New insight into melting and crystallization behavior in semicrystalline poly(ethylene terephthalate)

After isothermal crystallization, poly(ethylene terephthalate) (PET) showed double endothermic behavior in the differential scanning calorimetry (DSC) heating scan. During the heating scans of semicrystalline PET, a metastabl e melt which comes from melting thinner lamellar crystal populations formed between the low and the upper endothermic temperatures. The metastable mel t can recrystallize immediately just above the low melting temperature and form thicker lamellae than the original ones. The thickness and perfection depends on the crystallization time and crystallization temperature. The cr ystallization kinetics of this metastable melt can be determined by means o f DSC. The kinetics analysis showed that the isothermal crystallization of the metastable PET melt proceeds with an Avrami exponent of n = 1.0 similar to 1.2, probably reflecting one-dimensional or irregular line growth of th e crystal occurring between the existing main lamellae with heterogeneous n ucleation. This is in agreement with the hypothesis that the melting peaks are associated with two distinct crystal populations with different thickne sses. (C) 2000 John Wiley & Sons, Inc.