STEREOCOMPLEX FORMATION BETWEEN ENANTIOMERIC POLY(LACTIC ACID)S .6. BINARY BLENDS FROM COPOLYMERS

ABSTRACT: Binary blend films from copolymers Of D- and L-lactic acids were obtained by casting mixed solutions of the two copoly(lactic acid)s (PLA) with different D-contents (X(D)) and were studied by differential scanning calorimetry (DSC). We selected four cases from the combinations of binary (A-B) blends: mixing the same polymer [X(D)(A) = X(D)(B)], cross blending under the condition that X(D)(A) + X(D)(B) = 1, blending between a D-polymer (X(D) = 1) and other PLAs, and blending between a 1:1 DL-copolymer (X(D) = 0.5) and other PLAs. Racemic crystallites were exclusively formed between D-rich PLA and L-rich PLA when they had high optical purities. The melting point and enthalpy of the racemic crystallites decreased with decrease in the optical purity of D-rich PLA or L-rich PLA, suggesting that a D-unit in the L-rich polymer or an L-unit in the D-rich polymer disturbed the growth of racemic crystallites. This behavior was very similar to the homocrystallization in nonblended films. Homocrystallites were formed only if one component was crystallizable and the other component was noncrystallizable or if both components were crystallizable but had a similar X(D) content. The noncrystalline 1:1 DL-copolymer did not influence the crystallization of the other crystallizable polymer.