Comparison of different shear rheometers with regard to creep and creep recovery measurements

The performance of two shear rheometers with regard to creep and creep recovery measurements is investigated. The first one is a commercially available stress-controlled rheometer that uses an air bearing, and the second one is a magnetic bearing torsional creep apparatus that was built at the Institute of Polymer Materials in Erlangen. The creep and creep recovery measurements were performed in the linear-viscoelastic regime of two polyethylene melts at a temperature of 150°C. The creep compliances of the polyethylenes measured by both rheometers are in excellent agreement. The recoverable compliances of both polyethylene melts, however, have lower values in the case of the commercial rheometer than in the case of the magnetic bearing rheometer. The experimental parameters of creep recovery experiments and the features of the two bearings that are responsible for the different results of the two rheometers are discussed. The influence of the level of the applied shear stress on the short time behavior is investigated as well as the residual torque of the bearings which influences the long time region of the recoverable compliances. Also addressed are the influence of the momentum of inertia of the rotor and the bearing friction which is different for the two rheometers.