A study of the glass transition in the amorphous interlamellar phase of highly crystallized poly(ethylene terephthalate)

Abstract

The glass transition of poly(ethylene terephthalate) (PET) crystallized for 4 h at temperatures between 413 and 453 K was studied. Secondary crystallization processes were monitored by differential scanning calorimetry and the glass transition of the remaining interlamellar amorphous phase was studied by thermally stimulated depolarization currents measurements. Non-isothermal window polarization is employed to resolve the relaxation in modes with a well-defined relaxation time that is subsequently adjusted to several standard models. An analysis of experimental results reveals that cooperativity can be disregarded in the modelization of data. The evolution of modes during secondary crystallization, once primary crystallization has been completed, gives more weight to lower energy modes. As a consequence, secondary crystallization tends to lower the glass transition temperature of the amorphous interlamellar phase, although remaining noticeably higher than in amorphous samples. The evolution of calorimetric scans of the glass transition is simulated from the obtained results and shows the same behaviour. Regarding the glass transition temperature of the material, it can be concluded that primary and secondary crystallization act in opposite directions even though the effect of secondary crystallization is much smaller. The interpretation of these results in terms of current views about secondary crystallization is discussed.