Origin of the Distribution of Potential Barriers for Methyl Group Dynamics in Glassy Polymers: A Molecular Dynamics Simulation in Polyisoprene

We have carried out molecular dynamics simulations of methyl group torsional librations in glassy polyisoprene at 150 K using the Insight and Discover codes from MSI Inc. with the Polymer Consortium Force Field. The model system was built using the MSI Amorphous Cell construction protocol with periodic boundary conditions. During the NVT molecular dynamics runs, the dihedral 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 generally assumed threefold approximation for the single particle methyl group potential. The density of states for methyl group torsional librations, as calculated from the time evolution of the dihedral angles, agrees quite well with previous inelastic neutron scattering results and shows a broad feature reflecting a distribution of potential barriers. This distribution is quantified in the framework of the threefold approximation. Performing similar simulations under different conditions for the nonbond interactions considered in the used force field, we conclude that the width of this distribution is mainly controlled by the nonbond interactions. Moreover, it turns out that these nonbond interactions also contribute significantly to the value of the average barrier for methyl group reorientation.