Determination of the onset of shear thinning of polydimethylsiloxane

Abstract

AbstractThe 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 start‐up 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. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008