(467c) Photonic Cellulose Nanocrystal (CNC) Coatings

Retention of cellulose nanocrystal (CNC) dispersions’ cholesteric microstructure in dried films can enable photonic materials that exhibit iridescence and selective reflection of circularly polarized light. Such materials would require uniform planar anchoring of the cholesteric helix; they could be used for optical filters, mirrorless lasers, and optical sensors. However, nonuniformities in the initial dispersion and microstructural changes during film casting and drying have made it challenging to obtain uniform optical reflection over more than a few microns. In this research, investigation of CNC drying kinetics and surface anchoring has enabled uniform planar orientation of the CNC helix in coatings on a lateral scale of few hundred micrometres. Uniform planar orientation and the associated selective reflection, was confirmed by microspectrophotometry, polarized light microscopy, and scanning electron microscopy. In addition to providing a foundation for the development of CNC based optical materials, this study also provides new scientific insights into the helical defects reported in natural chiral nematic photonic structures such as those of lomaptera beetles and pollia fruits.