Chain dynamics and relaxation in amorphous poly(ethylene terephthalate): A molecular dynamics simulation study

Chain dynamics in amorphous poly(ethylene terephthalate) have been investigated using molecular dynamics simulations. A previously developed model was adopted with some improvements. Torsional potentials were modified to reflect the results of quantum chemistry calculations and also to accommodate more precise experimental knowledge of the gauche/trans ratio of the glycol ethylene bond. The simulation system size was significantly increased. Autocorrelation functions (ACFs) for the system dipole moment, individual dipole moments, and phenyl ring normals were collected. The temperature range studied corresponds largely to the high temperature merged alpha and beta processes, but the lower end reached the high-temperature limit of experimental dielectric measurements of the beta relaxation. ACF relaxation times are in agreement with the position and activation energy of the experimental beta peak. Activation energies of conformational transition rates for the three dynamically flexible bond types are very similar to the effective torsional barriers. A disparity between the temperature dependence of relaxation times and conformational transition rates was observed, as well as the development of dynamic heterogeneity with respect to the locations and rates of conformational transitions of individual bonds. Correlations of transitions within a monomeric unit were monitored, and considerable correlation swa found between transitions at the phenyl link and the nearby glycol C-O bond.