Coupling between crystal melting and rigid amorphous fraction mobilization in poly(ethylene terephthalate)

A thorough analysis of the melting behavior of poly(ethylene terephthalate) (PET) is detailed in this contribution. Isothermal crystallization at 190°C followed by cooling to room temperature provides a three-phase structure composed of a mobile amorphous, a crystalline and a rigid amorphous fraction. A close connection between multiple melting and devitrification of the rigid amorphous fraction in PET is revealed by conventional and temperature-modulated calorimetry. Rearrangements of PET crystals at high temperatures involve recrystallization/annealing/crystal perfection following partial melting, which can occur only if the amorphous chain portions coupled to the crystal/melt phase boundary have sufficient mobility. Such mobility can be achieved above the glass transition of the amorphous chain segments coupled with the just-melted crystals. Combined analysis of the reversing heat capacity monitored during quasi-isothermal modulation with the thermal properties of the resulting structure suggests that annealing at temperatures below 210°C does not result in considerable reorganization and perfection of the crystal phase. The temperature of 210°C seems to be the point at which the rigid amorphous fraction coupled with the crystal phase attains sufficient mobility to allow development of crystals with increased perfection and thus higher thermal stability.