Crystallization from the melt of poly(lactide)s with different optical purities and their blends

Poly(lactide)s (PLA) with different optical purities (OF) were synthesized by copolymerization of D-lactide and L-lactide. Crystallization of D- and L-lactide-rich copolymers before and after 1:1 blending was studied from the melt over a wide range of annealing temperatures (T-a) using differential scanning calorimetry and polarizing microscopy. Melting temperature and crystallinity increased with an increase in OP for both the nonblended and the 1:1 blended PLAs and they lost their crystallizability when their OP became smaller than 76%, suggesting that the critical isotactic sequence length of PLA for crystallization was approximately 15 isotactic lactate units. Normal spherulite formation was observed for both the non-blended and the blended PLAs even when the OP of PLAs decreased to 76%, whereas the spherulitic size of the non-blended PLAs was larger than that of the 1:1 blended PLAs when compared at the same OP and T-a. The highest T-a for crystallization increased with increasing OP and was higher for the 1:1 blended PLAs than for the non-blended PLAs at a fixed OF, indicating that the crystallizable T-a range was extended to higher temperature by blending D- and L-lactide-rich PLAs. This is ascribed to stereocomplex (racemic crystallites) formation at high T-a from the 1:1 blended PLAs. Based on these results, the phase diagram for crystallization was depicted for the non-blended and the 1:1 blended PLAs.