Non-linear rheology of a face-centred cubic phase in a diblock copolymer gel

The viscoelastic behaviour of a poly(oxyethylene)-poly(oxybutylene) diblock copolymer in aqueous solution forming a face-centred cubic (fcc) micellar phase has been investigated using oscillatory shear rheometry. With increas ing strain amplitude, the micellar solution was observed to undergo a trans ition from linear to non-linear behaviour, characterized by strong shear th inning. The non-linear behaviour observed in the stress response was analyz ed by Fourier transformation of the waveform. Fourier analysis revealed tha t the high harmonic contributions to the shear stress response increased wi th strain amplitude and up to the 81st harmonic was observed for very large amplitudes. The onset of non-linear response as defined from the dependenc e of isochronal dynamic shear moduli on strain amplitude was found to be in good agreement with that defined by the appearance of a higher harmonic in the stress waveform. The amplitudes of the harmonic coefficients are compa red to the predictions of a model for the nonlinear rheological response of a lyotropic cubic mesophase based on the stress response to a periodic lat tice potential (Jones and McLeish 1995). It is found that the model is able to account for qualitative trends in the data such as the development of f inite higher harmonics with increasing strain, but it does not describe the full frequency and strain dependence of these coefficients.