The effect of thermal history on the crystallinity of different molecular weight PLLA biodegradable polymers

Crystallinity and viscosimetric molecular weights (Mw_v) of four l-lactic acid polymers (PLLA) having different initial molecular weight, i.e. 18 000, 31000, 156000 and 425000, have been determined as functions of the polymer thermal history. Treatments performed in a differential scanning calorimeter (DSC) at various rates showed the different capability of materials to crystallize, due to their different crystallization kinetics and depending on their molecular weight. In the case of cooling from melt, for instance, even at a cooling rate of 0·5 °C/min, only lower molecular weight polymers were able to develop crystallinity. In all materials, moreover, and depending on the initial molecular weight, a parallel degradation effect was observed, which is attributed to thermal cleavage occurring during the treatment.