RHEOLOGY, FLOW INSTABILITIES, AND SHEAR-INDUCED DIFFUSION IN POLYSTYRENE SOLUTIONS

All three viscometric functions, namely the shear viscosity and the first and second normal stress coefficients, are measured for several solutions made from nearly monodisperse polystyrenes. The molecular weights and concentrations of the solution components were chosen so that molecular theories could be tested in three different regimes: dilute, semidilute entangled, and concentrated unentangled. We find for dilute solutions that the dependence of the first normal stress difference on dimensionless shear rate, or Weissenberg number Wi, is similar to the prediction of beads-and-springs theories and that PSI, the negative of the ratio of the second to the first normal stress difference, is near zero, as predicted. For semidilute solutions, PSI decreases with increased Wi in qualitative agreement with the Doi-Edwards theory for entangled solutions. Surprisingly, PSI for the unentangled concentrated solutions is 0.10-0.20, in strong disagreement with the Rouse theory, which predicts PSI = 0. In many of these solutions at suitable Wi and temperature, we observe three different kinds of time-dependent phenomena. The first of these phenomena bears the earmarks of an elastic secondary flow and occurs at large Wi for PSI near zero, in agreement with theory. The second phenomenon, a second overshoot in normal stress, is consistent with shear-induced phase separation. And the third, a very gradual decrease in normal stress, is as yet unexplained.