  Contrary to some expectations, an experimental finding for a polymer that the
solution intrinsic viscosity $[\eta]$ or the melt viscosity is linear in the
polymer molecular weight $M$ does not indicate that polymer dynamics are
Rouselike. Why? The other major polymer dynamic model, due to Kirkwood and
Riseman [\emph{J. Chem.\ Phys.\ } \textbf{16}, 565-573 (1948)], leads in its
free-draining form to a prediction $[\eta] \sim M$, even though the polymer
motions in this model are totally unlike the polymer motions in the Rouse
model. In the Rouse model, the chain motions are linear translation and
internal ('Rouse') modes. In the Kirkwood-Riseman model (and its free-draining
form, derived here), the chain motions are translation and whole-body rotation.
The difference arises because Rouse's calculation implicitly refers only to
chains subject to zero external shear force (And, as an aside, Rouse's
construction of $[\eta]$ is invalid, because it concludes that there is viscous
dissipation in a system that Rouse implicitly assumed to have no applied
shear).
