Computer simulation study on sol-gel polymerization of Tetraethoxysilane(TEOS)

Abstract

This thesis is made up of two points of views on the sol-gel polymerization of $TEOS(Si(OC_2H_5)_4)$. First, the change of degree of polymerization and gelation time during sol-gel process of TEOS has been calculated with Monte Carlo simulation. Secondly, the structural changes during sol-gel process of silicic acid molecules$(Si(OH)_4)$ has been studied with Molecular Dynamics simulation. Chapter 2. The effects of the water/TEOS ratio, pH, temperature and alcohol/TEOS ratio on the degree of polymerization and gelation time in the acid-catalyzed sol-gel polymerization of TEOS have been studied systematically with Monte Carlo simulation. Although linear polymerization by bimolecular reaction was assumed, the reversibility of hydrolysis and condensation, the substitution effect and the effect of molecular separation were considered. The enhanced gelation with water/TEOS ratio resulted from increased water-producing condensation between fully hydrolyzed chain polymers. When pH varies, the fastest gelation occurs when the ratio of condensation to hydrolysis rate moves toward 1 as successive condensation after hydrolysis becomes possible. The accelerating effect of temperature enhances diffusion and increases the rate ratio of condensation to hydrolysis. Alcohol retards gelation due to its dilution effect. The qualitative results obtained in this model explain the experimental behaviors of sol-gel polymerization of TEOS, especially the change in the degree of polymerization, and gelation time with the experimental variables. Chapter 3. The structural change during sol-gel polymerization of $Si(OH)_4$ with temperature and pressure has been studied using molecular dynamics simulation. $Si(OH)_4$ system and $Si(OH)_4$ with $H_2O$ system were prepared to study water effect on structure. Intermolecular interaction distances obtained from radial distribution function results agree to other MD results. In $Si(OH)_4$ system, larger ring-like structures shows higher relative...