Elasticity of associative polymer solutions and slip at high shear stress

Abstract

The rheological behavior of several associative polymers in aqueous solution was examined by carrying out measurements in steady shear, oscillatory shear, and uniaxial extension. The associative polymers were commercial samples and the solution concentrations were in the 1% range, the range where close packing of the micelle-like clusters leads to bridging and the formation of large networks. In steady shear, solution viscosities varied from constant values over four decades in shear rate to shear-thinning over the same four decades. Measurements of the first normal stress difference show that the solutions were generally moderately elastic in steady shear, up to shear rates of 1500 s −1. These measurements also show that N 1 increased with shear rate in regions of shear thinning, which suggests that the thinning was a result of microstructure deformation, and not of microstructure breakdown as is currently thought. In small-amplitude oscillatory shear, it was found that the test fluids were linearly viscoelastic and that a one-or two-mode Maxwell model could be fitted to the data. Measurements of extensional viscosity yielded Trouton ratios in the range of 5–100. Hence the solutions were moderately elastic in extension as well as in shear. Some of the solutions were found to slip in a parallel-plate rheometer, and so slip was investigated using that instrument. From measurements of viscosity with various gap distances in the rheometer, it was found that slip occurred when the shear stress exceeded roughly 500 Pa. The viscometric data were analyzed assuming that there was a clear-liquid layer next to each plate, and the results indicate that the slip layer thickness was of order 20 nm, or about the size of a cluster.