Molecular dynamics simulation study of the alpha- and beta-relaxation processes in a realistic model polymer

Molecular dynamics simulations of a melt of freely rotating chains of 1,4-polybutadiene (FRC-PBD) have been performed over a wide range of temperature. Removal of the dihedral barriers in FRC-PBD allows for complete resolution of the Johari-Goldstein beta process from the primary alpha process in the simulation time window. We find that relaxation in the beta regime occurs as the result of large-angle excursions of all backbone dihedrals. While during the beta process each dihedral visits all available states, the complete relaxation of torsional autocorrelation function (alpha process) occurs only when the polymer matrix shows significant motion and each dihedral populates all states with equilibrium probability.