Molecular Dynamics Simulation of Atactic Polystyrene. 3. Short Range Order

A molecular dynamics simulation of bulk atactic polystyrene was performed, and the result was analyzed in detail to evaluate the intramolecular and intermolecular short range orders present. In this work the analysis was performed only on the results obtained from the united atom model, since, as was shown in the previous two publications, it gives much better agreement with experiment than the all atom model. Most of the study was performed with a system containing a single 80-monomer molecule, but some study was also made with a system containing two 40-monomer molecules. The short range order was examined by evaluating the radial distribution functions and orientational order parameters of pairs of aliphatic backbone segments and of phenyl group centers. In the case of intramolecular phenyl pairs, these quantities were evaluated individually according to the monomeric distance along the chain by which the two phenyls are separated. The conformation of the molecule was examined by evaluating the population distribution of backbone bond torsional angles in meso and racemic diads. The result resembles the conformational energy maps previously prepared by others, but some differences were also noted. The most prominent feature in the intermolecular correlation is the strong tendency for the phenyl groups to have a separation distance of about 6. A detailed analysis was given to define the relative orientation that the pair of phenyls adopt at this separation. Intermolecular pairs of backbone segments were found to be oriented mostly randomly at separation distances larger than 6 A