A COMPARATIVE MODEL FOR ANISOTHERMAL AND ISOTHERMAL CRYSTALLIZATION OF POLY(ETHYLENE-TEREPHTHALATE)

A new method for analysing and modelling anisothermal crystallization from a polymer melt is proposed in a way allowing comparison with isothermal conditions, and supposing that no variation of the process occurs in the whole temperature range. The model is based on a modified Avrami equation: 1 - U = exp{ - K(T)[(T(alpha) - T)/alpha]n}, where the starting point T(alpha), highly dependent on the cooling rate alpha, appears to be of major importance. Some methods for its determination are derived. Application to crystallization kinetics of poly(ethylene terephthalate) by differential scanning calorimetry is discussed and a comparison of both methods shows good accuracy. Moreover, anisothermal studies allow a more extended temperature range for K(T) determination and seem more closely connected with the current value of apparent n, interpreted in terms of a dual mechanism. Finally an equation describing K(T) variations is proposed and allows one to describe quantitatively any anisothermal process, particularly at the beginning, which is suitable for setting conditions.