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

Abstract

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 Mw=200 kg/mol (PS 200 k). Tests are performed on a cone-partitioned plate shear rheometer and cover a range of Weissenberg numbers (τdγ̇) 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 τdγ̇=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 δ2=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 δ2=1. The steady state and transient values of p21, N2, and N1 agree qualitatively well between both models and the experiment. The predicted normal stress ratio −N2/N1 is sensitive to the magnitude of δ2 in the TCR model, similar to the extinction angle. The MLD model yields |N2| and Ψ values lower than both experiments and the TCR model with δ2=1. From a comparison with the chain stretch time τs (0.065 s) it can be shown that the overshoot O of |N2| and p21 are linked to chain orientation, whereas O(N1) is associated with chain stretching. The magnitude of the overshoot for all shear rates increases as O(N1)<O(N2)<O(η) for PS 200 k. In comparison, a polydisperse polystyrene melt shows stronger shear thinning of −N2/N1 and an increase of the magnitude of the overshoot as O(N1)<O(η)<O(N2).