Rheo-impedance behavior of cellulose nanofibers produced by mechanical processing

CNFs are one of the renewable and the sustainable resources with low environmental impact and have various characteristics such as increased strength and weight reduction when added to resins. Since CNFs are one of the new materials that can fulfill the goals of the Sustainable Development Goals (hereafter abbreviated as SDGs), many researchers have been studying the nano-fibrillation of wood fibers. From the viewpoint of SDGs, it is necessary to avoid using a large amount of chemical agents and consuming a large amount of energy for the production of CNFs. To realize these requirements, it is important to find a way to industrially utilize CNFs containing insufficiently nanosized fibers, and for these purposes, it is essential to evaluate the physical properties of these CNFs from multiple perspectives. Cellulose fibers are intrinsically insulating materials, but how their electrochemical properties are changed by nano-fibrillization has been little studied. Therefore, we decided to clarify the relationship between the size of CNFs and the electrochemical impedance properties of the CNF suspensions containing un-fibrillated fibers, which were prepared by a wet refinement system. The fiber diameter remained constant as the number of mechanical treatments (hereafter referred to as the “number of collisions”) increased. On the other hand, the cumulative medium volume diameter, D₅₀, defined as the apparent fiber length (hereafter referred to as the “fiber length”, in microns), significantly decreases with the increasing number of collisions. The rheo-impedance |Z| of the CNF suspension remained nearly constant in the intermediate frequency range of 10³–10⁶ Hz, even if the internal structure of the system was deformed by the increasing shear rate. This means that the electrochemical properties of the CNFs are independent of the changes in the macroscopic aggregation structure. Furthermore, the internal resistance R₁ calculated from the impedance |Z| characteristics (Nyquist plot) became decreased with the increasing number of collisions, indicating a proportional relationship between the resistance R₁ and the CNF fiber length, D₅₀. This suggests that R₁ related to the resistance caused by the electrolyte in the suspensions or the protons dissociated by the hydration of the hydroxyl groups of the cellulose molecule as they move across the gaps between the microfibrils. Based on these results, it appears that the electrochemical properties of the CNF suspensions are independent of the changes in the macroscopic aggregation structure and simply depend on the fiber length, in other words, the electrochemical properties are a useful method for indirectly evaluating the fiber length of the CNFs.