Can classic Avrami theory describe the isothermal crystallization kinetics for stereocomplex poly(lactic acid)?

Abstract

Classic Avrami model and its modifications have found diverse applications in describing the thermal and phase behaviors of inorganic metals and organic polymers. The direct introduction of classic Avrami equation to offer quantitative analyses of crystallization kinetic parameters for enantiomeric poly(lactic acid) (PLA) blends may, however, lead to contradictory conclusions. As revealed by this study, during the characterization of isothermal melt and cold crystallization for stereocomplex PLA containing equal-weight poly(L-lactic acid) and poly(D-lactic acid), the kinetic parameters yielded by Avrami equation are not in line with the classic crystallization hypotheses or the direct morphological observations. The underlying mechanisms, to some extent, lie in the generation of stereocomplex crystals (SCs) during the cooling/heating which affects the subsequent crystallization dynamics. The huge gap between the melting enthalpies of 100% crystalline SCs (142 J/g) and homo-crystals (HCs, 93 J/g) is most likely responsible for the confusing kinetic parameters acquired from the deduction of Avrami equation, which is based on the integration of enthalpies as a function of crystallization time. This prompts for great care that the classic Avrami equation is not applicable to accurately describe the crystallization kinetics of stereocomplex PLA, given the generation of SCs prior to crystallization and the coexistence of HCs and SCs during crystallization.