Multi-Mode Mechanochromic Responses from Cholesteric Liquid Crystal Elastomer Tubes of Uniform Sheath

Abstract

Materials that exhibit varied optical responses to different modes of mechanical stimuli are attractive for complex sensing and adaptive functionalities. However, most mechanochromic materials are fabricated from films or fibers with limited actuation modes. Here, hollow tubes of a symmetric sheath are created using cholesteric liquid crystal elastomers (CLCEs) at the sub-millimeter scale. The oligomeric precursor is sheared in an elastomeric microchannel to form uniform thickness, overcoming gravity effect and Plateau-Rayleigh instability. In addition, the coloration is achieved to be faster and have higher reflectivity compared to that of solid fibers. The tube can undergo axial, circumferential, and radial strains upon extension and inflation. The combination of molecular anisotropy and geometry of the tube enables highly sensitive mechanochromic responses in both azimuthal and axial directions: inflation causes red-to-violet shift (≈220 nm) at a circumferential strain of 0.57. The inflation of a bent tube generates another mechanochromic mode with a higher sensitivity to strain. Finally, display of 26 alphabets is achieved using 5 tubes, of which the positions can be reconfigured, and curvature-dependent 3D photonic skins are demonstrated from tubes wrapped around 3D objects. The multi-mode mechanochromic tubes will find applications for soft robotics, adaptive displays, wearable sensors, and spectrometers.