Lignin modification for in-situ cured lignin-maleyl network in semi-crystalline polyamide/lignin shape memory composites

Lignin-based shape memory polymers (SMPs) have received increasing attention in the context of lignin valorization. This work designed a novel SMP with high content of lignin and high transition temperature (>150 °C) for shape memory. Soda lignin (SL) was first modified with maleic anhydride resulting in a lignin derivative (MAL) as confirmed by FTIR, 31P NMR, 2D-HSQC and TGA. The modification results in an elevated molecular weight and reduced Tg. SMPs were designed based on a penetrating lignin-maleyl network in a crystalline polyamide (PA) matrix, the former serving as the lock for the permanent shape and the latter as the reversible switch. The SMPs were prepared via procedures of solution mixing, film casting and in-situ thermal curing. Rheological study revealed that the precursors can be fully cured at 200 °C within 20 min with the aid of a bismaleimide spacer, which ensures a proper temperature window for curing. Compared to the PA/SL series, the PA/MAL series demonstrated improved mechanical properties and well-integrated interfacial morphologies, which enables efficient shape memory performance. This work provides a proof-of-concept study demonstrating that the design of crosslinked semi-crystalline polyamide/lignin dual-phase composites with modified lignin enables high-transition-temperature shape memory behavior for potential demanding applications such as in aerospace or mining engineering.