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in water showing strong birefringent nature but never reached the chiral nematic order
forming either tactoids or i ngerprints. Due to high polydispersity among length of
CNCs (extracted from tunicate by HCl acid hydrolysis) and viscosity of the suspen-
sions, it further showed lack of self-organization [113]. However, carboxylated CNCs
were obtained from cotton concentration of 5% (w/w) or more, and when these CNCs
grat ed with PEG formed chiral nematic mesophase similar to that of unmodii ed
CNCs with reduced i ngerprint pattern spacing (around 4 μm). Moreover, PEG-grat ed
CNCs showed drastic enhanced dispersion stability (even at higher solid content) and
the ability to redisperse into either water or chloroform from a freeze-dried form. Also,
they showed strong stability at higher ionic strength [6, 95].
15.4.4.2.2 Organic Medium
Heux
et al.
were the i rst to describe the self-organization phenomenon of CNCs in
aprotic solvents. Initially, surfactant coating was used for dispersion of CNCs and
obtained a chiral nematic structure with pitch value of approximately 4 μm, whereas in
the case of aqueous suspensions it is between 20 and 80 μm. In addition, higher concen-
tration (up to 36%) could be achieved due to steric stabilization exerted by surfactant
coating [114]. A detailed study of the structure of this phase (chiral nematic) has been
reported by the same group [115, 116].
15.4.4.2.3 External Field
In aqueous suspension, nonl occulated CNCs can be oriented under external i eld
whether it is magnetic or electric, due to the larger negative diamagnetic anisotropy
of CNCs as compared to cellulose and other particles like DNA [117, 118]. Sugiyama
et al.
were the i rst to demonstrate the orientation of crystalline cellulose (extracted
from tunicate) in dilute aqueous suspension under magnetic i eld of 7 T, and i lms were
prepared in which crystals oriented along the long axis perpendicular to the magnetic
i eld [117]. In this advancement, Kvien and Oksman demonstrated the alignment of
CNCs (2 wt%) in polyvinyl alcohol-based unidirectional reinforced nanocomposites
prepared by solvent-casting method, and nanocrystals were oriented perpendicular to
the homogeneous magnetic i eld (7 T). h e results revealed higher modulus (2 GPa)
of the nanocomposite in aligned direction as compared with transverse direction. h e
highly oriented nanocomposite is shown in Figure 15.9. [119].
Figure 15.9
FE-SEM pictures of (a) a fracture surface and (b) an etched surface of the CNCs
nanocomposite showing a highly oriented structure [119].
