Morphological evolution of immiscible polymer blends in simple shear and elongational flows

Abstract

The evolution of the morphology of the dispersed phase of immiscible polymer blends is studied in both shear and extensional flows. In the simple shear flow, predictions of the Lee and Park [H.M. Lee, O.O. Park, Rheology and dynamics of immiscible polymer blends, J. Rheol. 38 (1994) 1405–1425] and of the modified Lee and Park as well as the modified Grmela and Ait-Kadi [M. Grmela, A. Ait-Kadi, Comments on the Doi–Ohta theory of blends, J. Non-Newtonian Fluid Mech. 55 (1994) 191–195] models ([C. Lacroix, M. Grmela, P.J. Carreau, Relationships between rheology and morphology for immiscible molten blends of polypropylene and ethylene copolymers under shear flow, J. Rheol. 42 (1998) 41–62] are compared to stress growth data. The size of the dispersed phase is strongly affected by moderately finite strain imposed during the stress growth experiments. The morphological changes after stress relaxation are well predicted by the models for a polypropylene (PP), ethylene vinylacetate (EVA) and ethylene methylacrylate (EMA) blend [C. Lacroix, M. Grmela, P.J. Carreau, Relationships between rheology and morphology for immiscible molten blends of polypropylene and ethylene copolymers under shear flow, J. Rheol. 42 (1998) 41–62]. The prediction is less satisfactory for a polystyrene (PS)/polyethylene (PE) blend. The morphological evolution for the elongation flows of the PP/EVA/EMA blends has also been investigated. The morphologies of samples extracted before and after extrusion through an hyperbolic shaped (nozzle) die show that the elongational flow induces fibrillar structures. The extensional viscosity data, needed to solve the interface governing equations of the Lee and Park model, have been obtained from entrance pressure drop measurements. These equations are shown to qualitatively describe the transition from a spherical to a fibrillar morphology.