Microstructure and surface activity of mechanically-dispersed cellulose nanofiber aqueous sol

Cellulose nanofiber (CNF)-derived functional papers hold promise for application in various fields due to their unique properties such as gas barriers, high strength, transparency, etc. Mechanochemistry offers environmentally benign and sustainable synthesis of the functionalized CNFs (e.g., in combination with photocatalytic TiO₂). The CNFs could also favorably work to produce oxygen vacancies in TiO₂ that enables visible responsive photocatalysis. What microstructural and physicochemical changes then occur on the CNF sol during the milling treatment? In this study, changes in the microstructure of the CNF aqueous sol before/after planetary ball milling were investigated based on its rheological behavior, crystallinity, and diameter distribution. The surface activity of the CNFs was additionally characterized by water vapor adsorption. A decreased thixotropy hysteresis loop observed in the low milling speed (100 rpm)-treated CNFs indicated a weaker interaction among the fibers, but still having a three-dimensional structure. A further increase in the milling speed (300 rpm) could collapse them. A decreased X-ray diffraction peak intensity of the (200) plane observed in the 500-rpm-treated CNFs could indicate a split in the fiber’s bundle as well as shredding. The increased amount of water vapor adsorption in the 500-rpm treated CNFs also supports exposure of the new surface with hydroxyl groups derived from the glucose unit. Such newly formed hydroxyl groups can be effective reaction sites with, for example, the TiO₂ precursor and perhaps favorably works to improve the photocatalytic performance.