Flow Properties of Polymer Solutions. III. Non-Newtonian Viscosity and Relaxation Mechanisms with Long Relaxation Times

Abstract

Contributions of relaxation mechanisms with long relaxation times were examined for the shear stress κη(t, κ) after a sudden stop of steady shear flow of a 20-% polystyrene solution in chlorinated biphenyl, where κ is the rate of shear and t is the time of stress decay. The longest relaxation time τm(κ) and the corresponding strength ηm(κ) were evaluated from the slope and the intercept at t=0, respectively, of the asymptotic straight line at large t in the plot of logη(t, κ) vs. t. Those for the second longest relaxation time, τm(κ) and τm-1(κ), were obtained in a similar manner from the plot of vs. t. The relaxation times τm(κ) and τm-1(κ) were found to be independent of κ and the τm-1(κ) ratio τm(κ) was about 3. At the limit of zero rate of shear, ηm(κ) and ηm-1(κ) were approximately 30 and 45%, respectively, of the zero shear viscosity η0. As κ increased, ηm(κ) and ηm-1(κ) decreased more rapidly than the steady shear viscosity η(κ) did; the difference η(κ)−ηm(κ)−ηm-1(κ) was almost independent of κ. It was concluded that the nonlinear behavior, such as the shear-dependent viscosity of the polymer solution, is mainly due to the nonlinear behavior of the few relaxation mechanisms with long relaxation times.