MOLECULAR MODELING OF POLYCARBONATE .1. FORCE-FIELD, STATIC STRUCTURE, AND MECHANICAL-PROPERTIES

Molecular simulations of Bisphenol A polycarbonate were performed using a modified version of the Dreiding force field. In general, the simplicity of this generic force field was maintained. However, a few parameters were optimized by using ab initio calculations in order to generate better backbone torsional potentials. The validity of this modified force field was tested on a model compound similar to polycarbonate, 4,4'-isopropylidenediphenylbis(phenyl carbonate). The crystallographic data obtained from simulations of the model compound agreed well with the experimental data. The modified force field was then used in simulations of the glassy polymer. Model amorphous structures of Bisphenol A polycarbonate were built and optimized using periodic boundary conditions. The structure factor, S(k), was calculated from pair distribution functions of the model structures. The peak positions in calculated S(k) compare well with those obtained experimentally. The effects of the initial density on chain packing were also studied. Its impacts on the final density and energy are described. A general approach for calculating the stiffness matrix of any shape unit cell (triclinic system) is presented. A yield phenomenon for the model system was observed at about 10% strain. This value is comparable with experimental results (6-8%). The yield stress (0.25 GPa), however, was higher than experimental values, as was the Young's modulus at small strain. These results could be partially explained by the fact that the calculated mechanical properties represent an ideal structure and thus provide the upper limit values for the polymer.