Effect of the Cooling Rate on the Nucleation Kinetics of Poly(l-Lactic Acid) and Its Influence on Morphology

Although the crystallization kinetics of PLLA is slow enough to allow one to obtain amorphous samples by cooling from the melt at moderate rates, surprisingly, at the same time, the formation of crystallization nuclei is difficult to avoid. Their amount depends on the cooling rate from the melt: the higher the cooling rate, the lower the amount of available nuclei after reaching the glass state. Cooling at controlled rates of 5−300 °C/min was performed by making use of a relatively new high-speed calorimetry technology, high-performance DSC (HPer DSC). Subsequent isothermal cold crystallization of amorphous PLLA, at different temperatures distributed across the bell-shaped crystal growth curve, was shown to depend on the rate at which the glass state was attained, reflecting the number of nuclei formed. However, (only) after complete cold crystallization, the DSC heating curves and the resulting morphology measured by optical microscopy and AFM were shown to be independent of the previous cooling rate into the glass, i.e., of the number of nuclei formed during the cooling process. In cases where the DSC heating curves subsequent to isothermal cold crystallization show two melting peaks, their origin, recrystallization, was clarified by heating at rates varying from 10 to 300 °C/min. The morphologies of the cold crystallized systems have been assessed by AFM and were correlated with the calorimetric results. Using HPer DSC, isothermal cold and hot crystallization at the same temperature has been studied successfully.