Simultaneously Enhanced Stereocomplexation and Melt Strength by Dynamic Polylactide toward Heat-Resistant Green Foams

Abstract

Because of the poor stereocomplex (sc) crystallization and low melt strength, stereocomplexed polylactide (sc-PLA) is yet to fulfill its promise as a heat-resistant green foam. Herein, the strategy of dynamic PLA is proposed to promote sc crystallization and improve the melt strength of sc-PLA simultaneously. Maleic anhydride-grafted poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) were first blended to form sc crystallites, ensuring that the interchain cross-linking between PLLA and PDLA pairs occurred based on epoxy−anhydride chemistry. The prepared dynamic cross-linked sc-PLA enabled by Zn(II)-catalyzed transesterification can form exclusive sc crystallites during nonisothermal and isothermal crystallization, facilitated by the increased nucleation density and growth rate for sc crystallization. The inhibited separation of chains by interchain linking of PLLA and PDLA pairs and compatibilization induced by dynamic cross-linking due to the produced grafted multiblock copolymers of PLLA and PDLA by interchain cross-links shorten the diffusion path to form enantiomer chain pairs, reducing the kinetic hindrance of sc crystallization. The high viscosity of dynamic cross-linked sc-PLA at low frequencies indicates a significant improvement in melt strength, enabling the foamability with an expansion ratio of 5.9-fold by the supercritical carbon dioxide foaming technique. The obtained highly crystalline foam with exclusive sc crystallites can withstand temperatures up to 200 °C. The promoted sc crystallization and increased melt strength are realized simultaneously through dynamic cross-linking of PLLA and PDLA chains, opening up a new path for creating heat-resistant green foams.