Viscoelastic properties of aqueous concentrated polystyrene latex dispersions containing grafted poly(ethylene oxide) chains

Abstract

The viscoelastic properties of aqueous concentrated polystyrene latex dispersions containing grafted poly(ethylene oxide) (PEO) chains were investigated using oscillatory and steady-state shear stress-shear rate measurements. The relative viscosity-effective volume fraction results were fitted to the Dougherty-Krieger equation for hard spheres by adjusting the value for the adsorbed layer thickness Δ. The latter was found to decrease with an increase in volume fraction of the dispersion and near to close-packing considerable compression of the chains occurred. From the oscillatory measurements, the complex modules G ∗ , storage modules G′, and loss modulus G″ were obtained as a function of frequency at various latex volume fractions. The results showed that the dispersion changes from being more viscous ( G″ > G′) to more elastic ( G′ > G″) over a narrow range of volume fraction φ of the dispersion, i.e., when φ is increased from 0.465 to 0.5. Within the range of increase of φ, it is likely that the chains undergo some compression and interpenetration of the peripheries. When φ increased significantly above 0.5, the system becomes predominantly elastic and significant interpenetration and compression of the chains occur. Indeed when φ is further increased to 0.585 and 0.62, the moduli increase by several orders of magnitude and so does the dynamic viscosity. Under these conditions, the latex behaves as an elastic “gel.”