Study of Strain-Induced Crystallization and Enzymatic Degradation of Drawn Poly(l-lactic acid) (PLLA) Films

Abstract

Poly(lactic acid) (PLLA) melt-pressed films with low crystallinity were crystallized by stretching at a constant strain rate. The strain-induced crystallization and enzymatic degradation of drawn PLLA films in the presence of proteinase K at 37 °C was investigated using weight loss measurements, differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy. The results show that drawing has a significant effect on the crystallinity, molecular orientation, and enzymatic degradation. The absorbance ratio of the bands at 921 and 956 cm–1 (A921/A956) was chosen to determine the structural changes during strain-induced crystallization and hydrolysis. The DSC crystallinity and A921/A956 showed an increase with the draw ratio. Since we were unable to obtain the transition moment angle for the bands at 921 and 956 cm–1, the dichroic ratios were compared. It was found that the crystalline orientation develops rapidly at lower draw ratios whereas the amorphous orientation develops much more slowly. The enzymatic degradation of annealed PLLA films was reported, and surprisingly, quite different results were observed for the enzymatic degradation of oriented PLLA films. The extent of degradation was lower for the drawn PLLA film than for the undrawn melt-pressed PLLA film. The DSC crystallinity and A921/A956 of drawn PLLA films increased with the degradation time, suggesting an increase in the crystalline phase with degradation. This reveals that degradation occurs in both the free and the restricted amorphous region in the case of drawn PLLA films, whereas it occurs only in the free amorphous region in annealed unoriented PLLA films.