CALCULATION OF SHEAR INFLUENCES ON THE PHASE-SEPARATION OF POLYMER BLENDS EXHIBITING UPPER CRITICAL SOLUTION TEMPERATURES

Calculations were performed on the basis of a generalized Gibbs energy of mixing G(gamma), which is the sum of the Gibbs energy of mixing of the stagnant system and E(s), the energy stored in the system during stationary flow. With increasing shear rate gamma, the demixing temperatures shift to lower values (shear-induced mixing; diminution of the heterogeneous area), then to higher values (shear-induced demixing), and finally to lower values again before the effects fade out. The details of the rather complex phase diagrams resulting for a given shear rate are primarily determined by a band in the T/x plane (x = mole fraction) within which (partial derivative 2 E(s)/partial derivative x2)T < 0 (i.e., E(s) acts towards phase separation). There are two ranges of gamma within which closed miscibility gaps can exist: The more common outer islands are partly or totally situated outside the equilibrium gap (and within the above mentioned band). As gamma is raised they break away from the ''mainland'' at the upper end of the first region of shear-induced mixing and shift to T > UCST where they submerge. Bound to a suitable choice of parameters, a second kind of closed miscibility gaps, the inner islands, which always remain within the equilibrium solubility gap (and outside the band of negative curvature of E(s)) is additionally observed. This time the islands break away from the ''mainland'' at the lower end of the first region of shear-induced mixing where they also submerge. The present findings are compared with the results of previous calculations for LCSTs.