# Environmentally Stable Chiral-Nematic Liquid-Crystal Elastomers with Mechano-Optical Properties
## 4 Conclusions  
In conclusion, we proposed a design concept for synthesizing thermally, optically, and environmentally stable N* LC elastomers by employing cyanobiphenyl units in the elastomer as noncovalent cross-linking sites. Because of their low densities of covalent cross-linking sites, the N* LC (_x_) elastomers exhibited high stretchability and flexibility, which are favorable properties for realizing practical mechano-optical sensors. In addition, the thermal stability of the N* LC phase could be enhanced by increasing the concentration of cyanobiphenyl units that form noncovalent cross-links in the elastomer. Notably, this noncovalent cross-linking had almost no effect on the mechano-optical response behavior, such as the reflection peak wavelength and mechano-optical stability, but significantly enhanced the environmental stability. The N* LC (_x_) elastomers exhibited excellent mechano-responsive color change properties, with a hyposochromic shift of the reflection peak wavelength under an applied tensile strain and reversible recovery to the original shape and color owing to high elasticity. In addition, the hydrophobicity and large elastic temperature ranges of the N* LC (_x_) elastomers provided excellent resistance to water and ethanol. Based on the excellent physical properties of the N* LC (_x_) elastomers, including high stretchability, reversibility, and stability under harsh conditions, our material design concept of using noncovalent cross-linking sites in N* LC elastomers is expected to contribute to various mechano-optical sensor applications in the fields of wearable displays, electric skin, and soft robotics. The following are available online at [[https://www.mdpi.com/article/10.3390/app11115037/s1](https://www.mdpi.com/article/10.3390/app11115037/s1)]([https://www.mdpi.com/article/10.3390/app11115037/s1](https://www.mdpi.com/article/10.3390/app11115037/s1)), Scheme S1: Synthetic route for ISBCD, Text S1: Synthesis of chiral dopant monomer, Text S2: Preparation of PDMS film, Text S3: Cell preparation with rubbing treatment, Text S4: Preparation of N* LC(_x_) elastomers, Figure S1: 1H-NMR spectrum of ISBCD in CDCl3, Figure S2: 1\({}^{3}\)C-NMR spectrum of ISBCD in CDCl3, Figure S3: HMQC spectrum of ISBCD in CDCl3, Figure S4: POM image of N* LC (2.0) mixtures observed at room temperature before (a) and after (b) polymerization in a rubbing cell, Figure S5: DSC thermogram of I-ICP (2 nd scan, scanning rate of 10 \({}^{\circ}\)C min\({}^{-1}\)), Figure S6: DSC thermogram of N* LC (2.0) elastomer (2 nd scan, scanning rate of 10 \({}^{\circ}\)C min\({}^{-1}\)), Figure S7: DSC thermogram of N* LC elastomers containing different HAB/CN ratios: (a) 70/10, (b) 60/20, and (c) 50/30 (2 nd scan, scanning rate of 10 \({}^{\circ}\)C min\({}^{-1}\)), Figure S8: (a) Color change of N* LC (_x_) elastomers with different chiral dopant concentrations \(x\) (2.0-3.0 mol%) under an applied tensile strain of 0% (left), 40% (middle), and 80% (right), and (b) reflection peak wavelengths of N* LC (_x_) elastomers as a function of applied tensile strain (0-80%), Figure S9: (a) Schematic illustrations of film deformation by elongation along the _x_-axis the thickness direction and helical pitch along the _z_-axis, and the in-plane direction along the _y_-axis perpendicular to the elongation direction are compressed as the elongation strain increases) and (b) change in helical pitch _z_\({}_{x}\) (red circles and line) experimentally evaluated using the shift in the reflection peak wavelength and the theoretical thickness strain of rubbery elastomers (black line) under the applied tensile strain _z_\({}_{x}\) calculated using a Poisson ratio of 0.5, Figure S10: Photographs of an N* LC (2.0) elastomer film: (a) water droplet on the film surface, showing hydrophobicity and (b) contact angle of the droplet on the surface. Conceptualization, O.T. ; Investigation, K.K., T.S., S.K., K.H., N.A., A.S.; Formal Analysis, K.K., K.H., O.T. ; Writing--Original Draft Preparation, K.K., K.H., O.T.