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Summary: Fourier‐Transform rheology (FT rheology) was used to study the influence of the degree of branching on the nonlinear relaxation behaviour of polystyrene solutions. The results were compared with those obtained under oscillatory shear and step‐shear conditions. The different topologies could be distinguished using FT rheology where the other rheological measurements failed. Significant differences occurred under large amplitude oscillatory shear (LAOS) conditions as particularly reflected in the phase difference of the third harmonic, Φ3, which could be related to strain‐softening and strain‐hardening behaviour. Currently, this work is extended towards different topologies in polyolefins (e.g. long chain branched).Phase difference Φ3 as a function of the Deborah number De at γ0 = 2 for the polystyrene solutions measured at temperatures from 295 to 350.5 K.magnified imagePhase difference Φ3 as a function of the Deborah number De at γ0 = 2 for the polystyrene solutions measured at temperatures from 295 to 350.5 K.

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Large amplitude oscillatory shear (LAOS) was applied to a linear polymer melt in order to study the mechanical harmonic generation in the nonlinear regime. In the nonlinear regime mechanical harmonics at 3w1, 5w1, etc. are generated under oscillatory shear with a shear frequency w1. These higher harmonics can be analysed with respect to frequencies, amplitudes and phase if the time data of the torque is Fourier transformed. This experimental method (FT-Rheology) permits therefore to quantify the mechanical nonlinearities if sinusoidal shear rates are applied. This article describes the basic idea of this experimental method as well as the application to different anionically polymerized polystyrene melts. The dependence of the nonlinear, rheological properties with respect to the molecular weight Mn, the applied strain g0, the frequency w1 and the temperature was investigated. In addition to simple nonlinear theories we could also observe even harmonics at 2w1, 4w1, etc. in the FT-rheology spectra. The appearance of even harmonics was correlated with the appearance of a more complex nonlinear behaviour.