Beyond cellulose nanocrystals: photonic films fabricated from lyotropic liquid crystals

Photonic films prepared by self-assembly from sustainable resources are promising functional materials for applications in optics, sensing and coloring. Up to now, such films have mainly been fabricated from aqueous cellulose nanocrystal suspensions which require hydrolysis in concentrated sulfuric acid and highly controlled drying conditions. We present here for the first time that analog photonic films can be prepared from aqueous xanthan solutions which do not require any chemical treatment and only little control over the drying process. We achieve uniform structural color with different wavelengths of selective reflection by simply drying xanthan solutions of different starting mass fractions in closed containers. Surprisingly, the helical pitch in the dried xanthan films is significantly shorter than suggested by the helical twisting power which was determined from the cholesteric liquid crystal phase. This hints at distinct differences in the drying process of lyotropic liquid crystals containing stiff cellulose nanocrystals and semiflexible xanthan polymer chains.