Effect of cooling rate on the crystal/mesophase polymorphism of polyamide 6

The solidification of the quiescent polyamide 6 (PA 6) melt has been analyzed as a functionof the cooling rate in wide range between about 100 and 10^4 K min^1, by means of differentialscanning calorimetry, by recording of continuous cooling curves, and by time-resolved X-raydiffraction. The performed experiments allowed for the first time to establish the relationshipbetween the cooling rate, the crystallization temperature and the X-ray structure of PA 6.Exclusive formation of monoclinic alpha-crystals is only detected if the crystallizationtemperature is higher than about 430 K or if the cooling rate is slower than about 300 K min^1,respectively. The formation of alpha-crystals is increasingly replaced by development ofmesophase with increasing cooling rate, accompanied with a decrease of the temperature ofcrystallization/ordering. Finally, completely amorphous samples were obtained on coolingfaster than (510) 10^3 K min^1. The continuous decrease of the temperature ofcrystallization with increasing cooling rate, regardless the specific structure formed, precludesa primary effect of the nucleation mechanism on the alpha-crystal/mesophase polymorphism ofPA 6. A preliminary discussion of the effect of molar mass of PA 6 on the cooling-ratedependent polymorphism is also included.