Direct-Ink-Written Cholesteric Liquid Crystal Elastomer with Programmable Mechanochromic Response

Cholesteric liquid crystal elastomers (CLCEs) are unique anisotropic rubbers that can change their structural color in response to various stimuli such as heat, chemicals, electric fields, and mechanical stress. Methods such as anisotropic deswelling and surface alignment have been adopted to prepare CLCEs; however, they have limitations in creating spatially controlled CLCE geometries. In this work, a direct ink writing (DIW)-based 3D-printable CLCE that can be prepared by extruding viscous CLC ink is developed through a 3D printer nozzle, followed by photopolymerization. Interestingly, the helical axis is inclined to the printing direction by approximate to 32 degrees due to a combination of the shear-induced alignment causes during extrusion and the elongational force generated during deposition onto the substrate. This unusual helical axis distortion leads to both blue and red shifts of the reflection color depending on the direction of observation relative to the printing axis. Notably, the printed CLCE exhibits anisotropic mechanochromism upon stretching, because of the stretching-direction-dependent variations in the slant angle of the helical axis. This anisotropic mechanochromism can be harnessed to develop a unique CLCE-based strain sensor displaying intricate color patterns upon stretching, with significant application potential in encryption, anticounterfeiting, and structural health monitoring. Cholesteric liquid crystal elastomers (CLCEs) with slanted helices are prepared by direct ink writing-based 3D printing. The anisotropic mechanochromic response of the printed CLCE to being stretched relative to the printing direction has been observed for the first time, thereby enabling the creation of a unique strain sensor displaying intricate color patterns.image

Automated summary

In this study, a printable cholesteric liquid crystal elastomer (CLCE) was successfully prepared using direct ink writing-based 3D printing. By introducing shear and elongational forces during the preparation process, the helical axis of the CLCE was inclined, resulting in an anisotropic mechanochromic response when stretched. This unique property opens up new possibilities for the development of strain sensors with intricate color patterns.