Effect of Supercooling on Crystallization of Polyamide 11

The crystallization rate of polyamide 11 has been quantified in a wide temperature range between 320 and 450 K, using fast scanning chip calorimetry and differential scanning calorimetry. Different mechanisms of crystal nucleation/growth have been identified at temperatures below and above 370 K, causing a bimodal distribution of the crystallization rate as a function of temperature. Crystallization at low supercooling is connected with formation of triclinic alpha-crystals of lamellar morphology and ringed/banded spherulites. At high supercooling, formation of pseudohexagonal delta'-mesophase is observed. Because of the high nucleation density at low temperature, growth of the delta'-mesophase is nonspherulitic. The delta'-mesophase transforms on heating to alpha-crystals without affecting the superstructure. The study is completed by quantification of the cooling conditions to allow delta-crystal formation at low supercooling, delta'-mesophase formation at high supercooling, and complete vitrification of the melt. The interplay between nucleation density and mesophase formation according Ostwald's rule of stages is discussed as a consequence of immobilization of the amorphous phase/formation of a rigid amorphous fraction.