Linear viscoelasticity of incompatible polymer blends in the melt in relation with interfacial properties

A quite general characteristic of the rheology of incompatible polymer blends in the melt is their highly elastic behaviour at low fiequencies, corresponding to long-time relaxation processes. For emulsions of Newtonian liquids, this property can be readily connected to interfacial tension α : in a macroscopic flow, suspended droplets of radius R are subjected on the one hand to a viscous drag related to the viscosity µm of the matrix liquid and tending to deform their shape, and on the other hand to an elastic force of the order of α/R opposing the deformation. From a rheological point of view, the emulsion shows viscoelastic behaviour with characteristic relaxation times of the order of µmR/α. Blends of incompatible uncrosslinked polymers in the molten state can also be considered as emulsions, but the behaviour of the phases becomes viscoelastic by itself. A recent model, which accounts for the viscoelasticity of the phases, the polydispersity in size of the droplets and their hydrodynamic interactions, allowed us to explain the linear viscoelasticity of some selected polymer blends.