Comparison study of hydrolytic degradation behaviors between alpha '- and alpha-poly(L-lactide)

As one of the typical polymorphic polymers, poly(L-lactide) (PLLA) exhibits great dependences of physical/chemical performances on the crystalline structures. In this work, the dependences of hydrolytic degradation behavior of PLLA on the crystal form (alpha '- and alpha-form, respectively) and crystallinity (lower than 30 wt% and higher than 45%, respectively) during the hydrolysis process were investigated for the first time. To endow the samples with different crystal forms and crystallinities, the amorphous PLLA samples were annealed under different conditions. The crystalline structures of the samples were investigated through various characterization methods. The hydrolytic degradation measurements were carried out at 50 degrees C in distilled water. Specifically, the degree of the hydrolytic degradation of samples was qualitatively analyzed through detecting the concentration of hydrolysate in the medium using UV-Vis spectroscope. The results showed that for the samples with relatively low crystallinity (< 30 wt%), in the initial stage of the hydrolytic degradation process, sample with a-form but lower crystallinity exhibited more serious degradation compared with the sample with alpha '-form but higher crystallinity. While in the later stage of the hydrolytic degradation process, sample with alpha '-form exhibits more serious hydrolysis. However, for the samples with relatively high crystallinity (> 45%), sample with alpha '-form exhibits more serious hydrolysis during the whole degradation process. The microstructure evolution of samples was also measured and the results showed that the crystalline structure evolution of the PLLA samples was mainly determined by the initial crystal structures of samples before hydrolytic degradation.