Effect of thermally induced crystallization on enzymatic degradation of Poly (L-Lactic acid) (PLLA)

Poly (L-Lactic Acid) (PLLA) films were prepared by solvent casting method with dichloromethane. These films were isothermally annealed at various temperatures to prepare films with different crystallinities in order to study the effect of crystalline and amorphous morphology on enzymatic degradation. Crystallinity changes and thermal properties of annealed PLLA films were characterized using differential scanning calorimetry (DSC). FTIR spectroscopy was used to monitor the structural changes after annealing. DSC crystallinity increased with increasing annealing temperature except for the one annealed at 80°C. FTIR spectroscopy showed an increase in 921 cm -1 and decrease in 956 cm-1 with annealing temperature. The band ratio of 921 cm-1 and 956cm-1 increased with increasing annealing temperatures suggested that increase in crystallinity with annealing temperatures.;The enzymatic degradation was investigated in the presence of proteinase K at 37°C by means of weight loss, DSC, FTIR spectroscopy, HPLC and optical microscopy. The highest percent weight loss was observed for the film with the lowest initial crystallinity and the lowest percent weight loss was observed for the film with highest crystallinity, suggesting that degradation takes place predominantly in the free amorphous phase. HPLC showed no evidence for the presence of lactic acid suggested that degradation process does not occur by depolymerization mechanism as it occurs in hydrolytic degradation. It has been shown that the rate of degradation strongly depended on initial crystallinity and followed the first order kinetics. DSC and FTIR confirmed that there is no significant change in crystallinity with degradation time.