Molecular Dynamics Study of Wood-Inspired Polymeric Composite Interactions with Water

In this study, we analyze the hygromechanical behavior of composite consisting of amorphous cellulose, xylan, lignins reinforced or not with crystalline cellulose and treat the composites with polyethylene glycol as consolidant. Atomistic simulations is 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. To better understand the hygro-mechanical behaviour of biopolymer composites, Molecular Dynamics combined with Grand Canonical Monte Carlo simulations [1] is used to model sorption and sorption-induced deformation and determine the mechanical properties of wood-inspired biopolymer composites and a system approaching S2 configuration.<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 includes recent methodological developments towards good practices for such simulations and the recent insights on wood cell wall S2 layer hygromechanical behavior [2].<br/><br/>Composites show swelling-induced sorption and a mechanical weakening upon sorption. Due to the reinforcing effect of the crystalline cellulose fibre, the swelling and weakening of composites in longitudinal direction is supressed. Additional sorption is found to occur in the porosity created by the misfit between crystalline cellulose fibre and matrix, leading to a reduction of the pullout shear strength due to breakage of matrix-fiber hydrogen bonds by the water molecules. Adding polyethylene glycol to composites results in filling the gap between crystalline cellulose and matrix, leading to a reduction of the volumetric swelling and sorption, and an enhancement of the pullout shear strength.<br/><br/>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/>This work in inspired by wood, an orthotropic cellular biomaterial. As a plant and as a building material, water impacts the behavior of wood and particularly the cell wall material. As water molecules are adsorbed by the hydrophilic composite that makes up the cell walls, the induced fluid-solid interaction forces result in swelling of cell walls. Thus, this process, at the origin of wood-moisture relationships, lies at the nanoporous material scale. A second inspiration of the work is the treatment of waterlogged archaeological wood of shipwrecks, like the Varsa and Mary Rose, with PEG for its consolidation and stabilization, where PEG molecules replace the water making wood at museum conditions less susceptible to changes in humidity and able to sustain mechanical load.<br/><br/>[1] M. Chen, B. Coasne, R. Guyer, D. Derome, J. Carmeliet, Role of hydrogen bonding in hysteresis observed in sorption-induced swelling of soft nanoporous polymers, Nat Com., 2018 29;9(1):3507. doi: 10.1038/s41467-018-05897-9<br/><br/>[2] A. Shomali, W. Liu, C. Zhang, E. Schofield, B. Coasne, D. Derome, J. Carmeliet. Molecular, mechanisms involved in treatment of waterlogged archeological wood with polyethylene glycol: a hybrid Monte Carlo and molecular dynamics study, under review, 2024.