Time-resolved SAXS studies on crystallization behavior of poly(ethylene isophthalate-co-ethylene terephthalate)s

Abstract

Small-angle X-ray scattering measurements using synchrotron radiation were carried out for poly(ethylene terephthalate) and poly(ethylene isophthalate-co-ethylene terephthalate)s. In addition, differential scanning calorimetric measurements were conducted. Measurements were made both on polymers undergoing isothermal crystallization and during subsequent remelting. The primary and secondary crystallization behaviors are examined. Isophthalate units were found to be excluded from the crystals into amorphous layers during crystallization. No crystal thickening was observed during isothermal crystallization, which may be due to the relatively high chain rigidity. Secondary crystallization, detected predominantly at the later stages of crystallization, causes densification and shrinkage of the amorphous layer. Considering the results, it is proposed that secondary crystallization involves the formation of short-range molecular order in the amorphous layers of a lamellar stack as well as in the amorphous regions between lamellar stacks. This short-range-ordered phase has a lower density than the lamellar crystal formed by primary crystallization.