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

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Abstract
Poly(lactide)s (PLA) with different optical purities (OP) 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 (Ta) using differential scanning calorimetry and polarizing microscopy. Melting temperature and crystallinity increased with an increase in OP for both the non‐blended 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 Ta. The highest Ta for crystallization increased with increasing OP and was higher for the 1 : 1 blended PLAs than for the non‐blended PLAs at a fixed OP, indicating that the crystallizable Ta range was extended to higher temperature by blending D‐ and L‐lactide‐rich PLAs. This is ascribed to stereocomplex (racemic crystallites) formation at high Ta 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.