On imposing a constant shear rate over time on a polymer melt, the transient shear viscosity will increase, show an overshoot, and eventually reach a steady state value. The figure on the left shows this for various constant shear rates.
The figure on the right shows the steady state viscosity curve as a function of the shear rate.
In shear, the polymer chains will be oriented in the flow direction. Due to entanglements, the molecules will also be stretched. While the polymer molecules will be oriented further in the flow direction, it will become easier for them to slip and entanglements will disappear, resulting in a decrease of the stretch and a relaxation of stress. As a result, also the viscosity will decrease.
The first normal stress difference shows the same behaviour as the shear viscosity.