A comparison of torsional and capillary rheometry for polymer melts: The Cox‐Merz rule revisited

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Abstract
Rheological properties of high and low density polyethylene melts (HOPE and LDPE) were measured under two different deformation modes: sinusoidal strain (“dynamic”) and constant rate‐of‐strain (“steady‐state”). For the sinusoidal measurements, a torsional geometry was used, whereas the “steady‐state” measurements were carried out in both torsional and capillary flow. The complex and “viscous” components of the “dynamic” viscosity were compared with the “steady‐state” viscosities, corrected for both non‐Newtonian and end effects. For the HDPEs, the Cox‐Merz rule was found to be not valid. The corrected viscosity is closer in magnitude to the “viscous” component, and not to the complex viscosity. However, for the LDPEs, the corrected viscosity is equivalent to the complex viscosity. These results are compared to other correlations found by various authors.