Crystallization and hydrolytic degradation studies of poly(L-lactic acid) (PLLA)

Poly(L-lactic acid) (PLLA) films were prepared by melt-pressing technique. Thermally induced crystallization of PLLA by annealing was investigated using differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR). Effect of annealing temperature (Ta) on structure development was also monitored using FTIR and DSC. It was found that glass transition temperature (Tg) and crystalline melting temperature (Tm) substantially increased with increasing Ta. FTIR spectroscopic observation showed that the band at 921 cm-1 increased in intensity while the band at 956 cm-1 decreased in intensity with increasing Ta. Bands at 921 cm-1 and 956 cm-1 were therefore attributed to crystalline and amorphous phase, respectively. Bands at 2996, 2961 and 2945 cm-1 showed no change in absorbance with increasing Ta and they were used as reference bands to monitor the structural changes. Infrared spectroscopy was used as a means to estimate the crystalline fraction of PLLA. Calculated crystalline fraction obtained by DSC was correlated with that obtained by FTIR and the two are in good agreement.; The hydrolytic degradation of PLLA films in alkaline solution was investigated using weight loss and high performance liquid chromatography (HPLC). Degradation rate of PLLA films increased with increasing crystallinity. HPLC study showed that the percentage of lactic acid formed during degradation also increased with increasing crystallinity. The structural changes of PLLA films before and after the alkaline hydrolysis were characterized using DSC and FTIR. No significant change in heat of fusion of PLLA films was observed after the hydrolysis. It was found that Tg increased with increasing hydrolysis time and no significant change in Tm as a function of hydrolysis time was observed. FTIR study showed an increase in ratio of absorbance bands at 921 and 956 cm-1, suggesting that there is an increase in crystallinity with increasing hydrolysis time.