AbstractNon‐Newtonian flow of moderately concentrated solutions of polyvinyl alcohol (PVA) in water and polystyrene (PS) in toluene were measured with a Maron‐Krieger‐Sisko viscometer at various temperatures and concentrations. The usual dependences of the apparent and zero shear viscosities for two polymers on rate of shear, temperature, and molecular weight have been found. The log‐log plot of zero‐shear viscosity versus concentration can be represented by two straight lines intersecting at one point (critical concentrationcc). The critical concentration in volume fraction of polymer,v2c, multiplied by the chain lengthZis not constant but decreases with decreasingZ, although it has the same order of magnitude as the value ofZcv2obtained from the log‐log plots of viscosity versusZfor many systems of polar and nonpolar polymers. The productccρZ1/2(ρ = density of the solution) is constant and independent ofZ. To explain the constancy of this product, an equivalent sphere model is presented. When the volume fraction of spheres is assumed to be unity, the extension of molecules agrees very well with the unperturbed extension evaluated from intrinsic viscosity data
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