Chiral nematic cellulose nanocrystals and gluconic acid iridescent films with high flexibility and humidity response

Two novel chiral nematic cellulose nanocrystal (CNC) composite films composed of CNCs, gluconic acid (GA), polyvinyl pyrrolidone or polyacrylamide (CNC-GA-PVP and CNC-GA-PAM) were fabricated mainly by evaporation-induced self-assembly process. The visible iridescent color and chiral nematic structure of the obtained CNC-based films could be readily regulated by altering the content of GA in a wide color range. Both CNC composite films possessed good tensile strength. Worthy to be mentioned, the CNC-GA-PAM films exhibited rather high flexibility (elongation reached 222.53%) and could be curled into any shape, indicating the effective contribution of the polymer cross-linked networks from the non-ionic PAM. The reversible humidity responses were also observed for both CNC films with good sensitivity, probably due to the high hygroscopicity of GA. In addition, the CNC-GA-PAM films revealed exceptional water resistance without losing the regular chiral structure and structural color of CNCs. The influences of the content of tougheners and adding way of the polymers on the properties of CNC films were further investigated. This work presents a novel strategy for construction of multifunctional CNC-based nanomaterials for various applications. Graphical Abstract