Stress Relaxation of Suspensions of Powder in Polymer Solutions

Abstract

The relaxation modulus G (t) and the viscosity decay function η(t) after cessation of steady flow were measured for suspensions of Aerosil in polystyrene solution with a coaxial cylinder rheometer. Particle content were from 3 to 5wt%. Measurements were performed at various magnitudes of shear strain γ ranging from 0.046 to 2.4 and at various shear rates γ ranging from 0.024 to 1.21s^-1. The maximum relaxation time τ₁ and the associated relaxation strength G₁ were evaluated from G (t) through the Procedure X. Both τ₁, and G₁ decreased with increasing strain for all the suspensions studied. The Procedure X was also applicable to η(t) of 3wt% suspension, revealing that both τ₁ and η₁ decreased with increasing shear rate. However, it was not applicable to η(t) of 5wt% suspension, because η(t) did not decay to zero value even after very long time. As the behavior of G (t) and of η(t) were different from each other, detailed experiments were carried out. The stress relaxation after various shearing times under a constant shear rate was measured. When the shearing time was short, the stress relaxation function decreased with shearing time. On the other hand, when the suspension was sheared for more than a certain shearing time, the stress relaxation function began to increase with shearing time. And finally after long time shearing, i. e, after cessation of steady flow, it showed residual stress at the long-time region. It is suggested that G (t) and η(t) originate from two different relaxation mechanisms.