Cellulose Nanocrystal Chiral Nematic Composites with Wet Mechanical Adaptability

Cellulose nanocrystal (CNC)-based chiral nematic films are important optical materials due to their ability to selectively reflect circularly polarized light but are often limited in application by their instability in water. Improving the mechanical performance and stability of photonic CNC-based materials in water is therefore of great importance for advancing the utility of CNCs. Here, water-resistant covalently crosslinked chiral photonic composite films of CNCs were formed by the incorporation of an epoxy resin (ER). The resulting materials can adapt their stiffness and toughness through the mediation of inter-CNC and CNC–water interactions upon exposure to water. Dry films of CNC–ER exhibit improved mechanical strength and toughness, likely related to the uniform and continuous chiral nematic structures in which stress transfer is facilitated by interrod interactions. In water, the interrod interactions are switched off by establishing rod–water interactions, resulting in up to ∼4-fold increase in toughness relative to the dry composites. Moreover, the composite films display tunable chiral photonic properties across the visible spectrum and moderate red shifting of reflection in response to slight swelling in water. The switchable rod–rod/water interaction enabled by a hydrogen-bonded/covalent dual network opens a path to develop CNC-based materials with mechanical adaptability and responsive chiral optical properties.