Rheology of molten polyolefin interfaces: Slip in shear, hardening in extension

While barrier, optical, dielectric, and mechanical properties of multilayer polymer films have been studied extensively, there is comparatively little regarding the melt rheology of these multilayer films that would inform secondary processes such as thermoforming and biaxial orientation. Here we expand on our previous work regarding polyethylene/polypropylene (PE/iPP) solid-state adhesion to study the molten interface of 640 layer PE/iPP films. The interfacial tension of a metallocene linear low density polyethylene (mlE) and metallocene iPP (miP) system was measured by blending miP into mlE. The small amplitude oscillatory shear (SAOS) data was fitted with the Palierne model to extract an interfacial tension. Interfacial slip of the multilayer mlE/miP system was observed at shear stresses greater than 10 kPa. While neither mlE or miP homopolymer exhibited strain hardening behavior, the 640 layer mlE/miP system possessed a higher plateau extensional viscosity than anticipated as well as pronounced strain hardening behavior. These results suggest the molten interface has a significant impact in the secondary processing of extruded polyolefin films and may be an avenue to enhance thermoformability of iPP films.