Nanocellulose as a special type of cellulose has gained significant interest due to its unique properties: high aspect ratio, specific surface area, excellent mechanical properties, and relatively good surface reactivity. In particular, nanocellulose is widely used for its inherent flexible elongated structure and its capability to form physically entangled gel networks at low concentration. The effects of the level of carboxylation and calcium chloride (CaCl₂) concentration on the rheological behavior and microstructure for the carboxylated nanocellulose dispersion/hydrogel had been investigated.
It indicated that the nanocellulose dispersion exhibited sol state with low storage (G′) and loss (G′′) modulus when sodium hypochlorite (NaClO) concentration controlled to 2 mmol/g. But nanocellulose dispersion presented a form of gel at high NaClO concentration (6-12 mmol/g) and a critical state (similar values of G′ and G′′) was obtained with the NaClO concentration of 4 mmol/g. In addition, both the G′ and G′′ of nanocellulose dispersion were increased and then decreased with the increase of NaClO concentration. The highest modulus obtained when NaClO concentration controlled to 8 mmol/g, and a high fractal dimension (D_f) was also obtained at this concentration, indicating a compact network structure. The large amplitude oscillation shear (LAOS) suggested that nanocellulose dispersion exhibited Ⅰ behavior (strain thinning) but had a trend of III behavior (weak strain overshoot). The Fourier transform rheology, Chebyshev coefficient, and Lissajous curves were used to analyze the nonlinear behavior, implying a correlation between the nonlinear behavior and the network structure of the nanocellulose dispersion. This work would provide some guidance for regulating the properties of nanocellulose dispersion.
Besides, the effects of calcium chloride (CaCl₂) concentration on the creep-recovery, linear and nonlinear rheological behavior of nanocellulose gels had been investigated to quantify gel properties. The absolute zeta potential of nanocellulose gels were decreased as the CaCl₂ concentration increased, which was related to the electrostatic repulsion that origin from carboxyl group could be effectively screened with increasing CaCl₂ concentration. Rheological measurements further confirmed this result for nanocellulose gels, which revealed that the increased modulus and viscoelastic properties were obtained in the presence of CaCl₂. The rheological properties of nanocellulose gels were showed to depend on CaCl₂ concentration. The enhanced gel network structure was related to the Ca²⁺ ions that promoted crosslink between nanocellulose by salt bridge. This work highlighted the potential of using electrostatic complexation between nanocellulose and Ca²⁺ ions to form gels, and demonstrated the tunability of the rheological behavior by adjusting the concentration of CaCl₂.

TEMPO-periodate oxidation; Nanocellulose; Hydrogel; Rheological properties; Microstructure