Tuning the Iridescence of Chiral Nematic Cellulose Nanocrystal Films with a Vacuum-Assisted Self-Assembly Technique

Abstract

Iridescent films composed of the chiral nematic liquid crystal phase of cellulose nanocrystals (CNC) have attracted significant interest due to their fascinating optical properties. However, the current fabrication method, i.e., solution casting with a subsequent evaporation process, has significant limitations and therefore hinders the application of CNC iridescent films. In the present study, we demonstrate, for the first time, that vacuum-assisted self-assembly (VASA) can be used to fabricate highly oriented, large area, smooth, and structurally homogeneous CNC iridescent films. It was found that a long ultrasonic pretreatment is necessary for obtaining CNC iridescent films via VASA. Furthermore, it was also found that the iridescent color of the CNC films can be tuned by the sonication time, suspension volume, and degree of vacuum. By combining CD spectroscopy, SEM, and WAXD techniques, the internal structure of CNC iridescent films prepared by VASA has been investigated in detail. Moreover, the origin of the ultrasonic pretreatment effect on the self-assembly behavior of CNCs is also discussed.

The Self‐Assembly of Cellulose Nanocrystals: Hierarchical Design of Visual Appearance

“…This is typically achieved by evaporation‐induced self‐assembly, whereby a CNC suspension is evaporated under controlled conditions to give a solid film. However, a novel approach based on vacuum‐assisted self‐assembly has been recently reported, whereby vacuum membrane filtration of the CNC suspension was able to produce a filtration cake with a well‐ordered helicoidal order, with the pitch shown to be adjustable via the pressure differential.…”

Mind the Microgap in Iridescent Cellulose Nanocrystal Films

“…The optical properties of the films were found to depend upon the characteristics of the colloidal suspensions, which pass through different stages upon solvent evaporation and before the formation of the film. In order to use these films for optical applications, many attempts have been undertaken to control their optical properties and mainly their reflection characteristics . If casting followed by a slow solvent‐evaporation method is used, the resulting films show iridescent colors and its cross‐sections can present several domains with different pitch values.…”

Transparent Depolarizing Organic and Inorganic Films for Optics and Sensors

“…The chiral nematic structure of CNCs strongly resembles the natural organization of cellulose observed in some plant cell walls, and also strongly resembles the well‐known Bouligand structure of chitin, which is found in cuticles of beetles and exoskeletons of crabs . Many research groups have been investigating the chiral nematic structure of CNC films …”