Hydrolytic degradation behavior of stereo multiblock and diblock poly(lactic acid)s: Effects of block lengths

Stereo multiblock and diblock poly(lactic acid)s (PLAs), poly(DL-lactide) (PDLLA), and poly(DL-lactic acid), with block lengths comprising 3.8-61.9 lactyl units were synthesized. The effects of the block length sequence on the hydrolytic degradation behavior were investigated at 37 °C and pH 7.4 using amorphous (quenched) samples of these polymers, in comparison with quenched neat poly(l-lactide) (PLLA), poly(d-lactide) (PDLA), and PLLA/PDLA blend. 13C NMR spectra revealed that chain cleavage during the hydrolytic degradation occurred rapidly on the atactic segments connecting relatively long isotactic l-lactyl and d-lactyl segments. The rates of weight loss and hydrolytic degradation of stereoblock PLAs, PDLLA, and poly(DL-lactic acid) decreased with an increase in the average stereoblock length. The hydrolytic degradation of the stereo multiblock PLAs with average block lengths higher than 27 and 15 lactyl units was suppressed as compared to that of the PLLA/PDLA blend and the neat PLLA or PDLA, respectively. The stereocomplex crystallization occurred during the hydrolytic degradation of stereoblock PLAs and PDLLA with average stereoblock lengths higher than 7 lactyl units, in agreement with the results reported for the stereocomplex crystallization from the melt. The crystallization rate of stereoblock PLAs during the hydrolytic degradation increased on increasing the block length; however the rate of the hydrolytic degradation of the diblock PLA with the highest average stereoblock length was lower than that of the PLLA/PDLA blend.