Enhanced heat resistance and compression strength of microcellular poly (lactic acid) foam by promoted stereocomplex crystallization with added D-Mannitol

Owing to low melt strength and slow crystallization rate, poly (lactic acid) (PLA) foams are usually obtained by adding micro-/nano-particles, which inevitably sacrifices its green nature. Stereocomplex (SC) crystallites formed between poly (L-lactic acid) (PLLA) and poly (D-lactic acid) (PDLA) are appealing to endow PLA with promoted crystallization and superior heat resistance. Herein, microcellular PLA foams with high heat resistance and compression property were obtained using asymmetric PLLA/PDLA blends by a supercritical carbon dioxide (sc-CO2) foaming technology. To facilitate the formation of SC, D-Mannitol (DM) with plentiful hydroxyl groups was applied, which notably promoted the formation of SC and subsequently the crystallization of homocrystallites (HC). The PLLA/PDLA/DM blends foams revealed much smaller cell sizes and higher cell densities compared with the neat PLLA. Furthermore, the PLA foam with 0.7 wt% DM exhibited good heat resistance under a loading of 1200 times, which displayed no changes after treating at 150 °C for 10 min, while the PLLA was obviously deformed. Additionally, the specific compressive modulus of PLLA/PDLA/DM blend foam was enhanced by 330% compared with the pure PLLA. Such fully-biobased PLA foams with superior heat resistance and high mechanical performance could be potentially applied into thermoformed food packaging.