Origin of the distribution of potential barriers for methyl group dynamicsin glassy polymers: A molecular dynamics simulation in polyisoprene

We have carried out molecular dynamics simulations of methyl group torsiona l librations in glassy polyisoprene at 150 K using the Insight and Discover codes from MSI Inc. with the Polymer Consortium Force Field. The model sys tem was built using the MSI Amorphous Cell construction protocol with perio dic boundary conditions. During the NVT molecular dynamics runs, the dihedr al angle of each of the methyl groups and the positions of all of the atoms were recorded at 10 fs intervals. The results obtained support the general ly assumed threefold approximation for the single particle methyl group pot ential. The density of states for methyl group torsional librations, as cal culated li-om the time evolution of the dihedral angles, agrees quite well with previous inelastic neutron scattering results and shows a broad featur e reflecting a distribution of potential, barriers. This distribution is qu antified in the framework of the threefold approximation. Performing simila r simulations under different conditions for the nonbond interactions consi dered in the used force field, we conclude that the width of this distribut ion is mainly controlled by the nonbond interactions. Moreover, it turns ou t that these nonbond interactions also contribute significantly to the valu e of the average barrier for methyl group reorientation.