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.