Figure 1. (a) TEM image shows individual CNC crystallites, prepared by fast evaporation of a dilute CNCs suspension (about 0.05 wt%). (b) The calculated ratio of the anisotropic phase for varying CNCs volume fractions. The specific CNC concentrations chosen for freeze-casting are indicated by the coloured squares. (c) POM image of a 3.0 wt% CNC suspension, exhibiting the isotropic nature of the colloidal suspension. (d), (e) POM images of the anisotropic phase of a 10 wt% CNCs suspension, demonstrating liquid crystalline ordering into focal conic texture and fingerprint texture, respectively. All the images were taken under crossed polarized light. (f)-(h) Illustrations of the alignment in the CNCs suspension for their corresponding texture.
The 10 wt% and 3 wt% CNCs suspensions were then subjected to unidirectional freeze-casting in a cylindrical mold on a cold finger, with liquid nitrogen as the cooling source (Figure 2a).29 During the freezing process, the CNC nanoparticles accumulated between ice fronts and only formed a continuous 3D network when its volume fraction was higher than the percolation threshold. Figure 2b are the photographs of the CNC ice block obtained following unidirectional freeze-casting (left), and of the CNC aerogel (LROC-1) after freeze-drying (right). Sublimation of ice templates led to the formation of an aerogel-based material. Notably, the volume of LROC-1 was comparable to its corresponding ice block, suggesting that ice sublimation during the freeze-drying process caused little shrinkage. A collapse of the macroscopic structure was observed, when the volume fraction of the initial CNCs was below 0.006 (with the critical CNC concentration of 1.0 wt%, Figure S2). The self-supported network of the CNC monolith indicates that the colloidal CNCs are directly transferred into dry aerogel-like superstructures without a conventional hydrogelation process, as was the case for previously described aerogel fabrication methods.2,
In order to track the ice-assisted self-assembly process of CNCs, we characterized the anisotropy of a series of frozen colloidal CNCs blocks. It is well known that liquid water can freeze into solid ice with a crystalline phase and anisotropic properties.44 However, freezing of a multicomponent aqueous suspension causes phase separation that drives non-freezing components to the space between ice fronts. In turn, cellular structures form, while the mesostructural features depend on the nature of the aqueous suspension and freezing dynamics.29 Thus, the growth of ice fronts in CNC suspensions can promote a phase separation of CNCs liquid crystals. The frozen droplets of both the 3 wt% and 10 wt% CNC suspensions proved birefringent (Figures 2c and 2d), which can be ascribed to their anisotropic nature. Judging from the fact that the 3 wt% CNCs suspension is isotropic without liquid crystalline textures (Figure 1c), it is reasonable to assume that the birefringence observed in the frozen droplets arose from the anisotropic ice crystals or the alignment of rod-like CNCs. The POM image of the 10 wt% frozen droplets showed distinct birefringence colours (Figure 2d, similar to Figure 1d). Characteristics of fingerprint texture of the chiral nematic ordering of the CNCs were observed in certain areas of the 10 wt% frozen blocks (Figure 2e). This may suggest that the CNC nanorods between adjacent ice fronts are arranged in a chiral nematic liquid crystalline order. To support this hypothesis, the ice crystal of pure water was examined using polarized light microscopy (Figure S3); domains of textures that distinctly differed from those of the frozen droplets of the CNCs suspension were observed. Above all, we believe that the optical anisotropy observed in the frozen droplets of CNC-ice arises from the synergistic effect between the crystal structure of ice and the liquid crystalline alignment of CNCs.