Wood as a hydrothermally stimulated shape-memory material: mechanisms of shape-memory effect and molecular assembly structure networks

This study aimed to evaluate the shape-memory effect (SME) of wood (Populus x beijingensis W. Y. Hsu) and identify the net-points and switches in its molecular and morphological structures. During several cycles of deformation and subsequent recovery, a high shape recovery rate and ratio were maintained. The transverse compression tests of wet and dry wood reveal that the hydrothermal coupling stimulation can considerably reduce the strength of wood. The X-ray diffraction characterization of wood under hydrothermal stimulation shows that the role of network nodes in the SME of wood is influenced by temperature. The wavenumber shifting and changes in the intensity ratio of the characteristic Fourier transform infrared peaks showed that hydrogen bonds acted as switches for the water-stimulated shape-memory behavior. By taking into account viscoelastic relaxation, a kinetic model derived from nonequilibrium thermodynamic fluctuation theory was used to describe the shape recovery process. The effects of hydration on recovery kinetics, activation, and dynamic mechanical behaviors were also studied. To explain the shape-memory mechanism of wood under hydrothermal stimulation, a hybrid-structure network model based on a single three-dimensional switch network was proposed in this study.