Shear-induced phase separation in an associating polymer solution

Rheological and rheo-optical experiments have been performed under shear fl ow on 1% solution of an hydrophobic ethoxylated urethane (HEUR) with a mole cular weight of 35 000 and a C18 hydrophobic end-cap. Alter a Newtonian beh avior at low shear rates, followed by a slight shear thickening, a pronounc ed drop in viscosity is observed above a given shear rate. Creep experiment s confirms, that above a critical stress a rather long time is required to reach a steady state with a rather low viscosity. Under steady-state condit ions at 20 degreesC, birefringence first increases with shear rate until <( <gamma>)over dot> approximate to 5 s(-1), then decreases until <(<gamma>)ov er dot> approximate to 50 s(-1) and increases upward again. The decrease in molecular orientation is accompanied by a decrease of the transmitted ligh t intensity. On cessation of shear flow, the transmitted intensity increase s back to its initial value. This behavior is interpreted in terms of a rev ersible shear-induced phase separation.