The crossover between linear and non-linear mechanical behaviour in polymer solutions as detected by Fourier-transform rheology

The application of oscillatory shear strain leads, in the nonlinear regime, to the appearance of higher harmonic contributions in the shear stress res ponse. These contributions can be analyzed as spectra in Fourier space, wit h respect to different frequencies, amplitudes and phase angles. In this ar ticle, we present an application of this new characterization method to a s olution of the linear homopolymer polyisobutylene. The degree of nonlinear response during oscillatory shear is quantified using the normalized intens ity of the third harmonic contribution. We were able to show experimentally on polyisobutylene that there is an immediate onset of the non-linear resp onse even for very small shear strain amplitudes.