Rheo-SANS Study of an Aqueous Suspension of Poly(Methyl Methacrylate-co-Styrene) Particle

Abstract

For an aqueous suspension of poly (methyl methacrylate-co-styrene) (MS) particles, viscosity η and small-angle neutron scattering (SANS) intensity I(q) were measured under steady shear. The MS particles had partially screened surface charges. In the quiescent state, the particle distribution detected through the I(q) profile was isotropic and essentially liquid-like but also exhibited a weak, local order. Under steady shear, this distribution was distorted just moderately, and the η strongly decreased with increasing shear rate γ• . Thus, this thinning of η was related to the weak distortion of the particle distribution. Furthermore, the thermodynamic stress σ T estimated from the I(q) profile under the shear increased only weakly with increasing γ• , and the corresponding viscosity η T (=σ T /γ • ) was close to the η data. These results strongly suggested that the thinning of the MS suspension reflected the nonlinearity of σ T . The zero-shear viscosity η 0 of the suspensions was governed by an effective volume fraction φ eff (>φ) that included the volume of the electrostatic shell. In fact, the φ eff dependence of η 0 of the MS particles was in close agreement with the φ dependence seen for hard-core silica particles, 4) the latter being subjected to (almost) fully screened electrostatic interaction to have a negligibly thin electrostatic shell. This result suggested that the terminal relaxation of the MS suspensions corresponds to the relaxation of the thermodynamic stress, as similar to the situation in the hard-core suspensions: KeyThe thermodynamic stress reflects a strain/flow-induced distortion in the spatial distribution of the particles and relaxes through the Brownian motion of the particles. [8][9][10][11][12]