IRIDESCENT STRUCTURAL COLORATION AND SELF- ASSEMBLY OF CHIRAL NEMATIC CELLULOSE NANOCRYSTAL COMPOSITES

Abstract

Materials such as cellulose and its derivatives are attracting growing attention worldwide to recognize their potential in responding to increasingly complex technological requirements. The rising demand for multifunctional materials may be fulfilled with controllable functionalities by using self-assembling techniques. Photonic stimuli-responsive materials based on cellulose can shift color reversibly when responding to external inputs. We are interested in cellulose since it is one of the two structural components of lignocellulosic biomass, along with lignin, and has a crystalline structure. Additionally, the original fibre kind of architecture seen in wood is preserved by its crystallinity. This crystalline structure is significant for the optical response as it imparts a property known as birefringence. However, it's how this property interacts with light, how light can move at different speeds inside the structure, that causes the structural coloration. Cellulose nanocrystals (CNCs) are bio-renewable materials that self-assemble themselves into a chiral nematic ordering exhibiting iridescent colors and unique optical features. The inclusion of functional polymers, small molecules, or other active components into CNC precursors can enhance and stabilize their uniformity, allowing better stimuli interaction with the CNCs. Our strategy involves utilizing bio-mass waste to fabricate CNCs based photonic materials using different dopants. First, a standard optimized extraction process was adopted to isolate the CNCs from different sources. The resultant nanocrystals displayed variable morphologies such as spherical, rod, and needle shape. The hydrodynamic diameter, crystallinity index, decomposition temperature, liquid crystallinity, and storage modulus were also found to be dependent on the source used. Nanocrystals isolated from non-wood feedstock have shown a higher degree of polymerization of 108.2 and a high Crystallinity Index (C·I.) of 75%. The rod-like morphology with the liquid crystalline pattern was obtained at 3 wt.% concentration for CNCs isolated from sugarcane bagasse (SCNC). In the second work solvent test films based on self-assembled CNC structures are designed to detect and distinguish different organic solvents. The citric acid (CA) is added to tune the helical pitch (P) of the composite films. When added as stimuli, CA acts as a plasticizer, and the resulting films are mechanically durable. CNC/CA nanocomposite films can serve as discrimination sensors by showing an apparent color change when dipped in organic solvents due to different solubility and the magnifying effect caused by increasing CA concentration. The testing films were optically characterized with Polarized Light Microscopy (POM) and a UV–Visible spectroscopy (UV-Vis). In next work, we have proposed an approach to preserve and lock the self-assembled CNC films between the layers of a poly (lactic acid).