Effect of Pressurized Hydrothermal Treatment on the Properties of Cellulose Amorphous Region Based on Molecular Dynamics Simulation

Based on Materials Studio software, a cellulose chain with a polymerization degree of 20 and several water molecules were constructed to form a cellulose amorphous region-water model. The effect of pressure on the wood properties during hydrothermal treatment was investigated to explain the changes in the macroscopic properties from a microscopic perspective, thus providing a theoretical basis for wood heat treatment research. In this study, we performed dynamic simulations at atmospheric pressure (0.1 MPa) and pressurized (0.2 MPa, 0.4 MPa, and 0.6 MPa) conditions under a combination of NPT. In addition, five aspects were analyzed in terms of energy change: cell parameters and density, cellulose mean square displacement, number of hydrogen bonds, and mechanical properties. The results showed that pressurized hydrothermal treatment increased the densification of the wood, decreased the volume, and increased the density. As the pressure increased, the difference between the average value of the nonbond energy and the total potential energy gradually decreased, the bond energy decreased, and the interatomic repulsive force within the molecule gradually weakened. The increase in the number of hydrogen bonds enhanced the restraining effect on the arrangement of cellulose molecular chains and slowed down the movement of the cellulose chains. Young's modulus (E) and shear modulus (G) increased with increasing pressure, and Poisson's ratio (gamma) and K/G decreased with increasing pressure during the pressurized hydrothermal treatment of wood. The pressurized hydrothermal treatment increased the stiffness and decreased the toughness of the wood compared with those of the model with atmospheric pressure hydrothermal treatment.