For polymer materials, the viscosity depends on the rate of deformation. Although viscosity is time dependent until it reaches the steady state, which is a constant value for constant rate of deformation, very often it is only the steady state viscosity that is shown.
The steady state viscosity, as a function of the rate of deformation, can be described by the very general Power-law model, a general viscous fluid model.
Generally speaking, the steady state shear viscosity of polymer melts have a shear thinning behaviour.