RESULTS AND DISCUSSIONS
The starting aqueous CNC suspension contained CNCs with a rod-like morphology, and an average length (L) and diameter (D) of 220 nm and 15 nm, respectively (Figure 1a). The ζ-potential of the CNC suspension measured -57.6 mV, which indicated colloidal stability and electrostatic repulsion between the CNCs nanorods. As mentioned above, CNCs are a kind of lyotropic liquid crystal, thus, at critical concentrations, the CNC nanorods tend to orient parallel to each other, to simultaneously minimize the excluded volumes and maximize the packing entropy.40 In this case, the estimated critical volume fraction (φ) of the liquid crystalline CNCs based on Onsager theory is Thus, the theoretical value of φ for CNCs, extrapolated from the TEM results, is 0.27.
While the Onsager theory for excluded volume provides an intuitive way to describe the isotropic-to-anisotropic transition of the CNC lyotropic liquid crystals, it has been found that the critical value of the transition is much lower than predicted.42 In order to experimentally characterize the lyotropic property of the liquid crystalline CNC suspension, the initial pulp CNC slurry was diluted to give a series of concentrations ranging from 1.0 wt% to 12.7 wt%, with the corresponding volume fraction ranging from 0.006 to 0.081 (see the Supporting Information for details). In general, a suspension of anisotropic colloids shows an isotropic phase at low concentrations, but transits into a biphasic mixture, with co-existing isotropic and anisotropic phases, as particle concentrations increase. Figure 1b is the phase diagram of the concentration-dependent CNC suspension, which enables determination of the critical CNC concentration at which the transition from isotropic phase to anisotropic phase occurs, i.e., a top isotropic phase with a bottom liquid crystalline anisotropic phase. In addition, the diagram demonstrates that as the concentration increases, the overlap of excluded volumes causes the colloidal CNCs to fully align into a liquid crystalline order, which is in agreement with previous reports.42 We termed this anisotropic phase in the CNC suspension as chiral nematic liquid crystals.
Polarized optical microscopy (POM) images of the CNC suspension was completely dark for the 3% suspension (Figure 1c), indicative the fully isotropic nature of the CNCs suspension, and exhibited non-orientational ordering (Figure 1f). Increasing the CNC concentration induced a transition from isotropic to anisotropic state, with network-like defects, leading to bright views and a typical focal conic texture (Figure 1d and 1g). Further, the observed colour apparently arose from the birefringence and optical rotation, instead of selective reflection (Figure S1). Of note, the POM image of the 10 wt% CNCs suspension showed a fingerprint texture when confined in a liquid crystal cell (Figure 1e), suggesting that CNCs are arranged in a chiral nematic liquid crystalline order with the helical axis running parallel to the cell plane and the director winding anticlockwise (Figure 1h).43 The helical pitch of the chiral nematic CNC liquid crystals measured approximately 7.5±0.3 µm, two-fold higher than the periodicity of striped patterns.