Using force field simulations for the evaluation of the monomer parameters for the calculation of diffusion constants for long chain polymer melts

Force field simulations are employed in the evaluation of the monomer interaction diameter used in the lateral motion (LM) model for the dynamics of polymer melts. Interaction diameters are obtained for polystyene (PS), polyethylene (PE), poly(ethylene oxide) (PEO), and poly(dimethylsiloxane) (PDMS) from two- and three-chain configurations. It is found that these interaction diameters result in diffusion constants using the LM model that are in good agreement with experimental data. If the interaction diameter is known, the only adjustable parameter in the diffusion constant calculations is the bare monomer diffusion constant, D-0. The calculation of D-0 from short chain. short time force field molecular dynamics simulations is attempted for PE, PEG, and PDMS. The bare diffusion constant obtained for PE is found to provide accurate diffusion constants for long chain melts from LM calculations with no adjustable parameters. Values for D-0 were not obtained from the simulations performed for PEO and PDMS, because the monomers do not move significantly on the time scale of the simulations for these polar systems.