Transient stresses in immiscible model polymer blends during start-up flows

The transient rheological behaviour of immiscible polymer blends is studied during start-up flows. The stresses can be divided in a contribution from the component polymers and a contribution caused by the presence of an inte rface. By means of relaxation measurements, the component stresses after in ception of flow can be determined. In this manner, the different contributi ons to the stress can be compared separately with model predictions. For th e contribution of the components, one normally uses mixing rules that do no t take into account the morphology of the blend. As expected, a slight morp hology effect can be detected. The stress contributions from the interface are calculated combining the Doi-Ohta approach with different models that d escribe the droplet deformation. For flows at small capillary numbers, the Maffettone-Minale model leads to accurate stress predictions. With larger c apillary numbers, the normal stress profiles become more complex. The initi al part is rather well described by the model of Almusallam et al. [J. Reel . 44 (2000) 1055]. It is demonstrated that the morphological changes in the blend can be deduced in quite some derail from the measured stress profile s. (C) 2001 Elsevier Science B.V. All rights reserved.