SHEAR THINNING AND POLYMER DEFORMATION IN LARGE FLOW-FIELDS

We theoretically investigate polymer deformation and shear thinning, i .e., a decrease of intrinsic viscosity, in a dilute polymer solution a s a function of the applied shear rate (gamma) over dot. We use a bead -and-spring model with hydrodynamic interaction in the Rouse-Zimm fram ework, approximately accounting also for excluded-volume effects, and impose a constraint on the average mean-square spring length to preven t its stretching at large (gamma) over dot. When suitably normalized, both the intrinsic viscosity [eta] and the components of the mean gyra tion tensor [SS] depend on the single variable xi = (gamma) over dot t au(1)((0))/N1-upsilon, where tau(1)((0)) is the longest relaxation tim e for (gamma) over dot = 0, N is the number of chain springs and upsil on is the Flory exponent. The full shear-rate dependence is obtained n umerically, and compared with analytical results obtained under free-d raining conditions both for low and for very large shear rates. The sh ortcomings of the theory are also discussed, in particular a substanti al stretching under shear of the central springs, where the intramolec ular tension is largest, with a corresponding strong contraction of th e end springs due to the average character of the constraint.