Modulation of Tannic Acid on the Cholesteric Structure of Cellulose Nanocrystals

The chiral nematic phase structure, formed by the self-assembly of cellulose nanocrystals (CNCs) in an aqueous suspension and maintained in a solid film, shows great potential for optical applications. To achieve complex structures in optical devices, it is crucial to subject CNCs to specific shearing processes, such as spinning and printing. Understanding the structural and property changes of the CNC liquid crystal phase in these processes is of utmost importance. In this study, we investigated the effect of adding tannic acid (TA) on the rheological properties and cholesteric phase structures of CNCs/TA mixed suspensions. By calculating the surface site interaction points, we observed that TA can adsorb onto the surface of CNC rods in suspensions through hydrogen bonding. Through characterization techniques, such as polarized optical microscopy, rheology, and synchrotron SAXS, we examined the effects of TA addition on the microstructure and rheological properties of the CNC liquid crystal phase and clarified the change relating to the system composition. Under the same CNC concentration, the volume fraction of the anisotropic phase, the pitch, and the rod spacing of the cholesteric phase were not significantly affected by the addition of TA. However, the system viscosity was significantly reduced with the appropriate amount of TA (2 wt %), in a wide range of CNC concentrations (up to 15 wt % CNCs). The flow indexes (n) in Region I and Region III of steady-state shear curves of CNCs/TA systems (11-15 wt % CNCs) were compared. Moreover, we introduced the well-established theoretical models for liquid crystal polymers to tentatively interpret Region I of the CNCs/TA cholesteric phase and realized that increased numbers of smaller cholesteric-phase domains in the CNCs/TA system and interfacial modification by TA may contribute to the fluidity change. The feature of the domain texture of CNCs/TA systems is verified by polarized optical microscopy observations.