Figure 5. Morphology of the directional freeze-casted LROC-1
aerogel. Low magnification top (a) and oblique SEM views (b) of the
LROC-1, demonstrating the laminar structure of the aerogel. (c) High
magnification side SEM view of LROC-1, demonstrating the
aggregation-induced CNC layer. (d) Magnified SEM image of the layer in
(c), showing the periodic ordering of CNCs.
A mechanism for the ice-assisted assembly of CNCs, proposed based on the
experimental results, is illustrated in Figure 6. We speculate that
during the directional freezing process, the rod-like CNCs in suspension
are subjected to the elongation flow field due to the difference in growth
speed of ice crystals in length and thickness. This causes mechanical
deformation of ice-segregated CNC
networks.49 For
the isotropic CNC suspension, the extension flow can effectively align and
trap the rod-like CNCs into a nematic orientation with an ordered state.
Moreover, the formation of CNCs layers between ice crystals could
the nanoparticles into an arrested state, preventing them from losing
their orientation and relaxing back to isotropic state. For the dense
liquid crystalline CNC suspension, the CNCs are in an anisotropic state
with chiral nematic ordering. Once the freezing occurs, the resulting
extension flow can align the fluid CNC liquid crystals into a lamellar
arrangement, with ice crystals between them. For both of the
self-assembly processes, when the ice template is completely removed
during the sublimation process, their hierarchical structure as well as
the different orientation of the CNCs, are preserved.