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 (P₁-P₆) and up to two spherulitic forms packed with α- and β-crystal cells upon melt-crystallization at most temperatures. While the peaks P₁ and P₃ have been earlier assigned to the melting of the α-form and the peaks P₂ and P₅ are attributed to the β-forms, however, P₄ 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 α-forms. Correlation between the complex melting peaks and crystalline polymorphs in PHT was more successfully refined by employing combined techniques.