Neutron Reflectivity Measurements of Molecular Weight Effects on Polymer Mobility near the Polymer/Solid Interface
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Slip at Molten Polymer-Polymer Interfaces
Rui Zhao, Christopher W. Macosko Department of Chemical Engineering and Materials Science University of Minnesota 421 Washington Av. S.E. Minneapolis, MN55455, USA
ABSTRACT A number of researchers have reported an 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 well-defined 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 measured in a capillary rheometer and a rotational shear rheometer using parallel 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 of layers. From these results interfacial slip viscosity and velocity were estimated. Addition of diblock copolymer was able to suppress interfacial slip. INTRODUCTION Since the 1970’s, there have been reports of anomalously low viscosity for immiscible polymer blends. In some systems, they are even lower than either of the components. Most of these studies have been with capillary rheometer at higher shear rates and have often been reported as part of blend processing studies in technological journals. No fundamental investigation of this anomaly appears available. Here we report shear studies on well characterized blends and layered composites of the same polymers. Our results show that the anomalously low viscosity of blends is likely due to interfacial slip. Ultracki reviewed blends which showed anomalously low viscosity[1,2], as summarized in Table I. The maximum viscosity deviation is calculated as η -η dev. = ex av × 100 (1) ηav where ηex is measured blend viscosity, and ηav is calculated blend viscosity using log-additive rule Log ηav = φ1 Log η1 + φ2 Log η2 (2)
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where η1 and η2 are the viscosities of the components. The Flory-Huggins interaction parameter, χ, is also calculated using either interfacial tension measurement or solubility parameters[11-13], and also listed in the table. It appears that systems which are more incompatible (large χ) show larger viscosity deviation. Table I Blends
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