Enhancing the melt stability of polylactide stereocomplexes using a solid-state cross-linking strategy during a melt-blending process

Stereocomplexation between poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) provides a feasible route for improving the performance of polylactide (PLA), including mechanical strength, thermal stability and hydrolysis resistance. In recent years, several effective methods have been developed to prepare polylactide stereocomplexes (sc-PLA) from commercially available, linear, high-molecular-weight PLLA and PDLA. However, it is still a big challenge to attain pure sc-PLA in the melt-processed products because the prepared sc-PLA has a very poor melt stability, that is the ability to trigger the reformulation of stereocomplex (sc) crystallites after complete melting is significantly depressed, resulting in the formation of mixed homochiral (hc) and sc crystallites. Here we present a facile strategy to fabricate sc-PLA with good melt stability by low-temperature (180 °C) melt-blending of equimolar PLLA and PDLA in the presence of a trace amount (0.1–0.5 wt%) of a cross-linker. During the blending process, sc crystallites form rapidly, followed by a slight cross-linking of PLLA and PDLA chain couples in the mobile amorphous phase, whereas the chain couples in the crystalline phase hardly participate in the cross-linking reaction. The exclusive cross-linking of PLA chains in the amorphous phase not only allows for the introduction of abundant cross-linking points at the ends of the chain couples to prevent them from completely decoupling upon melting but also retains large amounts of long crystallizable PLA segments existing in the initially formed sc crystallites to impart the resulting sc-PLA with an excellent recrystallization ability upon cooling. The formation or reformulation of sc crystallites in the continuous melting and recrystallization process is found to be perfectly reversible, without any trace of hc crystallites.