Damping of acoustic waves in dilute polymer solutions

The shear viscosity eta_s(w) of dilute polymer solutions (of flexible coils) has been classically measured and interpreted in terms of the Zimm modes. We point out that the longitudinal viscosity eta_l(w) is the sum of two components eta_l=4*eta_s/3+eta_p, where eta_p is due to an effect of pressure on the coil size. We discuss eta_p for a crude model involving a dependence of both the Kuhn length a and the Flory parameter chi on pressure. We find that eta_p is comparable to eta_s for good solvent conditions, but that eta_p should dominate over eta_s near the theta point.