Stereocomplex formation of high-molecular-weight polylactide: A low temperature approach

A low temperature approach to prepare stereocomplex of high-molecular-weight poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) at temperatures much lower than those adopted in conventional melt blending, is presented in this paper. Wide-angle X-ray diffraction and differential scanning calorimetry results verified that complete stereocomplex crystallites without any evidence of the formation of homocrystallites can be achieved not only at equimolar PLLA/PDLA blends, but also at non-equimolar PLLA/PDLA (60:40) blends using such an approach. Furthermore, stereocomplex formation is more efficient for non-equimolar blends than for the equimolar one. Significant improvements in the crystallinity and melting temperature of stereocomplex crystallites and the thermal stability are achieved by this low temperature approach compared with either conventional melt blending or solution casting method. More interesting, fine powder form of stereocomplex can be obtained directly. The process takes advantage of the temperature window at which only the stereocomplex can grow and the homopolymers can no longer crystallize, and develops a feasible way to overcome the molecular diffusion issue in the stereocomplex growth. Thus this novel process provides a unique low temperature, solvent free processing route of preparing high degree stereocomplex crystallites using high-molecular-weight polylactides that has not previously been reported.