CALCULATION OF THE PHASE-SEPARATION BEHAVIOR OF SHEARED POLYMER BLENDS

The influence of shear on the phase diagrams of model blends of polymer A and polymer B, exhibiting phase separation upon heating, was calculated on the basis of the generalized Gibbs energy of mixing G(gamma), characterizing the steady state established at shear rate gamma, as the sum of G(z), the equilibrium Gibbs energy, and E(s), the energy the system can store while flowing. The results obtained for the two model systems A150/B200 and A75/B200 (the figures denote the molar masses in kg/mol) are qualitatively identical. With increasing shear rate the homogeneous region expands first, then shrinks below its equilibrium value within a certain gamma range, and finally becomes again larger than with the stagnant blends. For a given composition of such a polymer mixture two regions of shear dissolution at low and at high gamma values, respectively, are separated by a region of shear demixing; hence two inversion points of shear influences are theoretically expected for blends, in contrast to polymer solutions, for which one calculates only one. Experimentally, the inversion from shear dissolution to shear demixing is frequently observed and the opposite inversion was recently reported for the first time, but so far no evidence exists for the occurrence of both phenomena within the same system. Possible reasons for this fact are discussed.