Modeling of negative Poisson’s ratio (auxetic) crystalline cellulose Iβ

Energy minimizations for unstretched and stretched cellulose models using an all-atom empirical force field (Molecular Mechanics) have been performed to investigate the mechanism for auxetic (negative Poisson’s ratio) response in crystalline cellulose Iβ from kraft cooked Norway spruce. An initial investigation to identify an appropriate force field led to a study of the structure and elastic constants from models employing the CVFF force field. Negative values of on-axis Poisson’s ratios nu31 and nu13 in the x1-x3 plane containing the chain direction (x3) were realized in energy minimizations employing a stress perpendicular to the hydrogen-bonded cellobiose sheets to simulate swelling in this direction due to the kraft cooking process. Energy minimizations of structural evolution due to stretching along the x3 chain direction of the ‘swollen’ (kraft cooked) model identified chain rotation about the chain axis combined with inextensible secondary bonds as the most likely mechanism for auxetic response

Microporous materials with negative Poisson's ratios. II. Mechanisms and interpretation

For pt.I see ibid., vol.22, p.1877-82 (1989). In a previous paper the morphology of a microporous material made from expanded poly(tetrafluoroethylene) was described and results presented for its mechanical behaviour. The material was shown to be highly anisotropic and exhibited a large negative Poisson's ratio. In this paper a simple model for the microstructure is described to account for this effect. The model is based on an interconnected array of anisotropic particles that deforms so as to produce a large transverse displacement under longitudinal tensile loading. Very good agreement is found between the model and experimental results, providing an explanation for the variation of Poisson's ratio with tensile strain, in terms of changes in material morphology. </jats:p