Visualization Modeling And Molecular Dynamics Simulations Of Wood Nano Cellulose

As the most abundant form of biomaterial on earth, wood fiber is one of the most stable and the most popular polymer in industry. Cellulose is an essential component of wood fiber. The mechanical properties of cellulose can be characterized by their properties in both the ordered crystalline and disordered amorphous phases of the molecules. The amorphous cellulose regions contribute to the flexibility and the plasticity of the bulk microfibril, while the crystalline cellulose regions contribute to the elasticity of the materials. However, it is difficult to study the mechanical characteristics and deformation phenomenon of materials because the microstructure and properties cannot be completely understood only by experimental methods. The nano technology and molecular dynamics methods have been introduced and used in wood science that greatly broaden the research field of wood science and make the depth of research from cellular level to molecular level.This thesis studied the structural morphology of wood nano cellulose. The chemical method of acid hydrolysis has been used to separate cellulose crystals by removing the amorphous regions of cellulose, and then to generate the nano cellulose with mechanical method. The nano cellulose degree of crystallinity is so high that it is easy to form stable colloidal solution in water dispersing system. The nano cellulose prepared in experiments can be placed for a long time at room temperature without and layering or precipitation phenomenon.According to the structural features of nano cellulose, this paper presents the research work towards modeling the crystalline and amorphous cellulose structures with high-performance workstations and large scale clusters in Mississippi State University. The construction of wood nano cellulose models involved placing a specified number of14-β-D-glucose chains into the periodic simulation cells. The atom coordinates, length of the lattice, velocity distribution and environmental information can be extracted from these models. The open source code LAMMPS was used to write tensile deformation simulation program. After minimization and equilibrium, the models were under uniaxial tensile deformation simulation. The results calculated the interaction between the atoms, recorded the motion and stress of atoms, and analyzed the effect of hydrogen bonding fracture and recombination. With many simulations, appropriate timestep was determined to save calculation time and ensure accuracy. Post-processing analysis used MATLAB to analyze the simulation data and Atomeye to monitor the deformation.Molecular dynamic simulations were performed to investigate the deformation mechanisms with the use of the reactive force field ReaxFF. In contrast to previous molecular simulations in the literature, the reactive force field ReaxFF was used here to describe the atomic interactions, which allows dynamic bond scission and bond formation. The characterization parameters of wood nano cellulose were calculated by analyzing the simulation data, which was consistent with the results obtained in the literatures. The results showed that the reactive force field was suitable and reliability for carbohydrate researches.The stress-strain behavior due to deformation showed a typical transition from the linear and nonlinear elastic regions at low strains, to yield, then to stress softening and finally to strain hardening. This paper also qualitatively reproduced the dependence of the stress-strain behavior on strain rate, chain length, chain numbers and temperature. In addition, this work attempt to shed light on the mechanisms of deformation by examining the evolution of internal energy and chain conformations during mechanical deformation. The average length of intermolecular hydrogen bonds was observed to analyze the breaking and formation of hydrogen bonds. It can be concluded that chain slippage occurred in extension as evidenced by breaking of original and formation of new hydrogen bonds, thus resulting in poor recovery.This thesis discussed the research works towards the molecular simulation study used in wood nano cellulose. It was an interdisciplinary research of wood science. In order to improve performance of composite materials which be filled with nano fibers, these predictive models can be used to elucidate the interfacial compatibility between the cellulose and polymer and how deposited nanoparticles and nanophases on cellulose surfaces affect this interfacial strength. The work of this paper is of great significance for sustainable development of wood industry.