Thermal diffusivity and secondary crystallisation kinetics in poly(lactic acid)

Abstract

The non-steady state technique of laser flash analysis (LFA) was used to record the variation of thermal diffusivity with time during the cold-crystallisation of poly(lactic acid), a grade that contains 4% D stereoisomer. The measured diffusivity data was analysed in terms of the Avrami and Hoffman-Lauritzen models for polymer crystallization and this yielded kinetic data that was comparable to that which originated from DSC. Within the temperature range 88 to 109 °C, mechanistic ‘n’ values of 2.0 ± 0.1 were calculated and a nucleation constant of 6.58 × 10−5 K2 was determined. Measurements of thermal diffusivity were also recorded over a longer time-scale to determine whether thermal diffusivity was a useful indicator of secondary crystallization and this data was analysed using a modified Avrami equation that includes a root-time dependence of the secondary process. Secondary crystallisation rate constants (ks) increased with crystallisation temperature and yielded an activation energy for the secondary crystallisation process of 40 kJ/mol.