Stereocomplex formation in injection-molded poly(L-lactic acid)/poly(D-lactic acid) blends

Poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) blends were prepared by hand mixing, followed by injection molding at 210°C to produce tensile specimens. Thermal properties, crystalline structure, and mechanical properties were measured by differential scanning calorimetry (DSC), thermogravimetric analysis, wide-angle X-ray diffraction (XRD), and tensile testing. From the DSC tests of blends ranging from 10% to 30% PDLA in PLLA, the PDLA melting peak was absent and was replaced by a stereocomplex melting peak at 210°C, which was ~50°C higher than that for neat PLLA or PDLA. The reverse blending of PLLA into PDLA showed a similar behavior. Surprisingly, three melting peaks (for PLLA, PDLA, and the complex crystal) appeared in the 1:1 PLLA:PDLA pellet blends. However, the PLLA and PDLA powders (ground to less than 200 μm) and hand mixed, prior to injection molding, showed only small amounts of homocrystals and much higher fractions of stereocomplex crystals (18-44%). Compared to the hand mixed un-ground pellets, molded specimens from the PLLA and PDLA powders also exhibited higher tensile strengths (33-48 MPa) and moduli (1100-1250 MPa). Moreover, the stereocomplex formation was found to enhance the thermal stability compared with those of the pure PLLA and PDLA.