Enthalpy relaxation processes near the glass transition temperature of poly (ethylene terephthalate)

This study attempted to correlate the changes observed in thermal properties with the dielectric properties of poly(ethylene terephthalate) PET glasses. It was of particular interest to examine the effect of annealing below the glass transition temperature (Tg). Several amorphous PET film samples were annealed at various temperatures below the glass transition {70°C). The dielectric spectra showed significant changes in the relaxation occurring in the annealed samples. In addition, volumetric studies of the annealed glasses showed changes in specific volume. In differential scanning calorimetry studies the absorbed thermal energy in the transition interval of annealed PET paralleled the enthalpy relaxation occurring during the annealing process. The spherulite size on crystallization decreased following annealing in the subglass region, as observed by small angle light scattering (SALS). This phenomenon is attributed to an increase in nucleation density. The excess amount of enthalpy and free volume quenched into the glassy systems decreased with sub-Tg annealing, thus bringing the system closer to equilibrium