Inhomogeneous flow in a micellar solution of a diblock copolymer: creep rheometry experiments

The flow behaviour of a solution of a poly(oxyethylene)-poly(oxybutylene) diblock copolymer in water forming a body-centred cubic micellar phase is studied via creep rheometry. This indicates a plateau in a plot of stress versus shear rate, suggesting an inhomogeneous flow mechanism, possibly shear banding. Creep recovery measurements are performed as a function of stress, which reveals partial recovery at low stress but no recovery and purely viscous flow at large stress. The creep response of as-mounted (unoriented) and pre-sheared (oriented) samples is compared. The flow behaviour is identical for stresses above the plateau value, but differs at lower stresses due to incomplete sample alignment. At low stresses, shear thinning is observed for samples pre-oriented by shear at a stress above the plateau value. However, shear thickening is observed in creep curves at small stresses for samples pre-oriented at the same stress, suggesting micellar association occurs under these conditions. Small-angle X-ray scattering reveals that flow in the plateau region is in {110} planes oriented normal to the neutral direction with a close-packed 111 direction along the shear direction. However, at higher shear rates flow occurs in {110} and {211} planes oriented in the shear plane. The proportion of flow in the latter planes is observed to increase with shear rate, even though outside the plateau regime, suggesting that the high shear rate branch of the flow curve does not correspond to a unique orientation state of the bcc structure.