Recrystallization studies on isotropic cold-crystallized PET: Influence of heating rate

The nanostructural changes associated to the multiple melting behaviour of isotropic cold-crystallized poly(ethylene terephthalate) (PET) have been investigated by means of simultaneous wide- and small-angle X-ray scattering, using a synchrotron radiation source. Variations in the degree of crystallinity, coherent lateral crystal size and long period values, as a function of temperature, for two different heating rates are reported for cold-crystallized samples in the 100-190 °C range. The Interface Distribution Function analysis is also employed to provide the crystalline and amorphous layer thickness values at various temperatures of interest. Results suggest that samples crystallized at both low (Ta = 100-120 °C) and high (Ta = 160-190 °C) temperatures are subjected to a nearly continuous nanostructural reorganization process upon heating, starting immediately above Tg (≈80 °C) and giving rise to complete melting at ≈260 °C. For all the Ta investigated, a melting-recrystallization mechanism seems to take place once Ta is exceeded, concurrently to the low-temperature endotherm observed in the DSC scans. For low-Ta and slow heating rates (2 °C/min), a conspicuous recrystallization process is predominant within Ta + 30 °C ≤ T ≤ 200 °C. In contrast, for high-Ta, an increasingly strong melting process is observed. For both, high- and low-Ta, an extensive structural reorganization takes place above 200 °C, involving the appearance of new lamellar stacks simultaneously to the final melting process. The two mechanisms should contribute to the high-temperature endotherm in the DSC scan. Finally, the use of a high heating rate is found to hinder the material's overall recrystallization process during the heating run and suggests that the high-temperature endotherm is ascribed to the melting of lamellae generated or thickened during the heating run.