Resolving the mechanical response of liquid crystal elastomers – semi-soft elastic or auxetic

Liquid crystal elastomers (LCEs) display one of two distinct behaviours under deformations perpendicular to the nematic director: semi-soft elastic (SSE) or biaxial auxetic responses. The physical reason why an LCE should show one rather than the other response has so far remained elusive. Furthermore, while these responses have been observed individually, they have yet to be reported simultaneously or in chemically similar LCEs. Here, a series of monodomain side-chain LCEs with varying cross-link density is studied. At higher cross-link density, the samples display a clear auxetic (negative Poisson’s ratio) response, while at lower cross-link densities, the behaviour is consistent with the SSE response. At intermediate cross-link densities, the behaviour includes both auxetic and SSE natures. A theoretical framework is established from a modified Maier-Saupe model which quantifies the larger internal stresses in the higher cross-link density LCEs; a factor of ~3.3 greater internal stress is found between systems with a factor of 8 difference in cross-link density. We suggest that the internal field can, if sufficiently large, be a factor in causing the LCE to deform biaxially (and therefore auxetically), rather than uniaxially under strain. Using tensile measurements, cross-polarised microscopy and X-ray spectroscopy, we demonstrate that the deformation behaviour of these LCEs lies on a continuum and is a combination of the extent of the nematic ordering, robustness of nematic coupling, and relaxation dynamics. We show the importance of polymer dynamics in these systems, such that a reduction in the auxetic threshold occurs upon lowering strain rates or increasing temperature.