  We present an extensive experimental study of birefringence and
velocity-gradient components for a series of high molecular weight, flexible,
entangled polystyrene solutions subjected to transient start-up flows in a
co-rotating two-mill to several Weissenberg numbers. The time-dependent changes
in the polymer microstructure, as measured by a two-color flow birefringence
technique, is shown to be very distinctly coupled with the transient response
of the corresponding velocity-field, measured by a dynamic light scattering
technique. As expected, polymer deformations induce substantial modifications
from the Newtonian flow-field. As a result, measured values of both
velocity-gradient components as well as flow-type parameter reduce, thereby
drastically decreasing the extensional strength or measured Weissenberg number
at the stagnation-point relative to the velocity-field for a Newtonian fluid.
Although the impact of the dynamics of both polymer on flow and flow on polymer
are very complicated, their individual behaviors and the tight coupling between
them are shown to be invariant to the changes in polymer concentration and
molecular weight, provided that the number of entanglements per chain, $N_e$ is
held fixed. The dependence of the transient results on $N_e$ is very similar to
that found in the steady-state experiments on the same polymeric fluids
(Sanyal, Yavich, and Leal, manuscript in preparation, 2000). The results
presented in this paper are in close qualitative agreement with the recent
theoretical predictions (Remmelgas and Leal, J. Non-Newtonian Fluid Mech. {\bf
90}, 187, 2000) using a reptation-based vector model.
