Studying Wood-Water Relationships with Molecular Dynamics—How Far We Are Now?

Wood is an orthotropic cellular biomaterial. As a plant and as a building material, water impacts the behavior of wood. Wood is also a hierarchical material. At the base of this hierarchy lies the cell wall material. As water molecules are adsorbed by the hydrophilic matrix that make up the cell walls, the induced fluid-solid interaction forces result in swelling of these cell walls. Thus, this process, at the origin of wood-moisture relationships, lies at the nanoporous material scale.<br/><br/>The layer S2 of the wood cell wall is actually a composite polymeric material. Stiff crystalline cellulose bundles to form fibrils that are embedded by a profoundly organized matrix of different hemicelluloses, galactoglucomannan and glucuronoarabinoxylan, and of two states of guaiacyl lignin, non-condensed and condensed, considering here coniferous species. We have reproduced these assemblies and subjected them to different in-silico experiments. Using grand canonical Monte-Carlo and molecular dynamics, we have obtained adsorption and desorption isotherms of different polymeric systems. Water is known to rearrange the internal structure of the material of the S2 cell wall. The in-silico systems obtained at different moisture content can be probed to provide insights on the extent of this rearrangement in terms of configuration and in terms on its influence on the physical properties of the material.<br/><br/>Atomistic simulations used to mimic water adsorption and desorption in amorphous polymers, allow observations on mechanical behavior like swelling and shrinking, mechanical softening in compression and shear, and on the stick-slip behavior of the stiff fibrils pulled out of the matrix. We observed hysteresis not only in water sorption but also in mechanical properties. This hygromechanical behavior can also be observed in particular from the breaking and reforming of hydrogen bonds.<br/><br/>Atomistic modeling is an insightful tool for the in-depth study of the coupled effects of water sorption on hygric and mechanical properties of different polymeric components. Molecular modeling can contribute to support and complement experimental methods which yield, most frequently, indirect structural information. With molecular modeling, there is a freedom of investigating unlimited possibilities of configurations, ranging from individual wood polymer materials to composite structure resembling subunits of wood S2 cell wall. This presentation will cover recent methodological developments towards good practices for such simulations and the recent insights on wood cell wall S2 layer hygromechanical behavior.