Monodisperse polymer melts clarify rheology in complex flows in a similar way to that in viscometry. In simple rheological response, sharp monodispersity in molecular structure gives rise to a clear "reptation" peak in the loss modulus, and sharp stress overshoots in strong shear transients. These features are universal among entangled polymers when compared at equal degrees of entanglement and entanglement time scale. The corresponding features in the complex constriction flow described here include the highly symmetric stress fields seen in all nonstretching flows, even highly nonlinear ones, and the appearance of very characteristic features in both inflow and outflow when chains become stretched. The advantage of monodispersity at the molecular level is then understood as the separation of time scales for orientation relaxation (reptation) and stretch relaxation (Rouse), and the absence of the mixed states of chains that will naturally arise in a polydisperse blend. Some of these features occur only in the transient flow and not in steady state. The tube model constitutive equation captures these quantitatively, but the time at which they occur in the transient is affected (initially surprisingly) by upstream compressibility of the melt reservoir. Simulations that treat only isolated portions of the flow need to be presented with time-modified boundary conditions to correspond to real experiments.

The second advantage (for a scientific study, not for a real process) of very narrow polydispersity is the amplification of flow instabilities that break lateral symmetry. Both experiment and simulation indicate that strongly nonlinear flows are prone, especially in the outflows, to oscillations that direct the centerline alternately to one side, then the other, at a frequency that is set by the characteristic deformation rate in the channel.