Ultrastrong and superamphiphobic structural color composites via synergistic densification of cellulose nanocrystals and aligned nanofibrils

Abstract

Structural colors derived from chiral nematic cellulose nanocrystals (CNCs) hold great promise for sustainable photonics; however, their practical applications are severely impeded by intrinsic mechanical brittleness and extreme vulnerability to environmental hazards, particularly moisture and fire. Herein, we report an ultrastrong, superamphiphobic, and flame-retardant biomimetic composite film designed to fundamentally break this longstanding optical-mechanical trade-off. By encapsulating a fragile CNC photonic core between two robust, highly aligned cellulose nanofibril (CNF) skins derived from natural wood, a synergistic high-pressure densification strategy is employed to reconstruct the mechanical framework. The resulting "sandwich" composite not only perfectly preserves vibrant, tunable macroscopic iridescence but also achieves an unprecedented tensile strength of 474.4 MPa and a remarkable toughness of 21.15 MJ m-3—representing staggering ~50-fold and ~190-fold enhancements over pure CNC films, respectively. Furthermore, the integration of a hierarchical multiscale coating (F-FC-APP/SiO2) endows the composite with robust superamphiphobicity (contact angles >150° for water and varied oils) and excellent self-extinguishing flame retardancy. This rational structural and interfacial engineering strategy effectively shields the photonic core from physical and environmental degradation, paving a sustainable pathway for the deployment of durable, multifunctional structural color materials as eco-friendly alternatives to toxic chemical dyes in demanding optical, decorative, and load-bearing applications.