Injection-molded poly(L-lactic acid)/poly(D-lactic acid) blends: Thermal and mechanical properties

Poly(L-lactic acid) (PLLA)/Poly(D-lactic acid) (PLLA/PDLA) blends ranging from neat PLLA to neat PDLA were prepared by hand mixing, which removed the need for melt mixing, followed by injection molding at 200°C to produce tensile specimens. Thermal properties, crystalline structure and mechanical properties were measured by differential scanning calorimetry, thermogravimetric analysis, wide-angle X-ray diffraction and tensile testing. In blends ranging from 10% to 30% PDLA in PLLA, the PDLA melting peak was absent and was replaced by the stereocomplex melting peak at 210°C, which is about 40 °C higher than that for neat PLLA or PDLA. The reverse blending of PLLA into PDLA showed similar behavior. Surprisingly, three melting peaks (for PLLA, PDLA and the complex crystal) appeared in the 1:1 PLLA/PDLA blend because the source PLLA and PDLA initially had different degrees of crystallinity and did not mix well. However, the blends containing stereocomplex crystallites have higher thermal resistance and can be worked, without melting, at higher temperatures than the neat polymers: yet, the mechanical properties were not affected.