Figure 3. Photographs and POM images of the 10 wt% CNC liquid crystal before and after a freeze-thaw cycle, demonstrating formation of liquid crystalline CNC networks during and after freezing.
Further examination of the structural anisotropy of highly ordered aerogels, demonstrated that the solid LROC-1 aerogel samples displayed brilliant colours, similar to those observed for the 10 wt% CNCs liquid crystals (Figure 4a). In addition, fingerprint structures of the aerogel were apparent (Figure 4a), which served as a confirmation for long-range ordered structures.45
Generally, when normal light passes through a nematic-ordered sample sandwiched between two crossed polarizers, the intensity profile of the transmitted light can be expressed in the form of:
where I0 is the intensity of light passed through from the polarizer, θ is the angle between the polarizer and the director of long axis of the sample, Δn and L represent the birefringence and thickness of the sample, respectively, and λ is the wavelength of incident light.46
From this model, we can infer that when the director is (\(\theta=0\)) or perpendicular (\(\theta=90\)) to the polarization of the incident light, the corresponding light intensity is at its minimum value, which leads to complete extinction, whereas the maximum intensity is only obtained in the case of \(\theta=45\). High magnification POM images of two angular positions of the LROC-1 aerogel, which correspond to the 0° to 45° orientation, respectively (Figures 4c and 4d), showed no extinction and the intensity in both images was comparable, as was the range of birefringence colours, implying that the polarization plane of the incident light was rotated by the aerogel. This is a classic signature of twist,47 indicating that the LROC-1 CNC aerogel may feature a twisted order in its structure. In contrast, while the LROC-2 aerogel also exhibited anisotropic properties when viewed under crossed polarizers (Figure 4d), the images taken at the two different orientations (Figure 4e and 4f) differed, and showed extinction of light and decreased intensity. This implies that the plane of polarization for the incident light is either parallel or perpendicular to the long-range arrangement of the LROC-2 aerogel, indicating nematic ordering of CNCs along the growth direction of the ice fronts.