Rheological characterization and modeling of cellulose nanocrystal and TEMPO-oxidized cellulose nanofibril suspensions

Abstract

The production capacity of cellulose nanomaterials (CNM) is limited in part due to lacking standardized, rapid and reliable characterization methods for quality control. In this study, we demonstrate the potential of using rheology as a tool to address this challenge by developing detailed test protocols and a rheological model combining power law and the Cross model: $$\eta = a\dot{\gamma }^{ - b} + \eta_{\infty } + \frac{{\eta_{0} - \eta_{\infty } }}{{1 + \left( {\lambda \dot{\gamma }} \right)^{m} }}$$, where $$\dot{\gamma }$$ is the shear rate and $$\eta$$ is the viscosity. The test protocols, including sample preparation and viscosity measurement procedures, ensure obtaining robust and accurate data. The model, which combines the power law and the Cross model, describes the flow curves over the full range of shear rates across concentrations for both cellulose nanocrystals and TEMPO-oxidized cellulose nanofibrils. The model parameterizes the viscosity data for quick comparisons of different flow curves using all data rather than selectively using viscosity values at specific shear rates. Key model parameters correlate well with the suspension concentrations. We show that this model can be used to estimate the concentration of an uncharacterized CNM sample, and to estimate the salt concentration in a sample. Both applications are relevant to quality control.