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, usi ng the INSIGHT and DISCOVER programs from MSI Inc. with the Polymer Consort ium Force Field. The model system was built using the MSI amorphous cell co nstruction protocol with periodic boundary conditions. Starting from the se lf part of the van Hove correlation function, the incoherent intermediate s cattering 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 descri bed assuming decoupled segmental dynamics and rotations in a threefold pote ntial. 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 res ults and inelastic neutron scattering measurements. The glass transition wo uld thus hardly modify this distribution. (C) 2000 Elsevier Science B.V. Al l rights reserved.