Nonlinear shear rheology of polystyrene melt with narrow molecular weight distribution—Experiment and theory

Measurements of the shear viscosity and the first and second normal stress coefficients are shown at 175 °C for a nearly monodisperse polystyrene melt with M w =200 kg/mol (PS 200 k). Tests are performed on a cone-partitioned plate shear rheometer and cover a range of Weissenberg numbers (τ d γ̇) from 0.13 to 40. Experimental problems encountered are the axial compliance of the rheometer and the normal force capacity of the transducer. The later limits the maximum shear rate to τ d γ̇=40. Experimental data are compared with the models of Öttinger [termed thermodynamically consistent reptation model (TCR), Öttinger, H. C., J. Rheol. 43, 1461–1493 (1999)] for the convective constraint release parameter δ 2 =0, 1, and 2 and Mead, Larson, and Doi [termed Mead, Larson, and Doi (MLD), Mead, D. W., R. G. Larson, and M. Doi, Macromolecules 31, 7895–7914 (1998)] for δ 2 =1. The steady state and transient values of p 21 , N 2 , and N 1 agree qualitatively well between both models and the experiment. The predicted normal stress ratio −N 2 /N 1 is sensitive to the magnitude of δ 2 in the TCR model, similar to the extinction angle. The MLD model yields |N 2 | and Ψ values lower than both experiments and the TCR model with δ 2 =1. From a comparison with the chain stretch time τ s (0.065 s) it can be shown that the overshoot O of |N 2 | and p 21 are linked to chain orientation, whereas O(N 1 ) is associated with chain stretching. The magnitude of the overshoot for all shear rates increases as O(N 1 )