Effect of thermal history on nucleation and crystallization of poly(lactic acid)

In this paper, a successive heating and cooling protocol was designed to investigate the self-nucleation behavior of poly(lactic acid), PLA. The main objective of this investigation was to study the efficiency of the α and α′ crystalline modifications of PLA. This was carried out by comparing crystallization temperatures upon cooling after self-nucleation of samples previously crystallized at various isothermal temperatures ranging from 80 to 130 °C. During heating to the partial melting range, three different mechanisms were observed for crystallized samples. For samples crystallized below 100 °C, an exothermic peak was detected prior the main melting peak which is ascribed to the α′–α solid-state transition. For samples crystallized between 100 and 120 °C, a melt recrystallization mechanism was observed. Finally, for samples crystallized above 120 °C, only melting of the α phase was detected. Upon cooling after partial melting, it was found that samples comprising a mixture of α and α′ exhibited the highest crystallization temperature, the highest nuclei density, and the smallest spherulite size. Moreover, it was observed that samples that were isothermally crystallized between 100 and 120 °C, heated up to partial melting, and then cooled back to room temperature exhibited two peculiar crystallization peaks at 100 and 120° C. This phenomenon was ascribed to the formation of α and α′ crystalline phases as revealed by X-ray diffraction. In addition, by slightly changing the temperature within the self-nucleation temperature range, a change of the proportion of each peak was observed.