Rheological behaviour of polystyrene latex near the maximum packing fraction of particles

Additional developments in the comprehension of the rheological behaviour of polymer latices, especially near the high critical concentration ϕc, are presented for two polystyrene latices of average particle diameters close to 200 nm with different electrostatic properties. Not surprisingly, there is a rapid transition in the rheological characteristics over a narrow range of polymer volume fractions as the concentration of the disperse phase increases. By examining twelve different polymer volume fractions a unique value of the critical volume concentration, ϕc, was found for each latex. At this point, the steady shear viscosity, dynamic modulus, and dynamic shear viscosity change dramatically. Furthermore, these critical concentrations are well confirmed by the percolation theory for the dynamic zero shear viscosity as a function of volume fraction. The Cox-Merz rule is not obeyed by these dispersions at the concentrations greater than ϕc. By using a controlled strain Couette rheometer with a gap of 1.2 mm, shear thickening limits were also observed for both latices. The concentration dependence of the onset shear rate for shear thickening changes near ϕc for each of the two latices.