Meaning
Empirical observations indicate that the magnitude of complex viscosity at a specific frequency equals the steady shear viscosity at the same shear rate. Predictions of flow behavior from dynamic oscillation data are possible because the cox-merz rule links these two values. This relationship holds for most linear homopolymers and simple blends but often fails when long chain branching or high levels of filler are present.
Theoretical Basis
Molecular alignment under steady flow matches the response of the polymer chains under small strain oscillation. This alignment suggests that the same molecular mechanisms govern both steady state and dynamic deformation.
Experimental Application
Rheometer operators extend the effective shear rate range of measurements without encountering flow instabilities like edge fracture. By applying the cox-merz rule during a frequency sweep, the lab generates a viscosity curve that covers several decades of rate. This data helps moulders predict the pressure drop inside a die where shear rates are high.
The rule simplifies the characterization of new resin lots by reducing the number of separate tests required to build a full rheological profile.
Practical Limitation
Discrepancies between steady and dynamic data often signal the presence of complex microstructures within the melt. While the cox-merz rule provides a reliable baseline for virgin polyolefins, it typically overestimates the viscosity of materials containing cross-linked particles or liquid crystal polymers.
