REFINED CALCULATION OF THE PHASE-SEPARATION BEHAVIOR OF SHEARED POLYMER BLENDS - CLOSED MISCIBILITY GAPS WITHIN 2 RANGES OF SHEAR RATES

The phase diagrams of a blend of two homopolymers exhibiting a lower critical solution temperature (LCST) have been calculated on the basis of the generalized Gibbs energy of mixing (sum of the Gibbs energy of mixing of the stagnant blend and the energy the mixture can store during stationary flow) for various shear rates gamma. In the present paper the temperature dependence of the energy stored in blends was taken into account. The calculations yield a complex dependence: With increasing gamma the heterogeneous region of the phase diagram is first reduced (shear dissolution), then enlarged (shear demixing), and finally reduced again. So, with varying gamma, the influences of shear change sign twice; i.e., two inversions of the effects are observed. For very high gamma values the system behaves like in the quiescent state. Closed miscibility gaps can occur within two ranges of gamma: Within the first range the islands show up below T(c) (the critical temperature of the system) and merge into the original miscibility gap as gamma is raised; the first inversion point is located in this area. The second gamma range of islands (entirely located above T(c)) is observed within the regime of the second shear dissolution. The occurrence of islands can be suppressed for an appropriate choice of parameters and the previously published simpler behavior is regained.