Phase separation of polymer solutions in elongational flow

We have studied the phase separation behavior of polymer solutions in two-dimensional elongational flows that are sufficiently weak to avoid the coil–stretch transition. On the basis of a simple model that neglects hydrodynamic interactions, we find no shifts in the infinite molecular weight θ point, or coexistence curve. For finite chain lengths, there are flow-induced shifts of both quantities. These shifts are calculated to leading order in a loop expansion and are found to be order (Sζ)2N7/2, where S is the extension rate, ζ is the monomer friction coefficient, and N is the degree of polymerization. Our results are in qualitative agreement with experimental observations that find demixing promoted by flow.