  In order to address the selection mechanism that is responsible for the
unique lamellar orientation observed in block copolymers under oscillatory
shears, we use a constitutive law for the dissipative part of the stress tensor
that respects the uniaxial symmetry of a lamellar phase. An interface
separating two domains oriented parallel and perpendicular to the shear is
shown to be hydrodynamically unstable, a situation analogous to the thin layer
instability of stratified fluids under shear. The resulting secondary flows
break the degeneracy between parallel and perpendicular lamellar orientation,
leading to a preferred perpendicular orientation in certain ranges of
parameters of the polymer and of the shear.
