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-strectch transition. On the basis of a simple model that neglects hydrodynamic interactions, we find no shifts in the infinite molecular weight theta-point, critical point, or coexistence curve. For finite chain lengths, there are flow-induced shifts of all three quantities. These shifts are calculated to leading order in a loop expansion and are found to be of order (Szeta) sup 2 N sup (7/2), where S is the extension rate, zeta 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.