Year of Publication

2008

Degree Name

Doctor of Philosophy (PhD)

Document Type

Dissertation

College

Engineering

Department

Chemical Engineering

First Advisor

Dr. Stephen E. Rankin

Abstract

Modeling the competition between drying and curing processes in polymerizing films is of great importance to many existing and developing materials synthesis processes. These processes involve multiple length and time scales ranging from molecular to macroscopic, and are challenging to fully model in situations where the polymerization is non-ideal, such as sol-gel silica thin film formation. A comprehensive model of sol-gel silica film formation should link macroscopic flow and drying (controlled by process parameters) to film microstructure (which dictates the properties of the films).

This dissertation describes a multiscale model in which dynamic Monte Carlo (DMC) polymerization simulations are coupled to a continuum model of drying. 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. 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. The validity of the multiscale model is verified by the good agreement of the conversion evolution of DMC and continuum simulations for ideal polycondensation and first shell substitution effect (FSSE) cases.

Because our model allows cyclic and cage-like siloxanes to form, it is better able to predict the silica gelation conversion than other reported kinetic models. 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 gelation, shrinks the molecular size, increases the likelihood of skin formation, and leads to a molecular structure gradient inside the film. We also find that compared with a model with only 3-membered rings, the molecular structure is more complicated and the structure gradients in the films are larger with 4- membered rings. We expect that our simulation will allow better prediction of the formation of structure gradients in sol-gel derived ceramics and other nonideal multifunctional polycondensation products, and that this will help in developing procedures to reduce coating defects.

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