Macroscopic Control of Helix Orientation in Films Dried from Cholesteric Liquid‐Crystalline Cellulose Nanocrystal Suspensions

Abstract

The intrinsic ability of cellulose nanocrystals (CNCs) to self-organize into films and bulk materials with helical order in a cholesteric liquid crystal is scientifically intriguing and potentially important for the production of renewable multifunctional materials with attractive optical properties. A major obstacle, however, has been the lack of control of helix direction, which results in a defect-rich, mosaic-like domain structure. Herein, a method for guiding the helix during film formation is introduced, which yields dramatically improved uniformity, as confirmed by using polarizing optical and scanning electron microscopy. By raising the CNC concentration in the initial suspension to the fully liquid crystalline range, a vertical helix orientation is promoted, as directed by the macroscopic phase boundaries. Further control of the helix orientation is achieved by subjecting the suspension to a circular shear flow during drying.

Electric-field-induced shape transition of nematic tactoids

“…From a practical point of view, subjecting tactoids to shear flow has been shown to improve the optical properties of liquid crystal films by enhancing the alignment of the particles [17], and using electric fields can achieve this same effect in a much more controllable environment. Indeed, when doped with acrylamide monomers, these biphasic aqueous chitin suspensions could easily be polymerised under electric field, as was recently demonstrated in the case of clay suspensions [42].…”

“…These nematic droplets have characteristic shapes that have already been the subject of several theoretical studies [2][3][4][5][6][7][8] and experiments on systems such as vanadium pentoxide (V 2 O 5 ) [2,9], carbon nanotubes [10,11], rodlike viruses [12,13], F-actin in cells [14], chromonic liquid crystals [15], and cellulose nano-crystals [16][17][18]. These investigations showed that the typical tactoid shapes, for example the so-called spindle-like shape with a bipolar director field (see the right inset in Fig.…”

Fractionation of cellulose nanocrystals: enhancing liquid crystal ordering without promoting gelation

“…The second deviation-of critical importance for largescale control of the self-organization 18 and therefore for applications-is that, frequently, a completely liquid crystalline sample cannot be reached in practice because w 1 > w g , the CNC mass fraction at which the system turns into a macroscopic gel, which is often considered a kinetically arrested state 8,15,[19][20][21][22] . The rheological data that we present below confirm that the sample at w > w g is a physical gel; hence, the transition is a percolation phenomenon 23 , i.e., a continuous network of connected rods is established that spans the macroscopic sample and prevents the system from flowing.…”

“…Significantly, from an applied point of view, this brings w 1 well below w g , while in the original suspension, w 1 ≈ w g . We have thus gained access to a large CNC mass fraction range where the complete sample is in a nonarrested equilibrium chiral nematic phase, enabling much better control of the macroscopic order 18 . We conclude that the gelation at w g is due to the loss of colloidal stability, as the solvent counterion concentration has reached a critical value c g Na þ , inducing linear rod aggregation, which, as a secondary phenomenon, triggers percolation regardless of the L/d of the individual rods.…”

Biocompatible and Sustainable Optical Strain Sensors for Large‐Area Applications

“…[1] Many of the cellulose derivatives are commonly used in diverse industrial applications, such as food additives [2] and for biomedical devices [3] due to their non toxic and water soluble nature. Moreover, the self assembly nature [4] and responsiveness [5] of cellulosic biopolymers makes them extremely attractive for smart photonic applications [6,7] including sensing. [8,9] Among various types of cellulose and its derivatives, hydroxypropyl cellulose (HPC) encompasses all these desirable properties, which makes it an extraordinarily multifunctional and versatile material.…”