Comparison of eccentric rotating disk and oscillatory measurements of dynamic moduli in polymer liquids

Measurements to obtain the dynamic moduli G′(ω) and G″(ω) for several polymer liquids were made with the Rheometrics mechanical spectrometer using the eccentric rotating disk (ERD) and the forced oscillation methods. The ERD data were corrected for instrument compliance with the Macosko‐Davis formula. Oscillatory results were confirmed in several separate studies including testing of a Newtonian liquid and an elastic solid. Agreement between the two geometries on G′(ω) and G″(ω) was generally good (±5 percent) for |G*| less than 2 × 10⁶ dyne/cm². If no compliance correction was applied, departures occurred near a |G*| an order of magnitude lower, indicating strongly the need for applying a compliance correction to ERD data for most polymer systems. However, as |^G*| approached 10⁷ dyne/cm² (where compliance corrections of 50 percent or greater occurred), the two geometries yielded results which differed by 20 percent or more. No single variable was found which correlated the departures for different liquids. Nevertheless, the departures were such that values obtained in the two geometries for the plateau modulus of polyethylene, G N0 ≈ 2 × 10⁷ dyne/cm², agreed within about 10 percent.