Slip at polymer–polymer interfaces: Rheological measurements on coextruded multilayers

Abstract

Polypropylene (PP) and polystyrene (PS), with closely matched viscosity, were coextruded into 8, 32, and 64 alternating layers. The apparent steady shear viscosity of these multilayer samples was measured with an in-line slit rheometer and with a parallel-plate rheometer. In both cases the apparent viscosity decreased with the number of layers providing strong evidence for interfacial slip. The velocity difference across the interface (interfacial slip velocity) versus shear stress, ΔVI(τ) was calculated from the apparent viscosity measurements. ΔVI(τ) showed sigmoidal behavior: a region of very low slip (< 10−3 μm) at low shear stress, a strongly increasing region at τ > 103 Pa followed by a linear region ΔVI=τ/β∞. These data could be fit with a modified Ellis model. The same function fit the different number of layers and both slit and parallel-plate data indicating ΔVI(τ) is a material property of the PP/PS pair. Slip was also observed in PS/PMMA (polymethyl methacrylate) and PP/aPA (amorphous nylon) pairs. Addition of a premade P(S-b-EE) diblock copolymer to the PP/PS pair was able to suppress the interfacial slip but only after enough time for sufficient block copolymers to diffuse to the interfaces. The in situ formed graft copolymer of maleated PP with aPA at the interface of the PP/aPA pair directly suppressed the slip. Our results are in qualitative agreement with the theory of Furukawa (1989) and de Gennes (1992), which predicts fewer chain entanglements near the interface between two immiscible polymers, and thus a narrow region (∼ interfacial width) with lower viscosity. This lower viscosity is the cause of apparent interfacial slip. Goveas and Fredrickson (1998) extended these ideas to calculate the slip in a symmetric bilayer where both bulk and interface are Newtonian. Their results are in qualitative agreement with our observation but predict a stronger slip. Our results also indicate that the anomalously low viscosity reported by a number of researchers for immiscible polymer blends is due to interfacial slip. Blends of PP and PS show similar negative viscosity deviations to those reported but only at τ > 2000 Pa. No negative deviations were found for linear viscoelastic (low-stress) measurements.