Shear-induced heterogeneity in flocculating micro/nanofibrillated cellulose microstructures

Aqueous suspensions of micro/nanofibrillated cellulose are known to exhibit gel-like behavior at low concentrations. The physical gel properties are a result of extensive hydrogen bonding, fibril entanglement, and flocculation. In this work, we used dynamic oscillatory rheology and steady shear rheology to assess the microstructural behavior. Three phenomena were studied at length: (1) flow instability in the form of a deviation from typical shear-thinning viscosity at intermediate shear rates (1-10 s^-1), (2) reduced structural recovery after breakdown under intermediate values of oscillatory shear (5-8 Pa), (3) yield stress of the system as measured by two different rheological methods. These dynamic behaviors provide insight to the fundamental structure of the suspension, which behaves in the manner of a highly flocculated system. Furthermore, the properties of MNFC were compared with a system of (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO)-oxidized cellulose nanofibrils (CNF), which differ from MNFC in their smaller cross sections and negative surface charges. These properties result in increased stability of the TEMPO-CNF suspension. Between these two related systems, yield stress, floc dynamics, and critical concentration of gel formation were compared and analyzed.