Non-Newtonian viscosity in linear and star polymers

We theoretically investigate non-Newtonian viscosity and coil deformation o f linear and (regular) star polymers in dilute solution subject to large sh ear rates. A bead-and-spring model with preaveraged hydrodynamic interactio n, accounting also approximately for good-solvent expansion, is employed wi thin the Rouse-Zimm approach. We impose a constraint on the average spring lengths, so as to keep constant the average contour length of the arms unde r shear: this corresponds to assuming that the springs become increasingly stiffer. For any topology and a very large molecular mass, coil deformation modifies the hydrodynamic interaction, that goes to a maximum, and then de creases with a crossover from the Zimm to the Rouse regime with increasing shear rate. Correspondingly, the intrinsic viscosity decreases and then rai ses above its low-shear value. This behavior is however much less pronounce d under good-solvent conditions. At very large shear rate, the constraint o n the spring lengths becomes the dominant factor. This leads to a decrease of intrinsic viscosity with an asymptotic -2/3 power law for any draining c ondition. Simultaneously, the strongly elongated coil becomes fully aligned with flow.