# Environmentally Stable Chiral-Nematic Liquid-Crystal Elastomers with Mechano-Optical Properties
## 1 Introduction  
Stimuli-responsive photonic materials have attracted great interest owing to their tunable optical behavior, which is useful for applications in areas such as flexible electronics and soft robotics [1; 2; 3]. In particular, mechano-optical responsive materials change their optical properties under the application of physical stimuli, such as grinding, pressure, and tensile or shear strain [4; 5; 6; 7; 8; 9]. Liquid-crystal (LC) elastomers are promising advanced materials in which the orientation of the mesogens is strongly coupled to the polymer network; thus, electro-, mechano-, or thermo-responsive effects on the macroscopic shape and the microscopic molecular orientation can be observed simultaneously [10; 11; 12; 13; 14; 15; 16; 17; 18]. Notably, chiral-nematic (N\({}^{\star}\)) LC elastomers have unique optical properties based on the helical structure of the N\({}^{\star}\) LC system [19; 20; 21]. N\({}^{\star}\) LC elastomers spontaneously form a helically twisted molecular orientation, with the orientational direction of the mesogens rotating along the helical axis [22; 23; 24]. Thus, N\({}^{\star}\) LC elastomers exhibit Bragg reflection and selectively reflect circularly polarized light of a particular wavelength. The reflection wavelength, \(\lambda\), depends on the helical pitch, \(P\), of the N\({}^{\star}\) LC elastomers, as expressed by the following equation:  
\[\lambda=n_{\text{ave}}\cdot P, \tag{1}\]where \(n_{\text{ave}}\) is the average of the ordinary and extraordinary refractive indices of the LCs [25; 26; 27; 28]. In 2001, [PERSON] et al. reported a dye-doped N\({}^{\star}\) LC elastomer that acted as a mechanically tunable laser [29]. In this system, a mechanical stimulus applied to the elastomer changed the macroscopic shape in accordance with the change in the helical orientation. This pioneering work provided a basis for the development of N\({}^{\star}\) LC elastomers as stimuli-responsive photonic materials [30; 31; 32; 33; 34]. The helical orientation of an N\({}^{\star}\) LC elastomer is very sensitive to various external stimuli, such as heat [31], light [32], electric fields [33], and humidity [34]. For the development of mechano-sensing applications, a high sensitivity to multiple stimuli is unfavorable because the external stimuli can interfere with the observation of a pure mechano-optical response. Thus, stability in various environments, except under mechanical stimuli, is highly desirable for the development of N\({}^{\star}\) LC elastomers with highly sensitive mechano-optical response behavior. In recent decades, N\({}^{\star}\) LC network polymers with high cross-linking densities have attracted practical interest because of their thermally stable optical properties [35; 36; 37]. However, dense cross-linking is not appropriate for mechano-optical sensors because of the associated high glass transition temperature and low flexibility [38; 39]. Moreover, the cross-linker acts as an impurity that destabilizes the LC phase, leading to a decrease in the isotropic phase transition temperature [40]. Therefore, to utilize an N\({}^{\star}\) LC elastomer for mechano-optical applications, high thermal stability with a low cross-linking density is necessary. Herein, we report a useful design method for thermally stable N\({}^{\star}\) LC networked polymers with low densities of cross-linking sites. Our strategy for enhancing the thermal stability of network polymers involves introducing noncovalent interactions between the mesogenic moieties in the side chains of network polymers. In this study, we employed a cyanobiphenyl derivative as a side-chain mesogen because intermolecular interactions such as \(\pi\)-\(\pi\) stacking and dipole-dipole interactions between cyanobiphenyl mesogens are well-known to impart polymeric materials with high resistance to thermal treatment [41; 42; 43; 44].