Influence of physical aging on the molecular motion and structural relaxation in poly(ethylene terephthalate) and related polyesters

A combination of dielectric relaxation, dynamic mechanical thermal analysis , and positron annihilation measurements is reported on melt-cast films of poly(ethylene terephthalate), poly(ethylene naphthalate), and their copolym ers. The effects of change in the chemical structure of the polyesters are rationalized in terms of increased restriction of the mobility of the polar segments of the chain by the incorporation of the bulky naphthalate struct ure. All the quenched samples exhibit physical aging when raised to elevate d temperatures, the rate depending on the degree of undercooling used in th e aging experiments. The free volume surprisingly does not change significa ntly with temperature and leads to the suggestion that the reduction in the dielectric permittivity is a consequence of a reduction in the mobility of the local segments. The increased storage modulus is also consistent with a reduction in mobility. This proposal is further confirmed by the observat ion of a good correlation between the rates and extents of the physical agi ng as detected by dynamic mechanical and dielectric relaxation measurements . Similarities in the activation energies of the beta relaxation process fo r all the polymers investigated indicate that the dipole relaxation process es have a common origin and can be ascribed to motion of the linking polar entity. This study implies that aging is accompanied by an increase in orde ring within these polyesters.