A green strategy to regulate cellular structure and crystallization of poly(lactic acid) foams based on pre-isothermal cold crystallization and CO2 foaming.

Abstract

Bio-based poly(lactic acid) (PLA) foams show broad application prospects. However, PLA exhibits poor foaming ability and controllability due to its low melt strength and slow crystallization rate. The commonly used methods for enhancing the foaming ability of PLA easily harm its biodegradability and biocompatibility. This paper proposed a green strategy to regulate the cellular structure of PLA foams based on pre-isothermal cold crystallization. Before foaming, a pre-isothermal treatment was applied to control the crystallization of PLA, and the induced crystals were utilized to enhance melt strength and cell nucleation. SEM shows that a higher pre-isothermal temperature (Tc) leads to a larger spherulite size and higher crystal stability before foaming. The foaming experimental results demonstrate that, as Tc increases, the expansion ratio and cell size increase first and then decrease. This is because proper crystallization helps to improve melt strength and promote foaming, but excessive crystallization restricts cell growth. Finally, as Tc increases, the high melting temperature crystals of the foam gradually increase, while the crystallinity of the foam increases first and then decreases, which is attributed to strain-induced crystallization. The method proposed herein provides a green and simple route to fabricate PLA foams with tunable cellular structure and crystallization.