IN-SITU SMALL-ANGLE NEUTRON-SCATTERING FROM A BLOCK-COPOLYMER SOLUTION UNDER SHEAR

Small-angle neutron scattering profiles from a concentrated (65 wt %) polystyrene-polyisoprene block copolymer solution in dioctyl phthalate under shear were obtained both above and below the quiescent order-disorder transition (ODT). The ordered solution has a lamellar structure under quiescent conditions. The shear-induced structure was inferred from scattering measurements in two planes: the v-vxdelv plane and the delv-vxdelv plane (v is the fluid velocity direction and delv is the velocity gradient direction). Below the quiescent ODT, oscillatory shear produces lamellae that are aligned along the shearing surface, while steady shear results in a reorientation of the lamellae normals from the v-vxdelv plane to the delv-vxdelv plane. Above the quiescent ODT, steady shear induces order above a critical shear rate. The resulting scattering anisotropy obtained at different temperatures obeys a universal scaling law, and the critical shear rate increases exponentially with temperature.