Molecular Dynamics Study On Microproperties Of Cotton Cellulose During Tensile Process

Xinjiang is the most important cotton producing region in China.During the mechanical harvesting and subsequent processing of cotton,due to the squeezing and friction effects of mechanical devices,cotton fibers may stretch and break,reducing the quality of cotton and cotton products,which to some extent restricts the rapid development of cotton in Xinjiang.In addition,with the rapid advancement of industrialization,the demand for high-performance and green composite materials has become increasingly strong.However,most existing high-performance cellulose based composite materials only use cellulose as a reinforcing agent,which undoubtedly limits the advantages of nanocellulose in structure and performance.It is necessary to explore the inherent physical and chemical properties of cellulose in order to develop new composite materials.Moreover,the performance degradation caused by environmental moisture during the use of cellulose materials cannot be ignored in the application process.The article uses numerical simulation methods to study the tensile fracture behavior of cotton fibers during mechanical harvesting and subsequent processing,as well as its impact on mechanical properties.The results provide a theoretical research basis for clarifying the fiber fracture mechanism and micro friction mechanism during cotton mechanical harvesting and textile production and processing.At the same time,research will be conducted on the motion,aggregation,and microscopic evolution of cellulose chains and water in the cotton cellulose aqueous system to gain a more thorough understanding of the performance degradation behavior of cotton cellulose in the presence of water molecules,and to more effectively apply cellulose and cellulose based composite materials to daily production.The main research content and results are as follows:（1）Material Studio software was used to construct cotton cellulose amorphous system,crystal system and cellulose amorphous with water system according to the literature data,and optimize the energy of the system on the basis of the initial model to make its structure stable and close to the natural real state of cotton cellulose.（2）The uniaxial tensile behavior of cotton cellulose system under three strain rates(10^-4/ps,10^-5/ps and10^-6/ps)was simulated by molecular dynamics,and the stress-strain response curves of cotton cellulose system under different strain rates were obtained.The effect of strain rate on tensile fracture mechanism of cotton cellulose was analyzed.The dynamic simulation results show that the mechanical properties of cotton cellulose amorphous system increase with the increase of loading strain rate.Compared with the strain rate of 10^-5/ps,the elastic modulus of the system increased by 6.73 GPa at the strain rate of 10^-4/ps.In the process of stretching,the amorphous region of cotton cellulose mainly adapts to the external load through the stretching deformation of the macromolecular chain,that is,the deformation of the bond length and bond Angle,and the molecular chain does not break.The main reasons for the elongation of amorphous cellulose chains under different strain rates are different.At the same stretch length,the failure of cotton cellulose occurs in the crystal region,caused by the slippage and rearrangement of some molecular chains in the crystal structure.（3）The effect of water content on the tensile behavior and microscopic properties of cotton cellulose amorphous system was investigated,including the tensile mechanical properties,free volume,diffusion coefficient,glass transition temperature and the adsorption order of the preferred hydration site on cellulose.The results showed that the mechanical properties of the system increased first and then decreased with the increase of water content.When the water content is 4%,the mechanical properties of cotton cellulose were the best,and the elastic modulus and shear modulus increased by 7.6%and 9.4%respectively compared with that of dry cellulose.When the water content is 4%,the free volume of the system is the smallest,and its value is 697.79（?）³,which is 52.76%lower than 1476.97（?）³ when the water content is 0%.It indicates that the activity of cellulose chain is decreasing and the rigidity of cellulose chain is increasing.The mean square displacement and diffusion coefficient of water in the system are affected by the polymerization state of water molecules and the free water content.Compared with dry cellulose,the glass transition temperature of the system with 4%water content was increased by 72 K.Over the whole range of water content studied,hydroxyl O2,O3 and O6 on the cotton cellulose chain were dominant in the reaction with water compared to ether-based O4 and O5.In the 4%water content system,the molecular weight of water around the glycosidic bond O4 was the least,which caused the least damage to the original structure of cotton cellulose.