(177p) Making Mechanochromic Palettes of Cholesteric Liquid Crystal Elastomers for Direct Visualization of Deformation

In nature, diverse organisms often use color signals to communicate and interact with each other and their surroundings. Mechanochromic materials offer a unique way to blend artificial and natural realms by changing their colors in response to mechanical stimuli, in a way that mimics living organisms. Recently, mechanochromic photonic crystals (PCs) have gained attention for their potential in developing smart materials, strain sensors, and wearable technology. The structural color variation of PCs in response to the mechanical deformation results from the modulated periodicity of regular nanostructures. Soft material-based PCs in the form of elastomers or gels have been developed to realize immediate and reversible color changes across the entire visible spectrum.

Cholesteric liquid crystal elastomers (CLCEs) are particularly promising among mechanochromic PC materials because of their ease of processing and mechanical responsiveness. The periodic nanostructures of the CLCEs with a well-defined helical pitch exhibit vivid structural color. In this respect, innovative works have been done utilizing the mechanochromic response of CLCEs for anticounterfeiting, camouflages, textiles, and soft actuators. However, previous studies mainly focused on blue-shifted reflection colors that only provide information about the presence or absence of mechanical deformation. Relying solely on reflection color fails to provide accurate stress-related information due to a lack of distinction for deformation and limits effective color-based communication. Such a limitation arises from the inability to fully utilize the chiroptical properties of CLCEs. Given the dynamic nature of the real world, it is important to offer discernible color information depending on different types and degrees of deformation.

This study introduces a novel approach combining CLCEs and optical rotation in transmission mode to fully exploit the chirality inherent in the helical structures of CLCEs. The mechanochromic responses of CLCEs are investigated for two mechanical deformations—compression and uniaxial stretching—resulting in distinct optical rotation characteristics due to the deformed helical structures. Specifically, compressed CLCEs maintain their helical structures, while stretched CLCEs lose chirality by the uniaxial orientation of main chains. The linearly polarized light passing through the deformed CLCEs undergoes varying optical rotation, displaying different colors for compression and uniaxial stretching. As a proof of concept, we demonstrate a visual signaling system capable of showing different patterns or colors based on the types and degrees of deformation. This discovery contributes to designing new materials with tailored optical properties and mechanical functionalities for advanced applications such as strain sensors or soft robots.