Damping of acoustic waves in dilute polymer solutions

TL;DR

This paper investigates the damping of acoustic waves in dilute polymer solutions, highlighting the roles of shear and longitudinal viscosities and their dependence on solvent conditions and pressure effects.

Contribution

It introduces a model for the longitudinal viscosity eta_l as a sum of shear viscosity and pressure-induced effects, emphasizing pressure's influence near the theta point.

Findings

eta_p is comparable to eta_s in good solvents

eta_p dominates eta_s near the theta point

pressure effects significantly influence coil dynamics

Abstract

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.