation phenomena, and predict the occurrence of gradients of concentration, gelation, and structure in the films. The difference in gelation conversion between the top and bottom of a film is adopted as a measure of the strength of the molecular structure gradient across a film, and the influence of dimensionless numbers to describe reactive transport is discussed.