Secondary crystallization behavior of poly(ethylene isophthalate-co- terephthalate): Time-resolved small-angle X-ray scattering and calorimetry studies

Time-resolved small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) analyses were used to study the isothermal crystallization and remelting of poly(ethylene isophthalate-co-terephthalate)s containing 0-10 mol % isophthalate unit. For each of the polymers considered, evidence of the formation of both primary and secondary crystals was found in the SAXS pattern and its invariant Q, as well as in the DSC thermogram, which showed multiple melting endotherms. The melting of secondary crystals was found to shift significantly toward the high-temperature region with increasing crystallization time, indicating that the secondary crystals become thick and perfect over time. The thickness of the lamellar crystals was found to be very sensitive to the crystallization temperature, but no thickening was observed throughout the entire crystallization run, regardless of composition. The formation of secondary crystals, which favorably occurs during the later stages of crystallization, was found to cause a peak shift and an intensity increase in the SAXS pattern, a decrease in the SAXS invariant Q, and a decrease in the thickness of the amorphous layers in the lamellar stacks formed during primary crystallization. However, formation of secondary crystals was found not to be properly accounted for in the determination of the lamellar thickness from the SAXS patterns. The present results indicate that the secondary crystallization causes densification and shrinkage of the amorphous layers and that the resulting secondary crystals have lower electron density than the primary crystals. On the basis of the present results, we propose that secondary crystallization involves the formation of short-range molecular order in the amorphous layers of the lamellar stacks as well as in the amorphous regions between the lamellar stacks. This short-range ordered structure, which is likely a type of single thin lamella, thin lamellae, or fringed micelle-like order, has a lower electron density than the lamellar crystal formed by primary crystallization.