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

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. Film star: A new method for drying cholesteric liquid‐crystalline cellulose nanocrystal suspensions into solid films avoids phase coexistence and, with an applied circular shear flow, yields unprecedented structural control. The commonly observed mosaic‐like structure, with randomly varying orientation of the cholesteric helix axis, is replaced by a macroscopically uniform film with improved optical properties (see figure).