Translational and rotational rearrangement regions governing β relaxations in polymer glasses

We investigate the relaxation spectrum of polymer glasses via classical molecular-dynamics simulations of dynamical mechanical spectroscopy. We observed the distinct β relaxation preceding the α relaxation, associated with a clear dynamical heterogeneity both on the translational displacements and on the rotational angles for the covalent bonds. The heterogeneous dynamics comes from the spatially localized mobile particles, rather than the facilitated dynamics in the same molecules, highlighting the importance of intermolecular interaction on β relaxation. We find that the percolation of mobile clusters over the system corresponds to the β relaxation, verified both on the translational and the rotational motions. The consistent percolation is due to the strong dynamical coupling between them. However, the temperature point Tmax corresponding to the maximum dynamical heterogeneity is lower than the β relaxation temperature. It corresponds to the crossover point where the competition in the fluctuations between the mobile regions and immobile regions is the strongest. This is validated by the maximum structural anisotropy for the geometric shape of the rearrangement regions found at Tmax.