Effect of thermal-hydro-mechanical (THM) on wood shape memory effect and its mechanism

• THM at 200 °C eliminates SME with 93% dry shape recovery reduction. • THM disrupts 3D crosslinking network, enabling shape fixation of compressed wood. • High-temperature THM induces 73% hemicellulose pyrolysis. • Lignin cross-linking increases by 42% under 220°C treatment. • Covalent bond formation rises 2.1-fold after THM processing. This study investigates the three-dimensional (3D) crosslinked network of wood under thermohydromechanical (THM) treatment and elucidates the mechanisms by which THM disrupts this network, causing loss of the shape memory effect (SME). Above a critical threshold temperature of 200°C, high-temperature THM treatment markedly weakens or eliminates the SME. Specifically, after treatment at 220°C, dry and wet shape recoveries decreased by 93% and 79%, respectively, while shape springback stress dropped by 62.1%, as confirmed by mechanical analysis. Microstructural and chemical analyses revealed that THM disrupts the 3D crosslinked network of the cell wall through hemicellulose pyrolysis and lignin crosslinking, accompanied by a 10.1% decrease in crystallinity and a 35.6% expansion of the cellulose unit cell size at 200°C. These changes reduce the entropic elasticity and internal stress recovery capacity of the wood. The findings provide a scientific basis for achieving permanent shape fixation in compressed wood, with direct implications for industrial processing of wood-based products and the development of sustainable shape memory materials.