Rheological study of cellulose nanofiber disintegrated by a controlled high-intensity ultrasonication for a delicate nano-fibrillation

Herein, the rheological properties of 2,2,6,6-tetramethylpiperidin-1-oxyl radical-oxidized cellulose nanofiber (TOCNF) suspensions individualized using high-intensity ultrasonication were investigated. The surface charge density of the nanofibers and sonication time were 0.659–1.24 mmol/g and 30–600 s, respectively. With increased surface charge density, the minimum time required for disintegration decreased due to the repulsive force between oxidized nanofibers. Additionally, increased sonication time enhanced the TOCNF nanofibrillation, thereby forming networks between the nanofibers. Further disintegrating TOCNF increased shear viscosity and yield stress of TOCNF suspensions. Based on the crowding factor theory, the relationship between the average fiber width and sonication time was found at various surface charge densities. Ultrasonication was considered as an energy saving and precisely controllable nanofibrillation method. This research shows the change of fiber shapes during the nanofibrillation process, and suggests an estimation of disintegration degree by the relationship between the rheological properties and TOCNF morphology. Graphic abstract