Isotropic Liquid Crystal Elastomers as Exceptional Photoelastic Strain Sensors

A family of acrylate-based isotropic Liquid Crystal Elastomers (LCEs) exhibit stress- and strain-optic coefficients orders of magnitude greater than conventional polymeric and photoelastic materials. The three materials, composed of liquid crystalline and nonliquid crystalline monomers, show no nematic phase at any temperature. One of the materials has previously been synthesized with nematic symmetry, but here is instead templated with isotropic symmetry, demonstrating a previously unrealized idea proposed by de Gennes in 1969. Uniaxial strains applied to each material induce nematic ordering which we quantify using dye-absorption spectra and polarized Raman Spectroscopy. We deduce the coupling constants between the nematic liquid crystal order parameter and applied strain varies between 0.37 +/- 0.02 and 0.66 +/- 0.02-values large compared to other LCE systems. The combination of high strain-optic coefficients (0.048 +/- 0.003 to 0.11 +/- 0.01) and high compliances (245 +/- 18 to 1900 +/- 100 GPa(-1)) demonstrates that isotropic LCEs are exciting candidates for photoelastic coatings for assessing deformations across soft devices and biomaterials.