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 (γ) 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.

Six groups of cellulose-water-oxygen simulation models with oxygen concentrations of 0%, 2%, 4%, 6%, 8%, and 10% were established by molecular dynamics software to analyze the effect of oxygen concentration on the mechanical properties of wood cellulose during water vapor heat treatment in terms of the number of hydrogen bonds, the diffusion coefficient of water molecules, the mean square displacement of cellulose chains, and mechanical parameters. The results showed that the diffusion coefficient of water molecules increased steadily as oxygen concentration increased, which affected cell size and density to some extent. The mean square displacement of the cellulose chain at a higher oxygen concentration was larger than at a lower oxygen concentration, indicating that the cellulose chain became more unstable at high oxygen concentration. This trend was consistent with the amount of hydrogen bonds inside the cellulose chains. The analysis of mechanical parameters showed that Young’s modulus and shear modulus showed a trend of increasing and then decreasing with increasing oxygen concentration, and wood cellulose had good resistance to deformation and rigidity at 2% oxygen concentration. Therefore, during the heat treatment of wood, appropriately increasing the oxygen concentration will potentially improve the rigidity and distortion resistance of wood.

In this paper, a molecular model of cellulose amorphous region-water molecule was developed using Materials Studio software by applying the molecular dynamics method. The effect of low temperature on the properties of the lignocellulosic amorphous region, the main component of wood, was investigated in an attempt to explain the macroscopic property changes from a microscopic perspective and to provide a theoretical basis for the safe use of wood and wood products in low-temperature environments and other related areas of research. Dynamic simulations were carried out at 20 °C, 0 °C, −30 °C, −70 °C, −110 °C and −150 °C for the NPT combinations to obtain the energy, volume, density, and hydrogen bonding change trends of their models, respectively. The changes in the microstructure of the water molecule–cellulose amorphous region model were analyzed, and the mechanical properties were calculated. The results showed that the interaction between the amorphous cellulose region and water molecules was enhanced as the temperature decreased, the density of the models increased, and the volume decreased. The number of total hydrogen bonds and the number of hydrogen bonds between water molecule–cellulose chains increased for each model, and the decrease in temperature made the cellulose molecular activities weaker. The values of G, E, and K increased with the decrease in temperature, and K/G decreased with the decrease in temperature. It shows that the decrease in temperature is beneficial to enhance the mechanical properties of the amorphous region of cellulose and increases the stiffness of the material. However, the toughness and plasticity decrease when the temperature is too low.