A COMPARISON OF A UNITED ATOM AND AN EXPLICIT ATOM MODEL IN SIMULATIONS OF POLYMETHYLENE

We have carried out stochastic dynamics and molecular dynamics simulations of n-tridecane (C13H28) as isolated chains, in bulk melts, and in confined melts between solid surfaces, employing both a united atom (UA) model and an explicit atom (EA) model, in order to compare chain conformations, packing, orientational correlations, and self-diffusion predicted by the UA and EA models. The EA model, which explicitly takes into account all hydrogens, exhibits nearly identical results for chain conformations to those from the UA model. However, only the EA model, which shows considerably enhanced interchain packing and orientational correlations m the melts over those for the UA model, reproduces very closely the height and the width of the interchain peak in the experimental x-ray scattering profile. Dynamically, inclusion of explicit hydrogens decreases the self-diffusion constants in the melts by a factor of 6-8, resulting in reasonably good agreement with the experimental value. Moreover, in the melts confined between solid surfaces, the presence of explicit hydrogens leads to much more pronounced layering of both the monomer segments and the entire molecules, which are strongly oriented along the solid surfaces.