Small-angle X-ray scattering study of a poly(oxyphenylethylene)-poly(oxyethylene) diblock copolymer gel under shear flow

The structure and flow behaviour of a micellar ''cubic" phase is studied, u sing small-angle X-ray scattering (SAXS) and constant stress rheometry on a poly(oxyphenylethylene)-poly(oxy-ethylene) diblock copolymer in water. The predominant structure is a face-centred cubic (fcc) array of spherical mic elles, which under shear undergoes layer sliding to give a scattering patte rn from stacked hexagonal close-packed layers. A detailed analysis of the S AXS data indicates the presence of a fraction of grains with a structure di storted from a fee phase. The additional reflections that characterize this structure can be indexed to a rhumbohrdral unit cell, space group R (3) ov er barm, with the same volume as the fee unit cell. The rhombohedral unit c ell corresponds to a cubic cell that has been "stretched" along a [111] dir ection, and it is suggested that such a structure results from the gradient in shear velocity in the Couette cell employed. Shearing at high sheer rat es leads to a "smearing out" of the reflections, but upon cessation of shea r under these conditions a highly oriented SAXS pattern is obtained, which confirms the persistence of rhombohedral ordering. The shear-induced change s in orientation are correlated to a plateau observed in the stress plotted against shear rate, such a plateau being a sign of inhomogeneous flow.