New insights on moisture transport in wood

Wood is a well-used building material, for building structure, façade, windows, etc., where the capacity of wood to absorb water is not only an issue of dimensional stability and modified material properties, but also of durability. As water molecules are adsorbed into the hydrophilic matrix in the cell walls, the induced fluid-solid interaction forces result in swelling of these cell walls. The interaction of the composite polymeric material, that is the layer S2 of wood cell wall, with water is known to rearrange its internal structure, make it moisture sensitive and influence its physical properties. As the origin of moisture-induced processes is found at the S2 cell wall layer, we study the coupled effects of water sorption on hygric and mechanical properties of different polymeric components. Our aim is to understand all the ramifications of this intricate nanocomposite, with the specific aim of upscaling the results to cellular and macroscopic scales. In order to study the behavior of S2 layer, we analyze the different configurations of cellulose microfibril aggregates and S2 matrix using Molecular Dynamics (MD) simulations. These atomistic simulations are used to mimic water adsorption and desorption in amorphous cellulose, make observations on hysteresis and relate the hygromechanical behavior as observed from the breaking and reforming of hydrogen bonds. We upscale the observations using a poromechanical constitutive model. Further, upscaling to cellular scale is informed through accurate geometrical description using X-ray CT at different relative humidity. The ensemble of results documents the full co-occurrence of sorption and swelling. It shows swelling is reduced by the multilayer wall composition and the cellular structure of wood. This modeling methodology provides new insights in understanding wood material properties and behavior which cannot be directly determined from experiments and allows to explore new pathways for material development and durability of wooden components.