Thermal Analysis, X-ray and Electron Diffraction Studies on Crystalline Phase Transitions in Solvent-Treated Poly(hexamethylene terephthalate)

Crystal polymorphism, transformation, and morphologies in poly(hexamethylene terephthalate) (PHT) samples produced by solvent-casting from chloroform were examined by using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and in-situ transmission electron microscopy (TEM). Solvent-induced crystallization of PHT at room temperature yielded an initial crystal of $\gamma $-form, as confirmed by WAXD. Upon DSC scanning, the original $\gamma $-form in PHT exhibited three endothermic peaks, whose origins and association were carefully analyzed. The $\gamma $-form could be transformed to $\beta $-form at 125$^{\circ}$C via a solid-to-solid transformation mechanism. In addition, WAXD showed that $\gamma $- and $\beta $-forms co-existed in the temperature range of 100-125$^{\circ}$C. These mixed crystal-forms were further identified using TEM, and the selected-area electron diffraction patterns revealed that both $\gamma $- and $\beta $-form crystals co-existed and were packed within the same spherulite. Evidence of solid-solid transformation from $\gamma $-form to $\beta $-form in PHT was presented and discussed.