Abstract
The present work intends to study the variations in the rheological properties and aggregation behaviour of TEMPO-oxidised cellulose nanofibrils (CNF) aqueous suspensions, as a function of changes in concentration and systematic changes in the pH, by addition of acids with different anions. It was found that CNF suspensions form strong gels at mass fractions higher than 0.35 % and the gel point is ca. 0.18 %. On the other hand, aggregation is enhanced at acidic pH conditions due to lower charge repulsion among fibrils, leading to an increase of the suspension viscosity. However, distinct rheological behaviours were presented by CNF suspensions as different acids were applied. It was found that phosphate ions resulted in significant aggregation leading to formation of particles of large size and very strong gels, at pH 2.3; distinctly, the presence of acetate ions resulted in lower aggregation, lower particle size and weaker gels, at the same pH value.
Highlights
Cellulose nanofibrils (CNF) are nanomaterials with high aspect ratio
The present study reports the rheological behaviour of suspensions of TEMPO cellulose nanofibrils (CNF) as function of nanofibrils concentration and suspension pH, the latter effect evaluated by addition of different acids
Cellulose nanofibrils suspensions typically behave as polymeric so Carbohydrate Polymers xxx (xxxx) xxx-xxx lutions, forming an entangled network above a critical concentration, being that network destroyed by increasing shear forces, leading to viscosity decrease (Vadodaria, Onyianta, & Sun, 2018)
Summary
Cellulose nanofibrils (CNF) are nanomaterials with high aspect ratio. Typically, TEMPO-oxidised CNF consist of long fibrils (up to 2 μm) with very low thickness (less than 10 nm) (Nechyporchuk, Belgacem, & Pignon, 2016). Viscosity of CNF suspensions is controlled by entanglement of cellulose nanofibrils and its aggregation, being possible to tune the suspension properties by changing some conditions such as concentration, pH, ionic strength or added co-solutes (Fukuzumi, Tanaka, Saito, & Isogai, 2014; Hubbe et al, 2017; Sato, Kusaka, & Kobayashi, 2017) These phenomena can be used to control suspension’s viscosity and stiffness, being possible to induce bridging and flocculation of negatively charged fibrils by addition of polyelectrolytes, di- or trivalent salts or by decreasing pH to acidic values (Sato et al, 2017). The rheometry and dynamic light scattering results showed significant differences in shear viscosity, apparent yield stress and particle size, at the same pH value, as different acids were used
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