MOLECULAR-DYNAMICS SIMULATION OF ATACTIC POLYSTYRENE .1. COMPARISON WITH X-RAY-SCATTERING DATA

A molecular dynamics simulation of bulk atactic polystyrene was performed. A united atom model and an all atom model were developed, in both of which the bond lengths were held fixed and the phenyl group was represented by a rigid, planar hexagon. The degree of success of the simulation was examined by comparing the calculated X-ray scattering intensity curve with experiment. The united atom model reproduces the so-called ''polymerization peak'', and the calculated curve agrees well with one of the most recently published experimental data. The degree of agreement achieved is in fact comparable to the degree to which two published experimental curves agree with each other. The agreement in the radial distribution function between the calculated and experimentally derived one is good in the r range smaller than 9 angstrom but deteriorates somewhat for larger r. There are again uncertainties in the experimental radial distribution function itself at large distances, however, as can be seen by comparing experimental curves obtained by two different groups. In contrast to the united atom model, the all atom model was found to give a result which clearly disagrees with experimental results, and one of the commercially available software gave an even worse result. The nature of smearing that accrues in the experimentally determined radial distribution function, as a consequence of the differing q dependencies of C and H atomic scattering factors and the limited q range accessible to experiment, has been examined by calculating the radial distribution function directly from the simulation result.