Durable Hydrophobic Iridescent Films with Tunable Colors from Self-Assembled Cellulose Nanocrystals

Cellulose nanocrystals (CNCs) are known to self-assemble into a left-handed chiral nematic lyotropic liquid crystalline phase in water. When captured in the solid state, this structure can impart films with photonic properties that make them promising candidates in photonics, sensing, security, and other areas. Unfortunately, the intrinsic hydrophilicity of CNCs renders these iridescent films susceptible to moisture, thereby limiting their practicality. To address this issue, a novel strategy to prepare hydrophobic iridescent films from pre-assembled CNC films is reported here. These films underwent a swelling process, followed by esterification using acid anhydrides to render them hydrophobic. By increasing the alkyl chain length of the anhydride reagent, the hydrophobicity of the resulting iridescent films can be enhanced. They showed water contact angles ranging from 34° to 115° and demonstrated tunable structural color spanning from blue to red. Moreover, they also exhibited good durability when exposed to water for 24 h. This innovative method for producing durable hydrophobic iridescent thin films is expected to facilitate their use in water-proof photonic coatings, optical sensors, and other applications.