Preparation, characterization and computer simulation of poly(dimethylsiloxane) and its composites

This dissertation investigated different aspects of poly(dimethylsiloxane) (PDMS) elastomer. There are four major topics in this dissertation. The first two topics are studies of the reinforcement of elastomeric networks by filler particles. The third topic is the molecular dynamics simulation of the diffusive behavior of small gas molecules in polymers. The last topic is the development of a FORTRAN program for the conformational analysis of PDMS.; . Reinforcement of poly(dimethylsiloxane) networks by filler particles with various size distributions and surface properties. The effects of filler particle size and size distribution, the filler/polymer interfacial interaction, and the surface properties on the reinforcement of PDMS were investigated experimentally. The filled PDMS composites were prepared through either blending technique or in-situ precipitation process. The sizes and size distributions of in-situ precipitated silica filler particles have been successfully controlled through the concentration of catalyst in a multi-step process. It was observed that higher stresses could be obtained by using smaller filler particles. In-situ precipitation technique was proved to have advantages over conventional blending technique, in terms of providing improved homogeneity with less filler particles aggregations, stronger polymer/filler interfacial interactions, and consequently better mechanical reinforcements.; . Simulations on the reinforcement of poly(dimethylsiloxane) elastomers by randomly-distributed filler particles. Monte Carlo computer simulations were carried out on filled networks of PDMS, which were modeled as composites of crosslinked chains and randomly arranged spherical filler particles. The primary concern of the investigation was the effect of the excluded volume of these particles on the elastomeric properties of the polymers. Calculations were carried out for PDMS chains with different molecular masses between cross links, and for filler particles with different sizes and at various volume percentages. Distributions of end-to-end vectors for both unfilled and filled networks were obtained using Monte Carlo simulations based on rotational isomeric state (RIS) theory. More extended configurations, with a higher end-to-end distance, were observed for networks filled with smaller particles. The nominal stress f* and the modulus or reduced nominal stress (f*) were calculated from the distributions of end-to-end vectors using the Mark-Curro approach. Relatively small filler particles were found to increase the non-Gaussian behavior and to increase the normalized moduli above the reference value of unity.; . Molecular Dynamics simulation of the diffusive behavior of small gas molecule through polymers. Molecular Dynamics (MD) simulations were performed to investigate the diffusive behavior of CO{dollar}sb2{dollar} in PDMS and polyethylene. Self-diffusion coefficients of polymer segments and CO{dollar}sb2{dollar} molecules were predicted based on their mean-square displacements. It was observed that the diffusion coefficients were greatly affected by density and temperature. Activation energies for diffusions were predicted according to the Arrhenius behavior of polymer segments and small gas molecules.; . Development of a FORTRAN program for conformational analysis of poly(dimethylsiloxane). A FORTRAN program was developed for complete conformational analysis of relatively flexible PDMS chains. Various internal parameters such as backbone and side chain torsional angles, bond angles and bond lengths were considered as independent variables. Metropolis method was employed to investigate the distributions of the internal variables.