A structural model of hydrophobically modified urethane-ethoxylate (HEUR) associative polymers in shear flows

This paper describes the rheological behavior of a HEUR (hydrophobic ethoxylated urethane) associative polymer with C16H33 end groups at 2.0 wt % concentration in aqueous solution. Under normal steady shear, this solution exhibits Newtonian behavior at low shear rates and, as the shear rate is increased, passes through a shear-thickening region before exhibiting a sharp decrease in viscosity. Here we report superposition-of-oscillation experiments on steady-shear flows to examine the state of the network structure under different shear conditions. The technique involves applying a steady shear deformation to the fluid, and once the steady state is achieved, a small amplitude oscillation is imposed on the sample to measure the linear viscoelastic properties. We observe that within the shear-thickening region, the plateau modulus is larger than in the Newtonian region, suggesting that shear-thickening is the result of a shear-induced increase in the density of mechanically active chains, which may be due to incorporation of free micelles or higher aggregates into the network structure. In the shear-thinning region, the Maxwell relaxation time decreases with increasing shear stress or shear rate. Thus shear thinning is a consequence of a shear-enhanced exit rate of the hydrophobic end groups from the micellar junctions of the network. This is the first experimental evidence for shear enhancement of the relaxation rate of an associative polymer network.