Effect of moisture content on the microscopic properties of amorphous cellulose: a molecular dynamics simulations

Abstract

Performance degradation of cellulose and cellulose-based materials caused by water is an inevitable problem in application processes. In most studies, this was attributed to the fracture and rebuilding of the hydrogen bond network in the system; however, limited attention was paid to the movement, aggregation state, and specific property evolution of cellulose and water during this process. In this study, based on molecular dynamics simulations, the effects of moisture content on the microscopic properties of cellulose are investigated, including the mechanical properties, diffusion coefficient, glass transition temperature, microscopic motion of water molecules, and preferred hydration sites on cellulose. The results show that the mechanical properties of the system increase and then decrease as the water content in the system increases. When the moisture content is 4%, the mechanical properties of cellulose are the best, and the elastic modulus and shear modulus increase by 7.6% and 9.4%, respectively, compared with those of dry cellulose. The glass transition temperature of the system with 4% moisture content increases by 72 K compared with that of dry cellulose. The mean square displacement and diffusion coefficient of water in the system is affected by the water molecules’ polymerisation state and the free water content. In the entire range of water contents studied, hydroxyl groups O2, O3, and O6 of cellulose dominate the reaction with water compared with acetal oxygens O4 and O5. In the system with 4% moisture content, the number of water molecules around the glycosidic bonds O4 are the most minor and cause the least damage to the cellulose structure. A critical water content point of 4% is recommended, and this result is expected to provide a reference for maintaining the excellent and stable properties of cellulose and cellulose-based materials.