The polyester called PET, with a chemical structure comprising
terephthalic acid and ethylene glycol, is a widely used
thermoplastic, TP. The physical state of PET depends on its thermal
prehistory and phase structure. PET is a semi-crystalline polymer
that can be fully amorphous when rapidly cooled from the melt. At
fast cooling below the glass transition temperature, Tg, the motion
of macromolecules is insufficient to allow them to crystallize. Then
PET does not crystallize. If amorphous PET is further annealed
above Tg, it crystallizes. The phenomenon is called “cold”
crystallization [1]. If cooled slowly from the melt, PET forms
crystals that can be well characterized, e.g., applying the X-ray
diffraction method [2].