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.