Determination of the onset of shear thinning of polydimethylsiloxane

Abstract

ABSTRACT:The shear stress and the first normal stress difference were simultaneously measured in shear flow start-up experiments and a subsequent stress relaxation. The measured polydimethylsiloxane show shear thinning flow behavior with a first Newtonian region up to the shear rate at the onset of shear thinning. We found that the shear stress and the first normal stress difference have an equal shear rate at the onset of shear thinning. The shear stress at the onset of shear thinning does not depend on the molecular weight of the polymer and on temperature. Similar to the viscosity master curve, it is possible to create a normal stress master curve, outgoing from the first normal stress coefficient. The shear rate at the onset of shear thinning is estimated from the viscosity master curve and the shear rate at the onset of normal stress thinning from the normal stress master curve. The shear stress and the first normal stress difference have similar transient behavior with startup stress curves without maximum in stress growth experiments with shear rates from the first Newtonian region. The shear flow start-up experiment with the shear rate at the onset of shear thinning leads to the first indication of an overshoot of the start-up curve. The shear flow start-up experiments with shear rates from the shear thinning region show start-up curves with a maximum for the shear stress and for the first normal stress difference. It was found that the shear rate at the onset of shear thinning and the shear rate at the onset of normal stress thinning, determined by the shear flow start-up measurements, are equal to the values obtained from the master curves.

Model suspensions with rheological additives: Systems with plastic flow behavior

“…This silicone oil shows shear thinning flow behavior with zero-shear viscosity of 20 Pa s at 258C. The shear thinning region begins at 70 s 21 [35].…”

“…After the cessation of the shear flow, independent of the shear rate of the previous stress growth experiment, there is no structure available to oppose against the elastic springing of the strip bar. Consequently, the value of the residual stress is zero [35].…”

“…Polydimethylsiloxane exhibits start-up curves with or without a maximum [35] depending on the shear rate of the stress growth experiments. After the cessation of the shear flow, independent of the shear rate of the previous stress growth experiment, there is no structure available to oppose against the elastic springing of the strip bar.…”

Determination of the Yield Stress of Disperse Systems

“…Consequently, we can extend the definition of the yield stress to include all rheological systems-the rheological systems must have a three-dimensional network at low shear rates, but it is not necessary to be an elastic solid. The yield stress can be determined from the region of the flow curve, where the shear stress does not change significantly with the increase of the shear rate [19,[21][22][23][24][25][26][27] by extrapolation of the shear stress to the y-axis.…”

“…shear stress [19] and a three-dimensional (3D)-structure. They have consequently only a pseudo yield stress, which may be use for practical purposes [13].…”