Summary: The polymorphisms in poly(hexamethylene terephthalate) (PHT), along with their associated melting and spherulite morphologies, were examined by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and polarized‐light microscopy (PLM). The morphology and crystal cells were dependent on the temperature of crystallization. When melt‐crystallized at low temperatures (90–135 °C), PHT showed at least five melting peaks and two re‐crystallization peaks upon DSC scanning, and the samples displayed various fractions of both α and β crystals. However, only a single melting peak was obtained in PHT melt‐crystallized at 140 °C or above, which displayed a single type of β crystal. In addition, two different forms of spherulites were identified in melt‐crystallized PHT, with one being a typical Maltese‐cross spherulite containing the α crystal, and the other being a dendrite‐type packed mainly with the β crystal. This study provides timely evidence for a critical interpretation of the relationship between multiple melting and polymorphisms (unit cells and spherulites) in polymers, including semi‐crystalline polyesters.

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Summary: Thermal characterizations were performed to further discern the miscibility and qualitative interactions in blends of isotactic polystyrene (iPS) and poly(cyclohexyl methacrylate) (PCHMA). A method based on the enthalpy relaxation of the blends was used to overcome the difficulty or ambiguity in resolving closely‐spaced glass transitions of these two constituent polymers. Interactions between the blend components were further estimated by two additional methods: the blend's glass transition temperature (Tg) and the melting point depression. The blend's Tg method yielded a χ12 value ranging from −0.0016 (i.e., almost 0) to −1.98 (with the values depending on the amorphous PCHMA contents in the blends) in the temperature range of 95–110 °C, whereas the melting point depression led to χ12 = −0.039 at 240 °C. The interaction parameters obtained from these two methods are negative, confirming the miscibility with weak interactions. The results of these alternative thermal characterizations further clarified that the iPS/PCHMA blends, whose Tg's are too close to allow the use of conventional Tg criteria, are indeed completely miscible.Specific heat increment (ΔCP) at Tg for the iPS/PCHMA blends.magnified imageSpecific heat increment (ΔCP) at Tg for the iPS/PCHMA blends.