Xanthan-Induced Gelation in Cellulose Nanocrystal Suspensions: Altering Self-Assembly and Optical Purity

Abstract

Cellulose nanocrystals are charged rod-like nanoparticles that can spontaneously organize into cholesteric mesophases upon evaporation to produce photonic films with circular polarization. In this study, it is demonstrated that through the modulation of gelation and kinetics of phase separation via the addition of a strong gelating agent, xanthan gum, the cellulose nanocrystals produce flexible photonic films with improved optical purity in terms of circular reflection. The work reveals the self-assembly behavior as a function of xanthan gum through the volume depletion mechanism, quenching of the tactoids at the early stages of the self-assembly, and evaporation into cholesteric films. In dried films, this leads to significant reduction in the domain size and the absence of domain folding which is usually caused by the merger of large tactoids. Despite the reduction in metallic iridescence caused by the entrapment of cellulose nanocrystal tactoids, the films exhibit improved macroscopic color uniformity across 30–60° observation angles, which implies a combined effect of surface texturing and scattering of visible light induced by incorporating xanthan gum within the co-assembled structure. Overall the work provides new insights into the gelation mechanism in hybrid cellulose nanocrystal systems and presents an efficient approach to control the self-assembly and macroscopic color appearance.