Significant anisotropic deformation and optical shifts in stretched cholesteric liquid crystal elastomers.

Abstract

This study explores the opto-mechanical response of cholesteric liquid crystal elastomers (CLCEs) subjected to uniaxial stretching along the <i>x</i>-axis, perpendicular to their helical <i>z</i>-axis. A definitive crossover is observed in the strain (<i>ε</i>_<i>x</i>) dependencies of various optical and mechanical properties, such as the transmission spectra, degree of mesogen orientation, Poisson's ratios, and tensile stress. At low strains, CLCEs exhibit a blue shift in the selective reflection band due to a reduction in the helical pitch, accompanied by a decrease in reflection selectivity for circularly polarized light. Beyond a certain critical strain further pitch alterations halt. This strain regime is marked by substantial anisotropic lateral contractions without any <i>z</i>-axis contraction, as indicated by a Poisson's ratio (<i>μ</i>_<i>xz</i>) of zero. Within this intermediate strain regime, local directors predominantly reorient towards the <i>x</i>-direction within the <i>xy</i>-plane, resulting in a quasi-plateau of tensile stress. Approaching a higher critical strain a complete loss of reflective selectivity occurs. Past this threshold, while the mechanical responses resemble those of isotropic conventional rubber, they retain a periodic structure albeit without phase chirality. These observed features are accounted for by the Mao-Terentjev-Warner model, especially when the network anisotropy parameter is adjusted to match the critical strain magnitude associated with the cessation of selective reflection.