Polyethylene Melt Viscosity: Shear Rate-Temperature Superposition

The steady-state melt viscosities of both high density and low density polyethylene have been investigated for a series of samples over four decades of shear rate and a range of temperatures from 120°C (150°C for high density samples) to 300°C. In the case of low density polyethylene a broad range of molecular weight distributions was covered. Melt viscosity measurements were made using a capillary extrusion rheometer and a capillary with L/D=66.7 for which it was demonstrated that end-corrections were negligible. Using arbitrary reference temperatures, a superposition-temperature method was found to be applicable over the entire region of shear rates and of temperatures. A single set of superposition shift factors (for a given reference temperature) was found to be general for all low density polyethylene samples regardless of molecular weight distribution, and a second set was applicable to all of the high density samples. The dependence of the shift factors on temperature was described by a simple exponential, or Arrhenius, equation with activation energies of 11.3 kcal/mole and 6.3 kcal/mole for low density and high density polyethylene, respectively.