Isotropic Liquid Crystal Elastomers as Exceptional Photoelastic Strain Sensors

Abstract

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.

A pseudo-anelastic model for stress softening in liquid crystal elastomers

“…To accomplish this, we combined the neoclassical theory with the pseudo-elasticity framework for particle-reinforced rubber proposed in [43,44]. Since uniaxial stretch deformations are easier to reproduce in practice [12,13,89,90], we analysed theoretically the stress softening under cyclic uniaxial tensile loads, and calibrated a specific form of the pseudo-energy function to recent experimental data at various temperatures reported in [13]. Based on our numerical results, we conclude that the pseudo-energy function captures well the Mullins-like inelastic responses observed experimentally in LCEs, but that different parameter sets must be used at different temperatures.…”

Composites of functional polymers: Toward physical intelligence using flexible and soft materials

“…The examples outlined in this review are superficial in scope compared to the potential for LCEs. LCEs exhibit functionalities beyond what is covered in this review (e.g., dynamic/structural color change [40,41] and mechanical dissipation [42]). Additionally, LCE chemistry has benefited from the progress of polymer chemistry [9,43], which has enabled exchangeable covalent linkages [44], printable inks with tunable transition temperatures [26], and actuation blocking stress above 1 MPa (with work densities that range from about 30 kJ m −3 up to about 1200 kJ m −3 ) [24,45], among other features [46,47].…”

Nematic liquid crystalline elastomers are aeolotropic materials

“…Usually, when the elastic properties of a material are investigated, uniaxial deformations, which are easier to reproduce experimentally, are examined first [38][39][40][41]. For a purely elastic isotropic material, the shear modulus is then inferred from a universal relation between elastic moduli from the classical theory.…”