# Quantification of Optical Chirality in Cellulose Nanocrystal Films Prepared by Shear-Coating
## 3 Results and Discussion
### Swelling and Drying of CNC Chiral Films  
CNC chiral films have been proposed as humidity colorimetric detectors, taking advantage of the swelling properties of cellulose materials. The swelling of the glass-supported CNC chiral films was investigated by immersing the film in deionized water for 1 min and then measuring the reflectance as a function of the drying time at room conditions (25 \({}^{\circ}\)C and 45% relative humidity). Figure 7a shows an image of the sample just after immersion, and as can be seen, the central part looked light green. After 1.3 min of drying, that part turned to light blue (Figure 7b), and finally became a strong blue as shown in Figure 7c. The time evolution of the reflectance spectra is shown in Figure 7d. In these measurements, \(\lambda_{\text{max}}\) in the dry film is located at 395 nm and shifts to 477 nm after immersion in water. This shows that the average pitch increases by about 20% as the film swells. The swelling is similar to that reported for neat CNC free-standing films [1,29]. As the film gets dry, the band of selective reflection monotonically shifts to shorter wavelengths. The experimental time-dependence of \(\lambda_{\text{max}}\) is shown with the symbols in Figure 7e, and the line corresponds to a fitting with an exponential decaying function, \(\lambda_{\text{max}}=\lambda_{0}+A\text{exp}(-t/\tau)\), where \(\lambda_{0}\) = 395 nm is the wavelength for the reflection of the dry film, \(A\) = 85.1 \(\pm\) 2.1 nm and \(\tau\) = 1.20 \(\pm\) 0.06 min. The relaxation time \(\tau\) is of the same order of magnitude as reported for free-standing CNC films [30,31]. The introduction of highly hydrophilic molecules like glycerol [1,29] and polyols [32] in free-standing CNC films produces larger swelling. The improvement of the swelling capabilities of glass-supported CNC chiral films is the subject of ongoing research by our group.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \(\mathbf{A_{j}}\) & \(\mathbf{B_{j}}\) & \(\mathbf{C_{j}}\) \\ \hline \(n_{x}\) & 1.5773 \(\pm\) 0.0001 & 0.0035 \(\pm\) 0.002 & 0.00046 \(\pm\) 5\(\times\)10\({}^{-5}\) \\ \(n_{y}\) & 1.5750 \(\pm\) 0.0001 & \(-\)7.5\(\times\)10\({}^{-6}\)\(\pm\) 0.32 & 5.1\(\times\)10\({}^{-5}\) \(\pm\) 2\(\times\)10\({}^{-5}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Cauchy parameters for \(n_{x}\) and \(n_{y}\) in Equation (4).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \(\kappa\) & \(\mathbf{A_{j}}\) & \(\mathbf{E_{j}}\) (eV) & \(\mathbf{\Gamma_{j}}\) (eV) \\ \hline \(\kappa_{1}\) & 0.00055 \(\pm\) 1\(\times\)10\({}^{-5}\) & 2.787 \(\pm\) 0.008 & 1.41 \(\pm\) 0.01 \\ \(\kappa_{2}\) & 0.00199 \(\pm\) 1\(\times\)10\({}^{-6}\) & 2.982 \(\pm\) 0.001 & 0.497 \(\pm\) 0.003 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Gaussian parameters for \(\kappa\) (offset \(\kappa_{0}\) = \(-\)6.64\(\times\)10\({}^{-4}\)\(\pm\)10\({}^{-6}\)) in Equation (5).  
Figure 7: (**a**) Image of a CNC film after immersion in deionized water, (**b**) after 1.3 min, and (**c**) after 5.8 min. (**d**) Time-dependence of the reflectance for unpolarized incident light. (**e**) Experimental (blue circles) wavelength maxima of selective reflections as a function of drying time; the red curve corresponds to a fitted exponential decay of \(\lambda_{\text{max}}\).