MOLECULAR DYNAMICS STUDY ON THE EFFECT OF MOISTURE CONTENT ON THE MECHANICAL PROPERTIES OF AMORPHOUS CELLULOSE

The alteration of mechanical properties because of moisture is an inevitable problem in the practical use of cellulosic materials, as well as green and high-performance materials synthesized based on cellulose. Although researchers have analyzed and reported this issue from various aspects, it is necessary to report the variation of mechanical properties of the cellulose system and its causes in detail from the molecular level as well. Herein, the effect of moisture content on the mechanical properties of cellulose is methodically examined by molecular dynamics methods. The main reasons for the structural changes caused by the stiffness and activity space of the cellulose chains and the number of hydrogen bonds in the system are explained and discussed. The obtained results reveal that, in the simulated range of moisture content, low moisture (0 to 4%) exhibits a positive effect on the mechanical properties of the amorphous cellulose region, whereas the effect of high moisture content (4 to 8%) is negative. The mobility of cellulose chains first reduces and then intensifies as the number of water molecules increases, while the rigidity of the corresponding system first increases and then decreases. Additionally, the free volume of the system increases first and then decreases as the number of water molecules rises. The mechanical properties of the amorphous region of cellulose are proportionally correlated with the number of hydrogen bonds in the system. Based on these results, a moisture content of 2% can enhance the properties, increasing the H-bond density in the cellulose network.