Intrinsic viscosity of aqueous suspensions of cellulose nanofibrils

Abstract

Cellulose nanofibrils (CNF) from wood fibers are of increasing interest to industry because they are from renewable sources and are biodegradable. Owing to their high aspect ratio, they produce viscous suspensions and stiff gels that are strengthened by interfibrillar hydrogen bonds. In this study, the viscosity of aqueous CNF suspensions, at dilute concentrations ($nL^{3}<1$), was measured at various pH values by addition of HCl, and at various ionic strengths by addition of NaCl and $\hbox {CaCl}_{2}$. The results show that the primary electroviscous effect significantly increases the intrinsic viscosity. The intrinsic viscosity under conditions where the surface charge of nanofibrils is fully screened is in good agreement with the predictions of classical theory for dispersions of rodlike particles at low shear rates. Increasing the ionic strength up to $\kappa d\approx 1$ decreases the intrinsic viscosity; at $\kappa d>1$, the intrinsic viscosity increases because of fibril aggregation and increase of the effective volume fraction.