Detailed atomistic simulation of the segmental dynamics and barrier properties of amorphous poly(ethylene terephthalate) and poly(ethylene isophthalate)

We present results from detailed atomistic simulations concerning the structural, conformational, dynamic, and barrier properties of the amorphous (glassy and melt) phases of two polyisomers, PET [poly(ethylene terephthalate)] and PEI [poly(ethylene isophthalate)]. First, well-relaxed atomistic configurations of the two polyesters are generated following a succession of equilibration stages consisting of energy minimizations, temperature annealings and coolings, and compressions-decompressions, as proposed by Hofmann et al. (Hofmann D.; et al. Macromol. Theory Simul. 2000, 9, 293). This equilibration cycle is significantly extended here by subjecting the resulting configurations to an additional molecular dynamics (MD) simulation at a high temperature for 2 ns, affording extra relaxation at all length scales. With the statistically uncorrelated configurations generated via these extended equilibration runs, isothermal-isobaric (NPT) at P = 1 atm as well as canonical (NVT) (with the densities set at the corresponding experimentally measured values) MD simulations are performed in the melt state at T = 450 and 600 K to probe differences in the structure and segmental dynamics of the two polyesters. The simulations reveal significant differences in the local relaxation dynamics of the phenyl rings between the two polyisomers: In PET, these rings are found to exhibit significantly higher mobility than in PEI; this is attributed to the way in which phenyls are connected to their adjacent ester groups. Structural and conformational properties, on the other hand, are predicted to be practically identical in the two polyesters. Finally, transition-state theory (Gusev, A. A.; Suter, U. W. J. Chem. Phys. 1993, 99, 2228) is employed to calculate the rate constants of diffusive jumps between sorption sites, and hence the low-concentration self-diffusivity, of oxygen (O-2) molecules in well-relaxed atomistic configurations of the two polymers, whose densities exactly match the experimentally measured values. Calculated 02 diffusivities are found to be in excellent qualitative and quantitative agreement with experiment: 02 diffusivity in PEI is predicted to be 1.8 times smaller than in PET, in agreement with the experimental finding that PEI is 2-2.5 times less permeable to O-2 than PET in the amorphous state.