A number of researchers have reported na anomalous lowering of viscosity in immiscible polymer blends. Slip at the interfaces between the polymers has been proposed to explain these observations. Because of the complex morphology developed in melt blends it is difficult to test the slip hypothesis. However, using layer multiplication dies in coextrusion, two or more polymers can be alternatively combined into hundreds or even thousands of continuous layers generating a large amount of welldifined interfacial area. Polypropylene (PP) and polystyrene (PS) with closely matched viscosity were blended in a twin screw extruder and also coextruded into 2, 32, 128 alternating layers. The steady shear and dynamic shear viscosity of the blends was first measured in a capillary rheometer and a rotational shear rheometer using paralel plates geometry. While the steady shear viscosity of the blends was lower than that of both homopolymers, the dynamic shear viscosity of the blends was the same as that of the homopolymers. The pressure drop of the coextruded multilayer melts through a slit die was lower than that of both homopolymers and decreased with an increase in the number oflayers. From these results an interfacial slip viscosity was estimated. Addition of diblock copolymer was able to suppress interfacial slip.
展开▼
