Stereocomplexation and morphology of polylactides

Blends of isotactic polylactides of opposite configurations lead to the formation of stereocomplexes, provided the enantiomeric excess of the two homopolymers in contact is high enough. In this study, the stereocomplex formation between a poly(L-lactide) (100L) having an enantiomeric excess of 100% and a poly(D-lactide) having an enantiomeric excess of 80% (80D) has been investigated using differential scanning calorimetry and optical microscopy. These results have been compared to those obtained between two polylactides having both an enantiomeric excess of 100% (100L/100D blend). The melting temperatures of blends 100L/100D and 100L/80D are 230 and 208 degrees C, respectively. In both cases, the stereocomplex formation is preferred to the homopolymer crystallization and the stereocomplex controls the morphology of the blends over a wide range of concentrations. However, this control is more rigorous when the D component is 100% isotactic. This difference leads to a more complete crystallization of homopolymer 100L and to a greater influence of crystallization conditions in 100L/80D blends in comparison with 100L/100D blends. An epitaxial crystallization between the homopolymer 100L and the stereocomplex 100L/80D has also been observed at certain compositions.