Understanding the Mechanistic Behavior of Highly Charged Cellulose Nanofibers in Aqueous Systems

Mechanistic behavior and flow properties of cellulose nanofibers (CNFs) in aqueous systems can be described by the crowding factor and the concept of contact points, which are functions of the aspect ratio and concentration of CNF in the suspension. In this study, CNFs with a range of aspect ratio and surface charge density (380-1360 μmol/g) were used to demonstrate this methodology. It was shown that the critical networking point of the CNF suspension, determined by rheological measurements, was consistent with the gel crowding factor, which was 16. Correlated to the crowding factor, both viscosity and modulus of the systems were found to decrease by increasing the charge density of CNF, which also affected the flocculation behavior. Interestingly, an anomalous rheological behavior was observed near the overlap concentration (0.05 wt %) of CNF, at which the crowding factor was below the gel crowding factor, and the storage modulus (G′) decreased dramatically at a given frequency threshold. This behavior is discussed in relation to the breakup of the entangled flocs and network in the suspension. The analysis of the mechanistic behavior of CNF aqueous suspensions by the crowding factor provides useful insight for fabricating high-performance nanocellulose-based materials.