Dynamic response of block copolymer wormlike micelles to shear flow

Abstract

The linear and nonlinear dynamic response to an oscillatory shear flow of giant wormlikemicelles consisting of Pb–Peo block copolymers is studied by means of Fourier transformrheology. Experiments are performed in the vicinity of the isotropic–nematic phasetransition concentration, where the location of isotropic–nematic phase transition lines isdetermined independently. Strong shear-thinning behaviour is observed due to criticalslowing down of orientational diffusion as a result of the vicinity of the isotropic–nematicspinodal. This severe shear-thinning behaviour is shown to result in gradient shear banding.Time-resolved small-angle neutron scattering experiments are used to obtain an insightinto the microscopic phenomena that underlie the observed rheological response.An equation of motion for the order parameter tensor and an expression of thestress tensor in terms of the order parameter tensor are used to interpret theexperimental data, both in the linear and nonlinear regimes. Scaling of the dynamicbehaviour of the orientational order parameter and the stress is found when criticalslowing down due to the vicinity of the isotropic–nematic spinodal is accountedfor.