BLENDS OF ISOTACTIC AND ATACTIC POLY(LACTIDE) .1. EFFECTS OF MIXING-RATIO OF ISOMERS ON CRYSTALLIZATION OF BLENDS FROM MELT

Poly(L-lactide) (PLLA) and poly(DL-lactide) (PDLLA) blends were crystallized from the melt and their crystallization behaviors and morphologies were investigated using differential scanning calorimetry and polarizing microscopy. PLLA could crystallize from the melt in the presence of PDLLA when the PLLA content in the blend (X(PLLA)) was higher than 0.2. Spherulites were formed when X(PLLA) was between 0.6 and 1. The radius of the spherulites formed in the presence of PDLLA was larger than that of nonblended PLLA, probably because the coexisting PDLLA reduced the density of nuclei for the spherulites. Small crystallite assemblies were formed when X(PLLA) was between 0.2 and 0.5, and grew spherically from the nuclei when X(PLLA) was 0.5. Isotactic PLLA and atactic PDLLA seem to be miscible prior to crystallization, but PDLLA will be trapped between the lamellae of the spherulites or spherulitic assemblies during crystallization. PDLLA delayed the induction for crystallization of the blends. In spite of different morphology, the melting temperature and crystallinity of PLLA remained virtually constant when annealing was conducted for 600 min at X(PLLA) between 0.5 and 1. (C) 1995 John Wiley & Sons, Inc.