# Environmentally Stable Chiral-Nematic Liquid-Crystal Elastomers with Mechano-Optical Properties
## 3 Results and Discussion  
### Phase Transition Behavior  
The phase transition behavior of the N* LC (\(x\)) elastomer films was investigated using POM (Figure S4) and DSC (Figures S5-S7). In the monomer mixture for N* LC (2.0), the isotropic-to-LC phase transition was observed at 28 \({}^{\circ}\)C during the cooling process. From the planar texture shown in the POM images (Figure S4), we concluded that the LC phase observed in the monomer mixture was an N* LC phase. After polymerization, the N* LC phase observed in the monomeric state was fixed in the resultant elastomers. As summarized in Table 2, the LC temperature ranges of the elastomers with different ISBCD concentrations were similar, with an N* LC phase observed from \(\sim\)10 to 90 \({}^{\circ}\)C. These results indicate that the LC behavior of the elastomers was not affected by the chiral dopant concentration. ISBCD has been reported to have a very large helical twisting power (HTP = 43 \(\upmu\)m\({}^{-1}\) mol\({}^{-1}\)) [46]; therefore, only a small amount of ISBCD was necessary to induce a helical LC alignment. Because only a low concentration of chiral dopant was used, the thermodynamic properties of the elastomers were not affected. The N* LC phase of the elastomers was enantiotropic, indicating that the LC phase was reversibly formed during both the heating and cooling processes (Figure 2a). When the N* LC (\(x\)) elastomers were cooled from the isotropic phase, the N* LC phase exhibited the same optical texture immediately below the isotropic-N* LC phase transition temperature. Generally, polymer LCs tend to show large supercooling owing to their viscosity. However, in the current system, most of the mesogens were covalently bonded to the polymer main chains through a methylene spacer, and the mesogenic orientation was fixed by cross-linkers. We consider these structural features to be responsible for the high reversibility of the LC behavior and the quick recovery of the LC phase from the isotropic phase [48; 49]. To confirm this hypothesis, we prepared a linear LC polymer (l-LCP) without the cross-linking agent using the same polymerization procedure in the LC cell with polyimide alignment layers. As shown in Figure 2b, l-LCP formed a well-aligned homogeneous N* LC phase and exhibited an enantiotropic N* LC phase between 0 and 85 \({}^{\circ}\)C. As compared to the elastomers, l-PLC had a lower LC-to-isotropic phase transition temperature. In contrast to the N* LC (\(x\)) elastomers, which showed complete reversibility of the LC texture, l-LCP became opaque, and a different LC texture was obtained in the N* LC phase by cooling from the isotropic phase, as shown in Figure 2b. These observations clearly indicate that l-PLC formed N* LC polydomains after cooling from the isotropic phase and that the order \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{x} & \multicolumn{3}{c}{**Phase Transition Temperature (\({}^{\circ}\)C)**} \\ \cline{2-3}  & **Monomer Mixture 1** & **Elastomer 1** \\ \hline
2.0 & N* 28 I & G 9 N* 93 I \\
2.2 & N* 28 I & G 12 N* 90 I \\
2.4 & N* 27 I & G 11 N* 90 I \\
2.6 & N* 27 I & G 8 N* 91 I \\
2.8 & N* 27 I & G 12 N* 95 I \\
3.0 & N* 26 I & G 10 N* 92 I \\ \hline \hline \end{tabular} \({}^{\dagger}\) Abbreviations: G, glassy; N*, chiral nematic; I, isotropic. The phase transition temperatures were determined by POM for the monomer mixtures and DSC for the elastomers.  
\end{table} 
Table 2: Phase transition temperatures of N* LC (\(x\)) monomer mixtures and elastomers with different ISBCD contents. parameter of the LC phase was lower than that just after polymerization. Therefore, we concluded that the mesogenic orientation in the N* LC (\(x\)) elastomers is stabilized and memorized by cross-linking. Next, to investigate the effects of the concentration of the CN monomer on the thermodynamic behavior, we synthesized N* LC (2.0) elastomers with different LC monomer ratios (HAB/CN). Here, the concentration of ISBCD was fixed at 2.0 mol%.