Two-dimensional Fourier transform rheology

A two-dimensional Fourier transformation (FT) theology experiment is presented that separates the relaxation dynamics of different contributions to the stress relaxation, here specifically for entangled polymers. This experiment has the overall form of discrete large amplitude oscillations and consists of multiple step shear experiments. It is called large amplitude step shear oscillations, LASSO. The time-dependent nonlinear material response follows the discrete periodic excitation and can therefore be Fourier transformed, which results in a spectrum of harmonics for each delay time between the steps. The FT is used here to correlate the different step shear experiments. The method was applied to a slightly entangled, polydisperse polyisobutylene solution where a small deviation of the time strain separation is detected, even at times exceeding the Rouse time by orders of magnitude. On the other hand, the time temperature superposition seems to work for all the harmonic decays within the spectrum. When the time window between the shear steps is reduced such that slow relaxation processes still possess memory of prior steps, an increase of the nonlinear contributions of the fast, completely relaxed, relaxation modes is observed. This is in qualitative agreement with reptation models where polymer stretching and orienting are considered coupled processes. (C) 2001 The Society of Rheology.