A STRUCTURAL MODEL OF HYDROPHOBICALLY-MODIFIED URETHANE-ETHOXYLATE (HEUR) ASSOCIATIVE POLYMERS IN SHEAR FLOWS

This paper describes the rheological behavior of a HEUR (hydrophobic e thoxylated urethane) associative polymer with C16H33 end groups at 2.0 wt % concentration in aqueous solution. Under normal steady shear, th is solution exhibits Newtonian behavior at low shear rates and, as the shear rate is increased, passes through a shear-thickening region bef ore exhibiting a sharp decrease in viscosity. Here we report superposi tion-of-oscillation experiments on steady-shear flows to examine the s tate of the network structure under different shear conditions. The te chnique involves applying a steady shear deformation to the fluid, and once the steady state is achieved, a small amplitude oscillation is i mposed 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-thickeni ng is the result of a shear-induced increase in the density of mechani cally active chains, which may be due to incorporation of free micelle s or higher aggregates into the network structure. In the shear-thinni ng 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 ju nctions of the network. This is the first experimental evidence for sh ear enhancement of the relaxation rate of an associative polymer netwo rk.