Polymers with laterally attached rods exhibiting negative Poisson's ratio

Abstract

Materials having negative Poisson's ratio (auxetic) exhibit the unusual property of expanding when uniaxially stretched and getting thinner when compressed, thus contrasting with conventional materials which show the opposite behaviour. Liquid crystalline polymers (LCPs) have been proposed to exhibit this behaviour, which arises as a result of a mechanism which involves re-orientation of laterally attached rod-like units which in the unstressed state are aligned in the direction of the main polymer chain, whilst in the stressed state they rotate to the orthogonal direction. This deformation mechanism results in a lateral expansion due to an increase in the inter-chain distance when the polymer is stretched. In this dissertation, molecular mechanics and molecular dynamics simulations were performed, using the PCFF force-field, on polymeric systems based on A. C. Griffin's LCPs in order to understand better the behaviour of these systems when they are stretched. These simulations suggest that when such polymers are stretched along the direction of the main chain (the Z-direction), the systems are conventional at small strains but auxetic in some planes at higher strains. In particular it was found that although certain polymers got thicker in a certain plane (the YZ plane) but narrower in another plane (the XZ plane), a negative Poisson's ratio was still exhibited through the overall increase in thickness of the cross-sectional area.