(312j) Mechanochromic Liquid Crystal Blends

Cholesteryl ester liquid crystals (LC) self-assemble into periodic helical structures which give rise to their unique, responsive optical properties. The responsive liquid crystals undergo color change due to the pitch variation when subjected to external stimuli such as mechanical deformation. In this study, the liquid crystal composites (LCC) were prepared based on an emulsion process. Firstly, the liquid crystals were mixed with PVP dissolved in ethanol to form an emulsion. This emulsion was then blended with a TPU/THF solution to in which the LC is stabilized by both PVP and TPU. LCC films, fibers, and patterns have been fabricated by draw-down coating, electrospinning, and 3D printing. When the solvents evaporate, the original spherical LC droplets that are embedded in the TPU matrix deform into oblate shape exhibiting a unique color due to the curvature effect. The water contact angle on the LC composite film (80.4°) is very close to the one on the TPU surface (83.1°) and much higher than that on the PVP/LC surface (22.6°), indicating the LC droplets are likely embedded inside the TPU matrix. The LC films and patterns were subjected to cyclic stretching/release process. The color change is dependent on the strain as well as the strain rate. We hypothesize the color change is the result of the pitch change upon stretching as well as the recovery of the TPU matrix and the LC droplets. This emulsion-based approach without complex LC chemistry and 3D printing provide a promising platform for achieving lightweight, flexible, stimuli-responsive materials.