A rapid thermally responsive shape-memory EVA/NBR/LNBR thermoplastic vulcanizates based on a coordination-crosslinked system

Novel thermoplastic vulcanizates (TPVs) based on a coordination-crosslinking system composed of ethylene-vinyl acetate copolymer (EVA), nitrile butadiene rubber (NBR) and liquid nitrile butadiene rubber (LNBR) were prepared via dynamic vulcanization, and a simple and efficient strategy for fabricating thermally responsive shape-memory polymers (TSMPs) was developed. After etching, both EVA/NBR/LNBR TPV and EVA/NBR TPV exhibited distinct sea-island surface microstructures. The incorporation of LNBR into NBR effectively reduced the particle size of the cross-linked rubber phase in EVA/NBR/LNBR TPV, significantly enhancing the mechanical strength of the TPV. EVA, serving as the continuous phase, mainly contributed to the shape fixity capability of the dynamic vulcanizate system. In contrast, the highly elastic NBR/LNBR rubber particles acted as the dispersed phase, providing the primary driving force during shape recovery. The introduction of LNBR into NBR resulted in a slight decrease in the shape-fixity ratio (SF%) of EVA/NBR/LNBR TPVs, but led to a remarkable increase in shape-recovery ratio (SR%). By adjusting the mass ratio of EVA to the NBR/LNBR blend, the shape fixity and shape recovery behaviors of the EVA/NBR/LNBR TPV can be effectively controlled under thermal stimulation. Increasing the EVA content in the TPV improved the SF% but significantly reduced the SR%. The shape-memory performance of EVA/NBR/LNBR TPVs was strongly temperature-dependent. When both the fixing and recovery temperatures were set to 85°C, the EVA/NBR/LNBR TPV with a mass ratio of 80/20 exhibited SF% and SR% values above 94.0%, with a shape-recovery time of 15∼20 s, demonstrating excellent shape-memory behavior.