Alcohol-induced gelation of cellulose nanofibrils dispersions

Cellulose nanofibrils (CNF) are nanoscale sized fibrils obtained from cellulosic materials, especially wood pulp. Due their biological nature, they are inherently green materials, renewable and biodegradable, also abundant, cellulose being the most abundant natural polymer. CNFs, due the colloidal dimensions, present a versatile material to form films, nanocomposites or dispersions. TEMPO-oxidized CNFs, negatively charged flexible fibrils, can be easily dispersed in water, and caused to aggregate and gel by physical-chemical modifications of the dispersing media, salt, co-solvents, etc. In this work, we focused on
CNF gelation induced by addition of co-solvents, namely methanol, ethanol, 1-propanol and 2-propanol. These systems were investigated both at macroscopic level, via rheology, and at nanoscale level, via small-angle X-ray scattering. Rheological data showed that all the alcohols studied are capable of inducing CNF gelation. The addition of alcohol leads to an increase of viscosity, producing shear-thinning fluids, up to a certain critical alcohol concentration at which the gel forms. This critical concentration, and the gel’s shear modulus, depends on the type of alcohol, the more hydrophobic alcohols (log P) have a lower gelation onset and higher shear modulus. Preliminary analysis of the
SAXS data suggests the alcohol affects the cross-section of the scattering centred at higher-q end instead of changing the structure of the larger structures observed at the lower-q range. That could suggests that the gelation is driven by fibrils aggregating aligned along their longitudinal axis resulting in thicker fibrils, but still forming long, flexible objects.