Study on crystallization of nylon-6 (polycapramide): Part 2. Effect of molecular weight on isothermal crystallization kinetics

Abstract

Further studies of the kinetics of primary isothermal crystallization of fractions of nylon-6 with viscosity-average molecular weights ranging from 17 000 to 49 000 are reported. The results were analysed in terms of the Avrami equation to give rate constants and the mechanism of crystallization. In accordance with theoretical assumptions the crystallization rate decreased as molecular weight and temperature increased within the investigated range of molecular weight and crystallization temperature. The relation between the logarithm of half-time of crystallization and the molecular weight is linear. The Avrami exponent n depends on the crystallization temperature and molecular weight of the fractions. Above 210°C the Avrami exponent n was constant and equal to 4–6 for various fractions, corresponding to spherical and sheaf-like growth from sporadic nucleation. Below 210°C, n was equal to 2.8–5, corresponding to spherical and sheaf-like growth from pre-determined nuclei. It has been found that the Avrami exponent n depends on the molecular weight starting from the limiting value of 32 500. For fractions with a lower molecular weight, the exponent n was not dependent on that parameter. Above that value exponent n decreased as the molecular weight increased for all crystallization temperatures. For all samples, assuming n = 4 and rise of crystallization temperature is accompanied by rise of melting temperature, linear relations for log K against T m T(ΔT) and T 2 m T(ΔT) 2 were obtained.