Instrument compliance effects revisited: linear viscoelastic measurements

Torsional compliance of the torque transducer can be an important issue in linear viscoelastic (LVE) measurements when the sample stiffness is high relative to the instrument stiffness. We evaluated compliance effects of the ARES 2K-FRT on LVE measurements by systematically comparing the results of the frequency sweep mode obtained with 25 and 8 mm plates, respectively. In addition to the transducer, the test fixtures do contribute significantly to the system compliance. Without correction, the upper limit for the complex modulus ∣ G ^∗ ∣ is approximately close to 4 × 10⁵ Pa at 10% uncertainty, when using 25 mm plates. This limit is lower than the plateau modulus \(\left( {G_N^0 } \right)\) of most polymers. Therefore, instrument compliance can lead to significant errors for \(G_N^0 \) and wrong scaling for G ^″ in the plateau and Rouse regions. The respective roles of transducer and tool compliances are discussed. The FRT transducer compliance is corrected in real time in the instrument firmware. Tool compliance is a common problem for all rheometers when measuring stiff samples.