Thermodynamic and kinetic thermal analyses on dual crystal forms in polymorphic poly (Heptamethylene terephthalate)

Thermal analyses were performed for determining the equilibrium melting temperatures T°_m of the respective α- and β-crystal in melt-crystallized polymorphic poly(heptamethylene terephthalate) (PHepT) using both linear and nonlinear Hoffman- Weeks (H-W) methods for comparison of validity. These two crystals in PHepT do not differ much in their melting temperatures. The equilibrium melting temperatures of the α- and β-crystal as determined by the linear H-W method are 98 °C and 100.1 °C, respectively; but the nonlinear H-W method yielded higher values for both crystals. The equilibrium melting temperatures of the α- and β-crystal according to the nonlinear H-W method are 121 °C and 122.5 °C, respectively. Both methods consistently indicate that T°_m of the β-crystal is only slightly higher than that of the α-crystal. Such small difference in T°_m between the α- and the β-crystal causes difficulties in judging the relative thermodynamic stability of these two crystals. Thus, kinetics of these two crystals was compared using the Avrami and Ozawa theory. The crystallization produced by quenching from T_max=110 °C and 150 °C shows a heterogeneous and homogeneous nucleation mechanism, respectively. The lower T _max=110 °C leads to heterogeneous nucleation and only α-crystal in PHepT, whose crystallization rates at same Tc are much higher than crystallization rates by quenching from T_max =150 °C leading to either α- or β-crystal with homogeneous nucleation.