Shear Thinning: Determination of Zero‐Shear Viscosities from Measurements in the Non‐Newtonian Region

Experimental information on the viscosities, η, of polymer solutions and of polymer melts as a function of shear rate is modeled by means of an approach that describes the diminution of ln η as a function of shear stress, τ, in terms of an exponential decay. The approach uses the following three adjustable parameters: the zero‐shear viscosity of the system, a characteristic shear stress, quantifying its susceptibility toward shear thinning, and a dimensionless parameter stating the magnitude of the effect. This procedure gives access to the Newtonian behavior also in cases where direct measurements are impractical or impossible; it discloses two phenomena not reported so far: a qualitative change in the efficacy of τ at a characteristic concentration and indicates the occurrence of two different disentanglement mechanisms in thermodynamically unfavorable solvents. Relative viscosities of polystyrene solutions in toluene as a function of shear stress τ at 25 °C; the polymer concentrations are indicated. The intercepts of the curves with the ordinate yield the zero shear viscosities.