Facile Anisotropic Deswelling Method for Realizing Large‐Area Cholesteric Liquid Crystal Elastomers with Uniform Structural Color and Broad‐Range Mechanochromic Response

Abstract

Cholesteric liquid crystal elastomers (CLCEs) are soft and dynamic photonic elements that couple the circularly polarized structural color from the cholesteric helix to the viscoelasticity of rubbers: the reflection color is mechanically tunable (mechanochromic response) over a broad range. This requires uniform helix orientation, previously realized by long‐term centrifugation to ensure anisotropic deswelling, or using sacrificial substrates or external fields. The present paper presents a simple, reproducible, and scalable method to fabricate highly elastic, large‐area, millimeters thick CLCE sheets with intense uniform reflection color that is repeatably, rapidly, and continuously tunable across the full visible spectrum by stretching or compressing. A precursor solution is poured onto a substrate and allowed to polymerize into a 3D network during solvent evaporation. Pinning to the substrate prevents in‐plane shrinkage, thereby realizing anisotropic deswelling in an unprecedentedly simple manner. Quantitative stress–strain–reflection wavelength characterization reveals behavior in line with theoretical predictions: two linear regimes are identified for strains below and above the helix unwinding threshold, respectively. Up to a doubling of the sample length, the continuous color variation across the full visible spectrum repeatedly follows a volume conserving function of the strain, allowing the CLCE to be used as optical high‐resolution strain sensor.

From Equilibrium Liquid Crystal Formation and Kinetic Arrest to Photonic Bandgap Films Using Suspensions of Cellulose Nanocrystals

“…The network formation corresponds to the kinetic arrest in a drying CNC suspension. While the original anisotropic deswelling method employed long-term centrifugation during solvent evaporation to ensure the required unidirectionality of sample compression [180,181], thereby orienting the helix perpendicular to the resulting elastomer film, we recently demonstrated that the same effect can be achieved without centrifugation if the solution droplet is pinned to the substrate to avoid horizontal compression [182], just as for the case of drying CNC suspensions. Moreover, when a CLCE is stretched in the film plane, perpendicular to the helix axis, the perfect sinusoidal modulation of the optic axis is lost.…”

“…Moreover, when a CLCE is stretched in the film plane, perpendicular to the helix axis, the perfect sinusoidal modulation of the optic axis is lost. Just like for films produced by drying CNC suspensions in which m is not vertical, an alternating arrangement of rapid and slow director rotations results, removing the circular polarization of the reflected light such that the color is seen almost with equal strength through leftand right-handed circular polarizers [182,183].…”

Reconfigurable and Spatially Programmable Chameleon Skin‐Like Material Utilizing Light Responsive Covalent Adaptable Cholesteric Liquid Crystal Elastomers

“…Previous freely‐standing CLCE films have demonstrated control over color changes through thermal and mechanical strain, yet lack the ability to permanently reconfigure the optical characteristics as demonstrated here. [ 13,18,19 ] We demonstrate both reversible color and shape change through transient application of strain and also permanent reconfiguration of color and shape through photoactivated AFT‐based DCC. [ 20 ] To achieve these outcomes, films were prepared with all the necessary elements; crosslinking, an LCE host, a chiral LC monomer, a radical photoinitiator, and incorporation of AFT‐capable moieties throughout the network.…”

“…Other methods to achieve a helicoidal structure use anisotropic deswelling but this process takes ≈8+ hours. [ 13,19,25 ] The approach reported here resulted in a CLCE film with thickness exceeding 50 µm, contributing to variability in alignment but increasing the directionality of observable specular reflection. The resin was cooled between two glass slides to 60 °C, between the broad chiral nematic phase transition temperature of 40 °C and the transition from a chiral nematic to the isotropic phase (T NI ) at 70 °C, as indicated by differential scanning calorimetry (DSC) measurements performed on the resin (Figure S1, Supporting Information).…”

“…used an anisotropic deswelling method to align a freely‐standing CLCE film that uses uniaxial strain to dramatically deform the material and alter the visible reflection. [ 19 ] Although this method produces robust, highly aligned elastomers, this method requires 24 h of processing to occur, with 5 h for the first polymerization stage, whereas the procedure reported here requires only manual shearing of the material to achieve the necessary alignment to obtain a reflective surface in minutes with polymerizations occurring within seconds using light irradiation to initiate an acrylate homopolymerization.…”