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 speciļ¬c 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.