# Environmentally Stable Chiral-Nematic Liquid-Crystal Elastomers with Mechano-Optical Properties
## 3 Results and Discussion
### Mechano-Optical Response Behavior and Stability of N\({}^{\star}\) LC (x) Elastomers  
Toward the realization of mechano-optical sensors, we investigated the mechano-optical response behavior of the N\({}^{\star}\) LC (x) elastomers under tensile strain. Each of the N\({}^{\star}\) LC (x) elastomers showed selective reflection at a particular peak wavelength (\(x\) = 2.0, 835 nm; 2.2, 735 nm; 2.4, 691 nm; 2.6, 635 nm; 2.8, 584 nm; and 3.0, 504 nm; Figure 4a). As the concentration of ISBCD units increased, the peak wavelength hyposchromically shifted in accordance with Equation (1). As a representative example, we elongated the N\({}^{\star}\) LC (2.0) elastomer (Figure 4b,c). The film appeared faint red at 0% strain and changed to an intense orange color at 80% strain. The color change occurred under elongation, and the initial color was recovered after the removal of the applied strain, and this process could be repeated. The color change was evaluated quantitatively using reflectance measurements. The reflection peak wavelength of the N\({}^{\star}\) LC (2.0) elastomer was hyposchromically shifted from the near-IR region (835 nm) in the initial state to the orange region (616 nm) at 80% strain under tensile elongation. The other N\({}^{\star}\) LC (x) elastomers also exhibited similar mechano-optical response behavior, with the reflection peak wavelengths hyposchromically shifted under elongation (Figure S8). Figure 3: (**a**) Photographs of N\({}^{\star}\) LC (2.0) elastomers with HAB/CN = 40/40 (top) and HAB/CN = 70/10 (bottom) at different temperatures. Reflectance spectra of N\({}^{\star}\) LC (2.0) elastomers with (**b**) HAB/CN = 40/40 and (**c**) HAB/CN = 70/10 at different temperatures. (**d**) Relative reflectance intensity (\(R/R_{0}\); \(R\), reflectance at a certain temperature; \(R_{0}\), initial reflectance at 30 \({}^{\circ}\)C) and (**e**) reflectance peak wavelength of N\({}^{\star}\) LC (2.0) elastomers with HAB/CN = 40/40 (black circles and line) and HAB/CN = 70/10 (red triangles and line) as a function of temperature. The shift in the reflection peak wavelength can be explained by the isotropic deformation of typical rubbery materials. Such materials deform isotopically under incompressible conditions, leading to in-plane uniaxial elongation that causes the compression in both the in-plane perpendicular and thickness directions in the same manner. This phenomenon can be expressed by the following equation:  
\[\epsilon_{\text{theory}}=1-(1+\epsilon_{\text{x}})^{-1/2}, \tag{2}\]  
where \(\epsilon_{\text{theory}}\) is the theoretical strain in the thickness direction under the applied tensile strain \(\epsilon_{\text{x}}\). In our N* LC elastomers, we experimentally evaluated the thickness strain \(\epsilon_{\text{z}}\) by transforming Equation (1) into \(\epsilon_{\text{z}}=\Delta P/P=\Delta\lambda/\lambda_{0}\), where \(\Delta P\) is the change in the helical pitch and \(\Delta\lambda\) is the shift of the reflection peak wavelength under the applied tensile strain [52]. As shown in Figure S9, the experimental \(\epsilon_{\text{z}}\) values show good agreement with the theoretical \(\epsilon_{\text{theory}}\) values, which indicates that the N* LC (_x_) elastomers undergo isotropic deformation. Thus, an applied tensile strain can be evaluated quantitatively by merely measuring the shift of the reflection peak wavelength. Therefore, our proposed N* LC (_x_) elastomers have great potential as mechano-optical sensors with excellent flexibility and stretchability, unlike densely cross-linked LC polymers. Subsequently, we conducted a tensile test using the N* LC (2.0) elastomer. The elastomer exhibited a Young's modulus of 1.02 MPa and a fracture strain of 107%, indicating high stretchability. To further understand the mechano-optical stability, we evaluated the shift of the reflection peak wavelength during compressive/tensile strain cycles.