Stereocomplex formation between enantiomeric poly(lactic acid)s. 4. Differential scanning calorimetric studies on precipitates from mixed solutions of poly(D-lactic acid) and poly(L-lactic acid)

Polymer blends of poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA) were obtained by precipitation from their 1 g/dL methylene chloride mixed solutions into methanol under stirring and studied by differential scanning calorimetry (DSC). The DSC results showed that the racemic crystallization (stereocomplex crystallization) between PDLA and PLLA occurred in preference to the crystallization of each homopolymer, irrespective of the molecular weight of poly(lactic acid), in contrast to the polymer blends obtained by casting the mixed solutions of PDLA and PLLA with high molecular weights. However, when the mixed solution from PDLA and PLLA with a high molecular weight was poured into the nonsolvent without stirring or at 5 g/dL, the formation of racemic crystallites was greatly reduced and the homopolymer crystallites were formed in addition to the racemic crystallites. These results strongly suggest that the racemic crystallization does not take place in dilute solutions but during the precipitation process. The racemic crystallite has 1.4 kcal/mol as the enthalpy of fusion when the PDLA concentration in the mixed solution approaches zero.