Photonic Cholesteric Liquid-Crystal Elastomers with Reprogrammable Helical Pitch and Handedness

Abstract

The unique combination of the rubber-like property and the photonic helicoidal structure of cholesteric liquid-crystal elastomers (CLCEs) results in one-handed circular polarized light reflection, the wavelength of which is dictated by the Bragg relationship. Herein, a highly stretchable mechanochromic photonic CLCE film was fabricated by cross-linking mesogenic oligomers having thiol terminal groups, which further reacted to form disulfide (−S−S−) linkages. The mechanically stretched photonic CLCE film reflected both right-and left-handed circular polarized lights with a blue-shifted color. The helicoidal pitch and handedness controlled by the applied strain were programmed through a dynamic exchange reaction between the −S−S− linkages, thus realizing the patterning at selective regions. The pattern almost vanished under unpolarized daylight but was visible under circularly polarized light when the patterned photonic CLCE film had been heated above its isotropic temperature. The hidden patterns of the heat-treated CLCE film reappeared under unpolarized daylight when stretched, demonstrating a data encryption ability. These patterned photonic elastomers can be uniquely used in sensors, actuators, soft robotics, flexible displays, data encryption, and anticounterfeiting applications with a mechanochromic camouflage response.

Liquid crystal-based structural color actuators

“…In other work, disulfide (S-S) bonds were introduced in CLCE structural color actuators via a two-stage reaction, where LC oligomers with thiol terminal groups were synthesized via the thiol-acrylate Michael addition reaction in the first stage, followed by coupling the thiols to form dynamic S-S bonds in the second stage 68 . This CLCE film exhibited a redshift of the reflection color upon heating due to the thickness expansion.…”

Liquid Crystalline Elastomers Based on Click Chemistry

“…The thiol–Michael addition is particularly useful for preparing telechelic oligomers that can be subsequently aligned and polymerized. A variety of LCEs with different functionalities have been designed and prepared using this method. − For example, Gelebart et al synthesized an acrylate-terminated LC oligomer via the thiol–Michael reaction between a diacrylate monomer (in excess) and a dithiol chain extender . The oligomer was then copolymerized with an azobenzene-based dimethacrylate monomer to produce aligned LCE fibers that bent toward a UV light owing to the photoisomerization of the azobenzene molecules (Figure a).…”

Pigmented Structural Color Actuators Fueled by Near-Infrared Light

“…Stimuli-responsive structural color actuators based on cholesteric liquid crystals (CLCs), − cellulose nanocrystals, − hydrogels, − and opal/inverse opal-structured polymers − have been reported. Light has emerged as the most promising stimulus for structural color actuators as it allows wireless and remote activation with spatiotemporally selective capabilities. , For example, near-infrared (NIR, 0.76–1.5 μm)-light-responsive bending and color changes can be achieved using a bilayer hydrogel with an inverse opal scaffold or a cellulose nanocrystal/polyurethane bilayer. , However, achieving more complex color and shape changes using these materials remains challenging.…”