Figure 4. Anisotropic properties of the LROC-1 and LROC-2 aerogels. (a) Low magnification POM image of the LROC-1 aerogel, demonstrating typical birefringence texture of this material. The inset shows a fingerprint texture of the aerogel. (b, c) High magnification POM images of the LROC-1 at different orientations of the sample. (d) Low magnification POM image of the LROC-2 aerogel, demonstrating the anisotropy of the obtained material. (e, f) High magnification POM images of the LROC-2 at different orientations of the sample, implying high alignment of the CNCs.
Scanning electron microscopy (SEM) analysis was conducted to confirm continuous long-range ordering throughout the LROC-1 aerogel (Figure 5). As expected, the LROC-1 displayed distinct periodic, layered structures comprised of pure CNCs only (Figure 5a and 5b, low magnification). Oblique and high magnification views revealed much more elaborate structures (Figure 5c and 5d, respectively). The inter-lamellar spacing between each layer was approximately a few hundred nanometers wide, which was due to the sublimation of ice crystals (Figure 5c). The CNCs were squeezed into two-dimensional laminar structure, in a layered, long-range order. The high magnification view showed that the CNCs self-assembled into a twisted order, with structures similar to those previously reported for chiral nematic cellulose-based materials.48 The corresponding TEM image of LROC-1 also indicated that the CNCs within were highly locally aligned (Figure S5). However, the LROC-2 sample contained large domains of periodic columnar structures, of several microns in diameter, with a typically nematic ordering of the CNCs inside (Figure S6). In summary, significant differences were observed between the two types of aeorgels subjected to the ice-assisted self-assembly process, likely due to the influence of the initial CNC concentration.