Lamellar orientation of diblock copolymer solutions under steady shear flow

The steady shear rate dependence of the structure of two poly(styrene-d₈-b-2-vinylpyridine)s in α-chloronaphthalene, a common good solvent, was measured by small-angle neutron scattering (SANS) in a couette flow cell geometry in the ordered state, in which the solutions assume a lamellar structure (randomly oriented grains). SANS measurements revealed that steady shear flow induces macroscopic lamellar orientation at certain characteristic shear rates for the higher molecular weight sample (M_w = 3.6 × 10⁵). Lamellar normals are preferentially oriented along the vorticity direction at high shear rates. However, no shear effect was observed for the low molecular weight sample (M_w = 2.0 × 10⁵).