Co-assembling Polysaccharide Nanocrystals and Nanofibers for Robust Chiral Iridescent Films.

Assembling robust chiral biopolymer structures without compromising vivid optical iridescence is a grand challenge for biocomposite materials. Herein, we report a hierarchical nanocellulose nanostructure with a helicoidal organization co-assembled from chiral rigid cellulose nanocrystals (CNCs) and longer nanofibers isolated from the hydrolyzed wood pulp. This resulting highly iridescent chiral nanocellulose material is much tougher than traditional chiral CNC films. We found that the mixed nanocellulose are composed of needle-like nanocrystals and very long (up to 800 nm) flexible cellulose nanofibers (CNFs). Large-scale molecular simulation indicates that enhanced dynamic hydrogen bonding with labile networking facilitates mechanical reinforcement, owing to increased nanocrystal length, the co-assembly of nanofibrils in mixed bundles, and interchain entanglements. This study provides a novel strategy to transform the wood pulp residues into high-value-added photonic-bound polysaccharide materials. These hierarchical biomaterials can overcome the conflicting trends in designing balanced mechanical and optical performance of chiral biofilms and their conversion to robust chiral photonic materials with enhanced performance.