Effects of small temperature changes and strain rate on the elastoplastic behaviour of cellulose crystal at small deformations: A molecular dynamics approach

Abstract

This work investigates mainly the elastoplastic properties of cellulose crystal under deformations below the strain at ultimate stress in the z-direction using molecular dynamics (MD) simulations. Uniaxial tensile simulations were performed at strain rates of 10 −4 /ps, 10 −3 /ps, and 10 −2 /ps to study the effect of strain rate on the elastoplastic properties. The effect of temperature on the elastoplastic properties was also investigated for small temperature changes ranging from 298 K to 308 K. The elastoplastic properties obtained are based on the Ludwik modified model. These properties were obtained from MD simulation results using the least square method applied to the elastic and non-elastic regions distinctly. Moreover, it was observed for cellulose crystal that the strain hardening effect reduces with increasing strain rate and temperature. Also, at low strain rate in the z direction, a bump is observed on the stress-strain curve during the damage process of the cellulose crystal. This is due to the rearrangement of cellulose chains which results in cellulose recovery. It is shown that both covalent and non-covalent interactions contribute to this adaptability. In addition, cellulose crystal is observed to deform more isotropically with increasing strain rate. Moreover, an MD simulation of the uniaxial tensile test of CNT/cellulose crystal composite was performed. This composite demonstrated enhanced mechanical strength and Young's modulus compared to pure cellulose crystal. Notably, the composite exhibited a less isotropic behaviour and reduced strain-hardening characteristics, as indicated by its Poisson's ratio and the Ludwik parameters respectively. These results will help in the design of cellulose nano-crystal composites especially to exploit its plastic behaviour in applications such as piezoresistivity sensing and more. • The strain hardening effect decreased with increasing strain rate and temperature. • At low strain rate in the z direction, there is a rearrangement of cellulose chains which results in cellulose recovery. • Cellulose crystal deforms less anistropically with increasing strain rate.