| Abstract: | The abrupt changes in the linear expansion and dilatometric measurements of poly-(ethylene terephthalate) near 70°C indicate that the transition may be first order. DTA and DSC data show the normal heat capacity change at 70°C, while annealing for for 30 min. at 110°C induces further molecular order. Rapid quenching of PET after heating to 120°C yields an amorphous sample and measurement by depolarized light transmission as the sample is reheated shows rapid crystallization occurring near 80°C. With the occurrence of shrinkage near 68°C and apparent recrystallization at slow heating rates by depolarized light analyses, further support is added to the first-order character of this phase transition. Isothermal annealing for short times at temperatures between 50°C and 80°C produces further increases in molecular order. Cyclic and linear PET oligomers do not contribute to the occurrence of the 70°C transition. |
