Supercooling-controlled heterogeneous and homogenous crystal nucleation of polyamide 11 and its effect onto the crystal/mesophase polymorphism

Fast scanning chip calorimetry (FSC) was used for analysis of the kinetics of melt-crystallization of polyamide 11 (PA 11) in a wide range of temperatures between 60 and 170 °C. The crystallization rate exhibits a bimodal temperature-dependence which is associated with a transition from heterogeneous crystal nucleation at low supercooling of the melt to homogeneous crystal nucleation at high supercooling, with the cross-over occurring at around 115 °C. It is shown that the cross-over temperature depends on the particular PA 11 grade investigated, that is, on the presence of additives which may act as heterogeneous nucleators, supporting the notion that the minimum in the temperature-dependence of the crystallization rate is related to a change of the nucleation mechanism. Immediately after the isothermal crystallization experiment, the FSC samples were rapidly cooled to ambient temperature, for analysis of the phase structure using X-ray diffraction. The data suggest that formation of the δ’-mesophase mainly occurs in the temperature range of fast homogeneous nucleation in which crystal growth due to the high nuclei density and small distance between the nucleation sites is hindered. At higher crystallization temperatures the nuclei density is much lower, and growth of δ/α-crystals with a perfection depending on the crystallization temperature is observed. Regardless of the isothermal crystallization temperature, a distinct three-phase structure is not detected.