# Quantification of Optical Chirality in Cellulose Nanocrystal Films Prepared by Shear-Coating
###### Abstract  
Evaporation-induced-self-assembly is widely used to produce chiral cellulose nanocrystal (CNC) free-standing films reflecting left-handed polarized light. Research on supported chiral CNC films is rather scarce. The reflection and/or transmission of unpolarized light are the most common optical techniques used to characterize the selective reflection of CNC films whereas the use of techniques to quantify chiral properties is limited. Here, the fabrication of chiral CNC films supported on glass substrates by a shear-coating method, as well as a full characterization of their polarization properties, are reported. Optical chirality is evidenced in films, showing a brilliant blue structural color when viewed through a left-handed polarizer and darkness through a right-handed polarizer. Mueller-matrix data in the reflection and transmission modes are used to quantitatively characterize the structural origin of color in the films. The quantification of the linear and circular birefringence, as well as circular dichroism, is performed by analytical inversion of the Mueller matrix data in the transmission mode and regression analysis using Tellegen constitutive equations. The equivalence of the two methods to quantify the structural chirality in CNC films is demonstrated. The swelling of films in water and kinetics during drying is studied by reflection spectroscopy. cellulose nanocrystals; chirality; structural color; Mueller matrix; humidity sensor +
Footnote †: journal: Applied Sciences  
0021202130 June 202130 July 20  
phase separation is a phenomenon that can take anywhere from a few hours to several weeks [9]. CNC films formed from slow evaporation of a suspension in the nematic chiral phase preserve their arrangement [9]. The chiral nematic ordering is characterized by CNC preferentially oriented in pseudo-planes along a certain unit vector (the director). The director of adjacent pseudo-planes is rotated by a small constant angle about the helix axis. The pitch (\(\Lambda\)) of the structure corresponds to the distance separating two pseudo-planes when the director has completed a full rotation of 360\({}^{\circ}\). The helical microstructure in the chiral film enables selective Bragg reflection, which refers to the reflection of left circular polarized light in a spectral range centered at wavelength \(\lambda_{\text{max}}\)[10],  
\[\lambda_{\text{max}}=n_{\text{av}}\Lambda\cos\theta_{\text{t}}, \tag{1}\]  
where \(n_{\text{av}}\) is the in-plane average refractive index and \(\theta_{\text{t}}\) the angle of wave propagation in the film and is determined from the law of refraction, \(n_{\text{a}}\text{sin}\theta=n_{\text{av}}\text{sin}\theta_{\text{t}}\), where \(\theta\) is the angle of incidence and \(n_{\text{a}}\) is the refractive index of the surrounding ambient. However, there are many factors involved during the drying process that interfere with the collapse of the microstructure, leading to the production of defects in the films. To achieve a homogeneous pitch and helical axis orientation in areas of hundreds of square micrometers is a current challenge, and is the focus of attention for several research groups. Several approaches have been reported to manufacture homogenous CNC chiral films, like phase separation [11], application of orbital shear flow during drying [12], planar anchoring [13], or promoting tactoid annealing [14]. Obtaining a homogeneous microstructure, controlling both the orientation of the helical axis and the pitch, results in films with a reflection of left circular light in a narrow range of wavelengths, which is a desirable characteristic for their use in applications. In addition to film casting, the use of a doctor blade method for the preparation of CNC films on glass substrates has been reported [15]. In that work, birefringent films were obtained because of the alignment of the CNC in the direction of the applied shear stress [15].