Rheology and Phase Behavior of Lyotropic Cellulose Nanocrystal Suspensions

The dispersion microstructure and rheological properties of aqueous sulfonated cellulose nanocrystal (CNC) suspensions have been investigated. Between 3.07 and 10.4 vol % the suspensions phase separated into liquid crystalline and isotropic domains. At 12.1 vol %, no isotropic phase was visible, and the samples had the fingerprint texture characteristic of a cholesteric liquid crystal. Below 35 degrees C, temperature had little influence on rheology and phase behavior. However, between 35 and 40 degrees C there was a significant change in both the fraction of isotropic phase and the rheological properties. In contrast to many lyotropic suspensions, the steady shear viscosity did not go through a maximum with increasing concentration. Maxima were observed for complex viscosity, storage modulus, and loss modulus at concentrations that appeared fully liquid crystalline. Time concentration superposition was successful for the loss modulus but not the storage modulus. This suggests that the interface in biphasic samples affects the elastic relaxation but not the viscous response. At still higher concentrations, the fingerprint texture of the liquid crystal phase was absent, and the dispersions behaved as rheological gels.