Nonlinear Shear Rheology of Unentangled Polymer Melts

In the present work, we investigate the nonlinear shear rheology of unentangled polymer melts. We use linear polystyrenes with molar mass of 10, 20, or 30 kg/mol. The measurements of shear and normal stress are performed using cone-and-partitioned-plate rheometry. While the linear viscoelastic response is consistent with predictions of the Rouse model, in the nonlinear regime, the Cox–Merz rule is not fully validated, despite a universal thinning exponent of −0.5. These experimental results are analyzed using the recent shear slit model of Parisi et al. and molecular dynamics simulations. A new molecular picture is proposed to explain the origin of the transient stress overshoot, based on the concept of the advection time that marks the transition between affine and nonaffine deformation. Finally, a simple model is developed, by combining Rouse relaxation modes, chain confinement to shear slit in the velocity gradient direction and tension blobs in the velocity direction. The predictions of this model for shear viscosity are in excellent agreement with the experimental data and consistent with simulations.