Towards unraveling the moisture-induced shape memory effect of wood: the role of interface mechanics revealed by upscaling atomistic to composite modeling

The moisture-induced shape memory effect (SME) is one of the most intriguing phenomena of wood, where wood can stably retain a certain deformed shape and, upon moisture sorption, can recover the original shape. Despite the long history of wood utilization, the SME is still not fully understood. Combining molecular dynamics (MD) and finite-element (FE) modeling, a possible mechanism of the SME of wood cell walls is explored, emphasizing the role of interface mechanics, a factor previously overlooked. Interface mechanics extracted from molecular simulations are implemented in different mechanical models solved by FEs, representing three configurations encountered in wood cell walls. These models incorporate moisture-dependent elastic moduli of the matrix and moisture-dependent behavior of the interface. One configuration, denoted as a mechanical hotspot with a fiber–fiber interface, is found to particularly strengthen the SME. Systematic parametric studies reveal that interface mechanics could be the source of shape memory. Notably, upon wetting, the interface is weak and soft, and the material can be easily deformed. Upon drying, the interface becomes strong and stiff, and composite deformation can be locked. When the interface is wetted again and weakened, the previously locked deformation cannot be sustained, and recovery occurs. The elastic energy and topological information stored in the cellulose fiber network is the driving force of the recovery process. This work proposes an interface behaving as a moisture-induced molecular switch.