Enhanced Toughness and Thermal Stability of Cellulose Nanocrystal Iridescent Films by Alkali Treatment

Iridescent films constructed by self-assembly of rod-like cellulose nanocrystals (CNCs) are mechanically brittle and of poor thermal stability. Herein, we demonstrate that the toughness and thermal stability of vacuum filtered CNC iridescent films could be significantly improved by a simple alkali treatment. The results reveal that the NaOH treatment can simultaneously alter the condensed physical structure and surface chemical structure of CNCs in their liquid crystal state while the self-assembled CNC chiral nematic structure is still well retained. To be specific, the CNC was transformed from a higher crystallinity of form I to a lower crystallinity of form II, and the sulfate groups of CNCs were erased by alkali treatment, resulting in the remarkable enhancement in mechanical and thermal properties of CNC iridescent films. Of note, the unprecedented improvements in both tensile strength and toughness of CNC iridescent film have been achieved by alkali treatment. A sandwiched model with interdigitated molecular chains in amorphous layer as energy dissipation was proposed to explain the remarkable improvement in mechanical properties. This study shows that tailoring the condensed structure of CNC nanorods itself is a promising strategy to improve the performance of CNC iridescent films without blending with the second component.