A 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 that are largely decoupled from the dynamics of the polymer matrix, while the $\alpha $-relaxation exhibits strong coupling between matrix motion and polymer dihedral relaxation.