High-strength cellulose nanocrystal-based chiral photonic film

The twisted structure of cellulose nanocrystals (CNCs) endows them with the ability to form a cholesteric phase and, consequently, a photonic crystal film through evaporation-induced self-assembly (EISA). However, the physically force-dominated CNCs-based chiral photonic film (CCPF) exhibits obvious defects such as brittleness and water solubility. Here, we combine EISA with covalent crosslinking to prepare CCPFs via the Schiff base reaction between dialdehyde cellulose nanocrystals (ACNCs) and polyethyleneimine (PEI). Compared with the ACNCs film, ACNCs-PEI CCPFs demonstrate a significant increase in tensile strength (∼101 MPa) and elongation at break (1.31 %). After immersion in acidic, neutral, and alkaline solutions for 24 h, the as-prepared CCPFs can still retain their original shape. The CCPFs also exhibit left-handed chirality and excellent UV-blocking properties. Surprisingly, the abundant sub-fluorescent groups and crosslink-enhanced emission effect give CCPFs exceptional photoluminescence behavior. Overall, this work offers a method to produce high-tensile-strength CCPFs by combining covalent cross-linking with EISA, which can be extended to other hybrid CCPFs containing polyamine compounds.