Characterization and analyses on complex melting, polymorphism, and crystal phases in melt-crystallized poly(hexamethylene terephthalate)

Techniques of thermal analysis, wide-angle X-ray diffraction, and small-angle X-ray scattering were used to reveal relationships between complex melting behavior and various crystal forms in a polymorphic polymer. Correlation between the multiple melting endotherms and polymorphic crystalline forms in poly(hexamethylene terephthalate) (PHT) is a very intriguing one, which shows a maximum of six polymorphic melting endotherms (P-1-P-6) and up to two spherulitic forms packed with alpha- and beta-crystal cells upon melt-crystallization at most temperatures. While the peaks P, and P-3 have been earlier assigned to the melting of the alpha-form and the peaks P-2 and P-5 are attributed to the beta-forms, however, P-4 was yet to be resolved, which was absent in PHT melt-crystallized at lower temperatures. In this study, the fourth peak, P-4, was further resolved by high-temperature annealing of precrystallized PHT, and it was found to be associated with the most perfect and thus the thickest crystalline lamellae of alpha-forms. Correlation between the complex melting peaks and crystalline polymorphs in PHT was more successfully refined by employing combined techniques.