Stereocomplex formation between enantiomeric poly(lactic acids). 5. Calorimetric and morphological studies on the stereocomplex formed in acetonitrile solution

Precipitates from acetonitrile solutions of poly(D-lactic acid) (PDLA) + poly(L-lactic acid) (PLLA) mixtures were studied by differential scanning calorimetry and scanning electron microscopy. Precipitation was carried out under various conditions by changing the mixing ratio of the isomers, the polymer concentrations, the molecular weights of the polymers, and the solution temperature. It was found that the precipitates contained only racemic crystallites (stereocomplexes) consisting of equimolar amounts of D-monomer and L-monomer units, independent of the PDLA content in the initial acetonitrile solution. The enthalpy of fusion of the racemic crystallites was higher than that of the stereocomplexes obtained by casting a methylene chloride solution of the PDLA + PLLA mixture or precipitating a methylene chloride solution into methanol, provided the precipitation temperature of the acetonitrile solution was kept at 80-degrees-C. The precipitates were either platelets or micron-sized spherical particles, depending on the precipitation parameters. When the equimolar polymer mixture was allowed to crystallize in acetonitrile solution below 1 g/dL at 80-degrees-C, platelet-type precipitates were formed, while isolated and unified sphere-type precipitates were observed at 10 and 30 g/dL, respectively. It was concluded that the platelet-like and the sphere-like precipitates consist of racemic crystals from the PDLA + PLLA stereocomplex.