Controlled Stratification and Assembly of Cellulose Nanocrystals in Polymeric Films Toward Optically Active Coatings

Nature's most brilliant hues arise from the interaction of light with multilayered- structures of aligned building blocks. Mimicking this hierarchical organization in highly-ordered thin films of liquid crystalline species has attracted increasing attention for potential applications in sensors and optical switching displays. Due to its intriguing ability to organize into optically active materials, cellulose nanocrystals (CNCs) are attracting a strong interest in the scientific community. This study demonstrates that the shear-driven convective assembly technique can be used to stratify in a controlled fashion highly ordered multilayers of rod-like CNC embedded in a protective hydrophobic polymer matrix leading to optically active thin films. The films remain fully transparent even after stratifying 50 layers. Atomic force microscopy analysis reveals that over 87% of the CNCs in the upper layer aligned within ±20° of the withdrawal direction. Notably, the stratification does not disrupt the organization of the underlying layers. The films exhibit strong selective reflections with uniform and intense colors, dependent on the number of stratified layers. This scalable appraoch enables precise control over the optical characteristics of CNC-polymer composite films, presenting opportunities for environmentally friendly applications in pigment-free coatings, security papers, and optical devices.