Optical behavior of an associating polymer under shear flow

Flow birefringence and dichroism experiments are used to study the behavior of solutions of a hydrophobic ethoxylated urethane with a molecular weight of 35 000 g/mol and a C16 hydrophobic end cap in a concentration range aro und C*. Relaxation experiments after step shear have shown the existence of two relaxation processes. The first relaxation process with a characterist ic time, tau(short) approximate to 0.1 s, independent of concentration, has been attributed to the lifetime of an end group in a micelle and subsequen t relaxation through a Rouse process. The second relaxation process (tau(lo ng) approximate to 300 s), has been attributed to the relaxation of aggrega tes of polyoxyethylene chains. They induce a small dichroism in the solutio ns and damped oscillations in start-up shear flows at low-shear rates. Upon cessation of steady-shear flow, the two relaxations are still observed, th e amplitude of the short time one increasing with shear rate. The origin of this long-time relaxation, which is not observed in classical rheological measurements, is attributed to the presence of organic impurities remaining from the chemical modification of the polymer. Extensive purification of t he polymer leads to the disappearance of these impurities and consequently of the long relaxation process. (C) 1999 The Society of Rheology. [S0148-60 55(99)01306-1].