Crystallization kinetics and structure evolution of a polylactic acid during melt and cold crystallization

Abstract

The isothermal melt and cold crystallization and the evolutions of the mobile and rigid amorphous fractions during the crystallization are fully investigated for a polylactic acid using differential scanning calorimetry. The crystallization kinetics is analyzed through fitting the Avrami equation to two different sets of crystallinity data, one obtained from the crystallization extherms and one from the enthalpies of melting and cold crystallization. Both sets yield similar results, indicating the consistency between these two methodologies. The cold crystallization shows clearly faster crystallization rate and has a higher activation energy of 172.3 ± 8.9 versus 109.6 ± 8.8 kJ mol−1 for the melt crystallization. The Avrami exponent of the cold crystallization seems to decrease with increasing crystallization temperature, whereas the exponent of melt crystallization is relatively constant. In addition to the crystalline phase, properties of the mobile and rigid amorphous fractions are also studied and compared. With the increase in crystallinity, the glass transition temperature (Tg) of the mobile amorphous phase is found to decrease during the cold crystallization, whereas Tg decreases and then increases during the melt crystallization, indicating slightly different environment formed. However, no obvious difference is found regarding the heat capacity change at Tg nor the cooperative length scale associated with Tg. Furthermore, the rigid amorphous fraction is determined and found to be 30 % at the maximum crystallinity for the both cold- and melt-crystallized samples.