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.