Behavior of concentrated polystyrene solutions in large‐amplitude oscillating shear fields

Abstract

AbstractThe limits of linear viscoelastic behavior of polystyrene solutions have been investigated by subjecting them to large‐amplitude oscillatory strains, γ0. At strains less than one we find that the dynamic storage modulus G′(ω,γ0) and the dynamic loss modulus G″(ω,γ0) decrease quadratically with increasing strain. As a measure of the size of the linear viscoelastic region, we have determined the strain at which the moduli have fallen 5% below their zero‐strain values. This strain, γNL, is found to be independent of frequency ω at high frequencies but to increase with decreasing frequency at low frequencies. Behavior of this type is in qualitative agreement with the recent constitutive equation developed by Doi and Edwards. More specifically, we find that the rate of decrease of the storage modulus with increasing strain is quite similar to that predicted by their theory, but that the rate of decrease of the loss modulus is much slower. Some possible approaches for improving the agreement with experimental results are suggested. In the course of our work an interesting hydrodynamic instability was observed. The nature of this instability and methods to avoid it are discussed.