  We present the experimental results on a series of high molecular weight,
entangled polystyrene solutions subjected to a ``mixed'' shear and elongational
flow-type generated in a co-rotating two-roll mill. For steady-flows as well as
for start-up of flows from rest, we used two different optical methods to study
the dynamics of entangled polymeric fluids: two-color flow birefringence (TCFB)
and dynamic light scattering (DLS). Using the TCFB method, we measured the
birefringence, $\Delta n$, and the orientation angle, $\chi$, of the optic axes
in the solution and thereby the generalized viscosity function, $\eta$ (with
the use of stress-optical relations). The DLS method was applied to measure the
velocity-gradient, \gdot, and the flow-type parameter $\lambda$ for the polymer
solutions under flow conditions identical to the TCFB measurements. For low
deformation rates the symmetry of the flow-field was reduced with the use of
polymeric fluids compared to that seen with a Newtonian fluid. A molecular
constitutive Doi-Edwards-Marrucci-Grizzuti (DEMG) model which includes polymer
chain-stretching effects, has been used to numerically simulate the predictions
for $\Delta n$, $\chi$, and $\eta$ under steady flow conditions for the
polystyrene fluids with \gdot and $\lambda$, measured via DLS, as inputs to the
model. A detailed comparison of the DEMG model predictions with the
experimental results shows that the model works qualitatively as well as
quantitatively for the low and intermediate deformation rates, but fails at
high rates of deformation by predicting a stronger chain-stretching than
observed experimentally. The effect of polymer molecular-weight and number of
entanglements per chain are highlighted.

Soft Condensed Matter