Rheology of dilute and semidilute suspensions of spherical polyelectrolyte brushes

The viscosity, η, of dilute aqueous suspensions of spherical polyelectrolyte brushes is discussed. The spherical polyelectrolyte brushes consist of a solid polystyrene core of 100-nm diameter onto which linear poly(acrylic acid) chains are grafted. The relative viscosity, η/η_S (η_S is the viscosity of the solvent) of these suspensions is studied as a function of shear rate in the presence of different amounts of added salt. A marked dependence on shear rate is found, in particular when going to higher concentrations. Extrapolation to zero shear rate leads to the relative zero-shear viscosity, η₀/η_S, which can be described in terms of an effective volume fraction, φ_eff, for all salt concentrations under consideration. Moreover, the hydrodynamic radii derived from φ_eff coincide with data obtained by dynamic light scattering in the infinitely dilute regime. Data taken at higher concentrations point to a shrinking of the brush layer owing to mutual interaction.