  We study the dynamics of the phase behavior of a polymer blend in the
presence of shear flow. By adopting a two fluid picture and using a
generalization of the concept of material derivative, we construct kinetic
equations that describe the phase behavior of polymer blends in the presence of
external flow. A phenomenological form for the shear modulus for the blend is
proposed. The study indicates that a nonlinear dependence of the shear modulus
of the blend on the volume fraction of one of the species is crucial for a
shift in the stability line to be induced by shear flow.
