Slip at the Interface between Immiscible Polymer Melts II: Capillary Flow of Polymers with Unequal Viscosities

Abstract

Quantitative investigations of polymer/polymer interfacial slip are typically performed using rheological and optical techniques. Almost all of the investigations that use only rheological data to determine the slip velocity at a polymer/ polymer interface are restricted to drag flows of multilayers where the parallel layers are stacked on top of each other and sandwiched between parallel plates. In these investigations, the polymer/polymer interfacial slip velocities were estimated using the reduction in the measured viscosity when compared to the viscosity estimated by assuming that slip at the interface between the polymers does not exist (“no-slip”). Assuming a stable, essentially flat interface, the no-slip viscosity of the parallel multilayers undergoing drag flow can be derived and is equal to the volume fraction weighted harmonic mean of the viscosities of the individual polymers. Crucially, the derivation of the no-slip viscosity relies on the fact that the stress distribution across the multilayer sample is uniform. While this estimate for the no-slip viscosity is appropriate for this flow geometry, devising such estimates for other flows relevant for polymer processing (such as pressure driven flows) is not straightforward. In particular, the harmonic mixing rule 4-6) cannot be applied to the case of coextrusion since the shear stress distribution is not spatially uniform within the multilayer sample. Owing to this methodological limitation, investigations of polymer/polymer interfacial slip during coextrusion of polymers with unequal shear rate dependent viscosities using only rheological data are few. Therefore, systematic quantitative investigations of the effect of molecular weight and temperature on polymer/polymer interfacial slip during coextrusion have thus far not been possible. In order to overcome the above mentioned difficulty, we recently proposed a novel approach to estimate the slip velocity at a polymer/polymer interface during pressuredriven two-phase coaxial flow. The approach uses an adaptation of the Mooney method, which is usually used to investigate slip of a polymer liquid at a solid wall. Henceforth, we will refer to this new approach as the “modified Mooney method”. Conceptually, the main advantage of the modified Mooney method is that estimations of the values of the physical variables (such as the viscosity) under the assumption of no-slip are not necessary. Hence, at least in principle, the method can be used to determine the slip velocity at a polymer/polymer interface for two-phase coresheath samples undergoing pressure-driven coaxial flow even when the viscosities of the two coextruded polymers are unequal. Whether this can be accomplished in practice is yet be determined. In our earlier work, we established the validity of the modified Mooney method by comparing its results to that obtained using the deviation from no-slip method. In order to facilitate the determination of the slip velocity using the deviation from no-slip, we chose samples with identical shear-rate dependent viscosities over a range of shear rates. Consequently, in that work, the modified Mooney method was used only with polymers with almost identical viscosities. As mentioned earlier, the main advantage of the modified Mooney method is that it can be applied to investigate polymer/polymer interfacial slip even when polymers with Slip at the Interface between Immiscible Polymer Melts II: Capillary Flow of Polymers with Unequal Viscosities