Compatibility and Thermal and Structural Properties of Poly(l-lactide)/Poly(l-co-d-lactide) Blends

To explicate the interaction processes between poly(l-lactide) (PLLA) and poly(l-co-d-lactide) (PLDLA) in their blends, a series of high-molecular-weight PLDLA-m polymers (where m represents the percentage of l-lactate units) were prepared by ring-opening polymerization at different feed ratios of l- to d-lactides and blended with highly optically pure PLLA by solution blending. The compatibility and thermal and structural properties of the PLLA/PLDLA-m blends were investigated by differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FE-SEM) in detail. The PLLA/PLDLA-m blends with m = 60–20 wt % were only partially compatible because obviously double glass transitions were detected in DSC curves and multiphase structures were observed in SEM micrographs. The compatibility between PLLA and PLDLA-m decreased as the l-lactate unit content in PLDLA-m decreased. Crystallization of the PLLA phase in the PLLA/PLDLA-m blends was restricted by PLDLA-m at m = 95–40 wt % (the restriction was strongest at m = ∼70 wt %); however, it was enhanced as the l-lactate unit content decreased at m ≤ 40 wt %. The PLLA/PLDLA-m blends demonstrated weak-to-strong hydrogen-bonding interactions as the l-lactate unit content decreased at m ≤ 80 wt %. The PLLA/PLDLA-m blends can form not only a eutectic but also individual crystal structures of PLLA and PLDLA-m at m ≥ 80 wt %, while the formation of individual crystals of PLLA, PLDLA-m, and stereocomplexed polylactide (sc-PLA) is favored at m ≤ 15 wt %.