The effects of physical ageing on conformational changes of poly(ethylene terephthalate) in the glass transition region

In-situ FTIR studies on the conformational changes of poly(ethylene terephthalate) (PET) films with different thermal histories, i.e., quenched from the melt and sub-T-g annealed at different temperatures, were carried out while increasing the temperature through the glass transition region. The temperature dependencies of the infra-red spectra showed 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 abrupt changes of trans conformers were related with the temperature of sub-T-g annealing. An energy difference of 28.6 J/g between the trans and gauche conformational states was measured by the infra-red spectroscopic method. The energy absorbed during the abrupt conformational change represents only part of the energy of the endothermic peak observed by differential scanning calorimetry. The results are explained in terms of the formation of new cohesional entanglements during sub-T-g annealing.