Rheological Studies of Aqueous Concentrated Polystyren Latex Dispersions with Adsorbed Poly(vinyl alcohol) Layers

Abstract

The viscoelastic behavior of concentrated (20% w/w)aqueous polystryene latex dispersions (particle radius 92nm), in the presence of physically adsorbed poly(vinyl alcohol), has been investigated as a function of surface coverage by the polymer using creep measurements. From the creep curves both the instantaneous shear modulus, Go, and residual viscosity, ηo, were calculated. The yield value, τβ, was also measured as a function of surface coverage. All the rheological parameters decreased with increasing surface coverage. This was related to the reduction in particle interaction with increase of polymer adsorption. At full coverage the residual Go, ηo and τβ values were attributed to the combinea action of long-range electrostatic repulsion and steric interaction due to the presence of long, dangling tails. Rheological measurements were also carried out on flocculating, concentrated (25% w/w) aqueous polystyrene latex dispersions (particle radius 115nm). The flocculation was produced either by addition of Na2SO4 or raising the temperature of the dispersion at constant Na2SO4 concentration. Both the yield value and shear modulus were measured and the results obtained analyzed using two models, namely the floc rupture and elastic floc models proposed by Hunter and coworkers. Good agreement between the experimental yield values and those calculated using the elastic floc model was obtained, thus confirming its applicability to the case of flocculating, sterically-stabilized dispersions.