Methyl group dynamics above the glass transition temperature: a molecular dynamics simulation in polyisoprene

We have carried out molecular dynamics (MD) simulations of polyisoprene at 363 K, about 150 K above the experimental glass transition temperature, using the INSIGHT and DISCOVER programs 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. Starting from the self part of the van Hove correlation function, the incoherent intermediate scattering function was calculated for the protons in the main chain and in the methyl groups (MGs). The dynamics of the latter ones can be well described assuming decoupled segmental dynamics and rotations in a threefold potential. The limits of such an approximation are also discussed. We find the existence of a distribution of potential barriers for MG rotation which is very similar to that deduced from low temperature (150 K) MD-simulation results and inelastic neutron scattering measurements. The glass transition would thus hardly modify this distribution. (C) 2000 Elsevier Science B.V. All rights reserved.