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.