Robust Chiral Organization of Cellulose Nanocrystals in Capillary Confinement

Abstract

We showed large area uniformly aligned chiral photonic bioderived films from a liquid crystal phase formed by a cellulose nanocrystal (CNC) suspension placed in a thin capillary. As a result of the spatial confinement of the drying process, the interface between coexisting isotropic and chiral phases aligns perpendicular to the long axis of the capillary. This orientation facilitates a fast homogeneous growth of chiral pseudolayers parallel to the interface. Overall, the formation of organized solids takes hours vs weeks in contrast to the slow and heterogeneous process of drying from the traditional dish-cast approach. The saturation of water vapor in one end of the capillary causes anisotropic drying and promotes unidirectional propagation of the anisotropic phase in large regions that results in chiral CNC solid films with a uniformly oriented layered morphology. Corresponding ordering processes were monitored in situ at a nanoscale, mesoscale, and microscopic scale with complementary scattering and microscopic techniques. The resulting films show high orientation order at a multilength scale over large regions and preserved chiral handedness causing a narrower optical reflectance band and uniform birefringence over macroscopic regions in contrast to traditional dish-cast CNC films and those assembled in a magnetic field and on porous substrates. These thin films with a controllable and well-identified uniform morphology, structural colors, and handedness open up interesting possibilities for broad applications in bioderived photonic nanomaterials.

Angular optical response of cellulose nanocrystal films explained by the distortion of the arrested suspension upon drying

“…[93] The application of these general ideas can also be relevant to understand alternative self-assembly mechanisms involving unidirectional removal of the solvent such as in vacuum filtration or open ended capillaries. [94,95] The reaction was quenched by deionized ice and water after 30 min. Soluble cellulose residues and acid were removed by centrifugation (three steps at 20,000 g of 20…”

Integration of Optical Surface Structures with Chiral Nanocellulose for Enhanced Chiroptical Properties

“…This value is much higher than others reporting CNC orientation order parameters (within 0.6-0.8) induced by capillary forces or external fields. [29], [ 43 ] However, in contrast to the topmost layer, the chiral morphology is formed within the bulk of CNC film, beneath the nematic-like topmost layer. Indeed, the cross-section of interior layer of the CNC film exhibits a typical Bouligand morphology with layered trench trench organization, confirming the chiral organization within the bulk CNC film ( Figure 2c).…”

“…[25], [26], [27], [28] To improve structural uniformity and thus photonic performance, various approaches have been suggested including controlling the EISA assembly by confined geometries, drying at controlled humidity and temperature, and adding extrinsic components. [29], [30], [26], [31], [32] Progress has been limited because of due to the random organization of chiral tactoids with large variance in pitch lengths and orientation across the bulk films. [33] Such heterogeneous organization greatly compromises the CD properties of CNC films.…”

Retrieving the Coassembly Pathway of Composite Cellulose Nanocrystal Photonic Films from their Angular Optical Response

“…Before the kinetic arrest transition, the suspension is able to relax and adjust its cholesteric pitch, p (defined as its full‐turn periodicity), to the continuously increasing concentration upon solvent evaporation. Once the suspension is kinetically arrested, the pitch is expected to vary mostly as a result of volume decrease and macroscopic contraction along the helix axis, the latter being strongly affected by the cholesteric alignment, the sample geometry, and the boundary conditions 22–25. This essential step allows for the production of solid‐state CNC materials with very distinctive internal structure and corresponding optical properties 23,26–29.…”

“…Once the suspension is kinetically arrested, the pitch is expected to vary mostly as a result of volume decrease and macroscopic contraction along the helix axis, the latter being strongly affected by the cholesteric alignment, the sample geometry, and the boundary conditions. [22][23][24][25] Aqueous suspensions of cellulose nanocrystals (CNCs) are known to selfassemble into a chiral nematic liquid crystalline phase, leading to solid-state nanostructured colored films upon solvent evaporation, even in the presence of templating agents. The angular optical response of these structures, and therefore their visual appearance, are completely determined by the spatial arrangement of the CNCs when the drying suspension undergoes a transition from a flowing and liquid crystalline state to a kinetically arrested state.…”