THE EFFECTS OF PHYSICAL AGING ON CONFORMATIONAL-CHANGES OF POLY(ETHYLENE-TEREPHTHALATE) IN THE GLASS-TRANSITION REGION

In-situ FTIR studies on the conformational changes of poly(ethylene te rephthalate) (PET) films with different thermal histories, i.e., quenc hed from the melt and sub-T-g annealed at different temperatures, were carried out while increasing the temperature through the glass transi tion region. The temperature dependencies of the infra-red spectra sho wed that incremental changes of the population of trans conformers in the quenched sample are gradual, while rather abrupt changes occur in the sub-T-g annealed samples. The magnitude and temperature of the abr upt changes of trans conformers were related with the temperature of s ub-T-g annealing. An energy difference of 28.6 J/g between the trans a nd gauche conformational states was measured by the infra-red spectros copic method. The energy absorbed during the abrupt conformational cha nge represents only part of the energy of the endothermic peak observe d by differential scanning calorimetry. The results are explained in t erms of the formation of new cohesional entanglements during sub-T-g a nnealing.