Diffusion in non-ionic and ionic polymer solutions: Effects of shear rate and polymer concentration

Document Type

Article

Publication Date

1-1-1985

Abstract

Diffusion coefficients for ferricyanide ions in aqueous polymer solutions have been calculated from laminar flow mass transfer data obtained in a concentric annulus apparatus by the electrochemical voltammetric method. The electrolyte solutions containing one to five percent polyvinyl alcohol or polyacrylic acid were pumped through the annulus. Flow was longitudinal, and the shear rate ranged from 7 to 800 s -1. The diffusion coefficient for ferricyanide in the polymer solutions depended on polymer concentration in a different way for the non-ionic (polyvinyl alcohol) solutions compared to the polyelectrolyte (polyacrylic acid) solutions. Over the range of concentrations studied, the diffusion coefficient as a function of polymer concentration for polyacrylic acid solutions reached a minimum, increased to a maximum, and then decreased slightly. The polyvinyl alcohol data showed minima and maxima but no decrease, and the minimum occurred at a higher polymer concentration. For each type of solution, the diffusion coefficient decreased from five to twenty percent as the shear rate was increased from 100 to 800 s-1. The diffusion data can be related to polymer molecule uncoiling and stretching in the flowing solution and to solvation of the molecules present. In each case, a critical polymer concentration the concentration at which all the solvent is associated with the polymer and there is no free solvent was identified. This critical polymer concentration was in the range of 2 to 3 percent for polyvinyl alcohol solutions and 3 to 4 percent for polyacrylic acid solutions. The Stokes-Einstein ratios calculated from the diffusivity and rheological data show that the effect of shear rate on diffusivity cannot be explained entirely in terms of changes in apparent viscosity. © 1985, Taylor & Francis Group, LLC. All rights reserved.

Publication Title

Chemical Engineering Communications

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