Hydrodynamics of semidilute polymer solutions

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Abstract
Hydrodynamic equations for semidilute polymer-solvent solutions are derived for both Rouse and reptation dynamics, with emphasis on the coupling between concentration fluctuations and polymer elastic stress. For the Rouse model, we project microscopic Fokker-Planck equations down to the hydrodynamic variables plus a long-lived strain variable. Also, we construct a more general two-fluid model of polymer plus solvent which shows how elastic stresses may cause diffusion currents. One important consequence of this Letter is the extension of work by Helfand and Fredrickson on Rouse solutions in weak shear to the more relevant entangled case.