Thermal, spectroscopy, and morphological studies on polymorphic crystals in poly(heptamethylene terephthalate)

Polymorphism and its influential factors in poly(heptamethylene terephthalate) (PHepT) were probed using differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and wide angle X-ray diffraction (WAXD). PHepT exhibits two crystal types (α and β) upon crystallization at various isothermal melt-crystallization temperatures (Tcs) by quenching from different Tmaxs (maximum temperature above Tm for melting the original crystals). Melt-crystallized PHepT with either initial α- or β-crystal by quenching from Tmax lower than 110 °C leads to higher fractions of α-crystal, but crystallization from Tmax higher than 140 °C leads to higher fractions of β-crystal. In addition to Tmax, polymorphism in PHepT is also influenced by crystallization temperature (Tc = 25–75 °C). When PHepT is melt-crystallized from a high Tmax = 150 °C (completely isotropic melt), it shows solely β crystal for higher Tc, and solely the α-crystal for Tc < 25 °C; in-between Tc = 25 and 35 °C, mixed fractions of both α- and β-crystals. However, by contrast, when PHepT is melt-crystallized from a lower Tmax = 110 °C, it shows α-crystal only at all Tcs, high or low.