Thermal Reversible Gelation of TEMPO-Oxidized Cellulose Nanofibers Dispersed in Propylene Glycols

TEMPO-oxidized cellulose nanofibers (CNF) have attracted a great deal of research interests recently, due to their environmentally friendly nature, excellent mechanical properties and abundant surface functional groups for direct applications or modifications. They also show good dispersibility in water due to these hydrophilic functional groups, and strong electrostatic repulsion between the fibers. Our lab has successfully dispersed CNF in ethylene glycol (EG) and propylene glycol (PG), having different hydrophobicity. The CNF/EG suspension was found to behave similar to the aqueous CNF suspension, while CNF/PG suspension exhibited unusual gelation behavior beyond overlap conc., as revealed by the long-time oscillation monitoring. This thermal-irreversible gelation process can be greatly accelerated at high temperature, and the formed gel can be reversed back to viscous liquid upon severe stirring. We argue that the PG is a poor solvent for CNF compared to EG and water, and thus with relatively higher hydrophobicity. The increasing temperature appears to increase the hydrophobicity of PG, leading to more hydrophobic aggregation between CNF and thus stronger gelation tendency.