Enhancing stereocomplexation by the synergistic effect of d-mannitol and CO2 foaming: Toward high-performance and fully bio-based polylactic acid foams.

Although polylactic acid (PLA) stereocomplex (SC) crystallites have superior mechanical strength and heat resistance compared to homocrystallites (HC), the preparation of PLA foams with a high content of SC crystallites and a fine cell structure remains challenging. Herein, d-Mannitol (DM) with plentiful hydroxyl groups was utilized as an efficient bio-based nucleating agent for the SC crystallization of PLLA/PDLA blends. The results demonstrated that the incorporation of DM significantly promoted SC crystallization under both non-isothermal and isothermal conditions. This preferential SC crystallization was confirmed by FTIR spectroscopy and attributed to the hydrogen-bonding interactions between PLA chains and DM molecules, which could shorten the diffusion path for enantiomer chain pairing and reduce the nucleation and kinetic hindrances during SC crystallization. Notably, the nucleation effect of DM, combined with CO2 plasticization and the biaxial tension generated during foaming, synergistically promoted the SC crystallinity of PLA-based foams from 22.6 % to 46.5 %. Owing to the enhanced cell structure and increased SC crystallinity, the specific storage modulus of PLLA/PDLA/DM foam at 200 °C was 11.8 MPa/(g/cm3), 11.8 times greater than that of PLLA/PDLA foam. This work provides a facile yet effective strategy for preparing fully bio-based PLA foam with excellent mechanical properties and high heat resistance.