Structural Coloration of Solid Films Prepared from Xanthan Lyotropic Liquid Crystal Solutions

Inspired by the helical self-assembly of chiral molecules in nature, iridescent films as promising photonic materials have been yielded by drying cholesteric nanoparticle suspensions or polymer solutions. To date, biobased cellulose nanocrystal suspensions are the main source for fabricating such films, which involve the hydrolysis of raw materials with a huge consumption of concentrated sulfuric acid and rigorous control of the drying process. To enhance sustainability, we here demonstrate that structurally colored, nonbrittle films can be produced via drying ultrasonically treated xanthan water solutions without additional chemical treatment and under mild evaporation control. By adjustment of the duration of ultrasonication, the chain length of xanthan gum can be tuned, leading to a distinct change of its cholesteric liquid crystal behavior. Surprisingly, the evolution of the pitch from the cholesteric liquid crystal to the dried film does not follow the same mechanism as that known from cellulose nanocrystal suspensions but exhibits a much more pronounced shrinkage. We analyze this behavior by applying various theoretical models and discuss the impact of chain flexibility and the point of gelation.