Intrinsic nonlinearity from LAOStrain-experiments on various strain- and stress-controlled rheometers: a quantitative comparison.

Strain-controlled large amplitude oscillatory shear (LAOStrain) experiments on a polyisoprene melt and a polyisobutylene solution were conducted on four different rheometers. The results are compared using nonlinear quantities such as the normalized intensity of the third harmonic ( I) and the intrinsic nonlinearity in order to assess the reproducibility of the experiments. Two of the investigated instruments were strain-controlled rheometers, another two, were advanced stress-controlled rheometers. Since the stress-controlled rheometers are able to conduct strain-controlled tests when employing an active deformation control loop, the two different rheometer types could be compared. Experimental details like the gain of the deformation control loop, and the method of temperature control have been shown to play crucial roles in achieving reasonable reproducibility across the different instruments. Furthermore, deviations from the quadratic scaling of I with the strain amplitude and the influence of instrument inertia on nonlinear quantities were observed for one of the stress-controlled instruments. The standard deviation of the intrinsic nonlinearity Q( ω) at a specific angular frequency as determined by measurements on the same instrument was found to be 8 % or lower. The relative deviations of Q across different instruments were instead up to 12 % in the investigated frequency range with an exception for a specific instrument and one of the samples, where the deviation was considerably larger. [ABSTRACT FROM AUTHOR]