Crystalization and solid-state structure of random polylactide copolymers:: Poly(L-lactide-co-D-lactide)s

This paper presents a continuation of our earlier research on the crystallization and solid-state structure of polylactide copolymers. The focus here is on random copolymers containing predominately L-lactide and small amounts (1.5, 3, and 6%) of D-lactide. As expected, degrees of crystallinity and spherulite growth rates decrease substantially with increasing D-lactide content in the copolymers. The importance of defect arrangement (isolated vs paired stereochemical defects) was demonstrated by comparison to our earlier research on L-lactide/meso-lactide copolymers. At a given degree of supercooling, measured lamellar thicknesses decrease significantly with increasing R stereoisomer concentration: e.g., by more than a factor of 2 (compared to poly(L-lactide)) for the 6% D-lactide copolymer. The results of small-angle X-ray scattering experiments indicate that a significant amount of noncrystalline material resides between lamellar stacks. Equilibrium melting points were estimated for the copolymers using the Gibbs-Thomson approach, and the values conform with predictions of the model of Wendling and Suter in the exclusion limit. Taken together with the significant reduction in lamellar thickness and crystallinity, these results point to substantial rejection of D-lactide (and meso-lactide) defects from S stereoisomer crystals. However, experiments by others on similar copolymers suggest that a significant amount of R (or R-R) isomers can be included in S crystals under certain crystallization conditions. Some speculation about the origin of these differences is presented.