Vitrification of the Entire Amorphous Phase during Crystallization of Poly(butylene terephthalate) near the Glass-Transition Temperature

The isothermal crystallization kinetics of poly(butylene terephthalate) at low temperatures near the glass transition was investigated by using chip-sensor fast scanning calorimetry. The Avrami analysis revealed that the Avrami index underwent a systematic change from 4 to less than 2 in the low-temperature peak of the crystallization rate, and significantly low crystallinity was achieved in the primary stage near the glass-transition temperature. The mechanism was ascribed to the inhibition of the crystal growth by the rigid amorphous fraction (RAF) that is constrained by crystals, as proposed by Schawe for other crystalline polymers. A mobile amorphous fraction (MAF) as opposed to RAF can be evaluated by temperature-modulated fast scanning calorimetry. Near the glass-transition temperature, MAF was confirmed to transform into RAF with the progress of crystallization, and all amorphous fractions became rigid.