CALCULATION OF SHEAR INFLUENCES ON THE PHASE-SEPARATION BEHAVIOR OF POLYMER-SOLUTIONS IN THE REGION OF THEIR LOWER CRITICAL SOLUTION TEMPERATURE - CREATION OF CLOSED MISCIBILITY GAPS

Model calculations concerning the phase separation of sheared solutions were performed for a system with an exothermal theta temperature of 333.3 K and 5000 polymer segments per chain. To determine the demixing conditions for a certain shear rate gamma, a generalized Gibbs energy G-gamma = G(z) + E(s) was used. The Flory-Huggins equation, with a concentration- and temperature-dependent interaction parameter, served as an analytical expression for G(z), the Gibbs energy of mixing at zero shear. E(s), the energy that can be stored during stationary flow, as a function of composition, temperature, and gamma was represented by typical empirical relations. The results of these calculations yield two peculiarities, which are typical for lower critical solution temperatures: (i) At some compositions a temperature interval of homogeneity separates the equilibrium solubility gap from a flow-induced region of immiscibility (bananalike extension of the equilibrium two phase area); this phenomenon is in accord with measurements reported in the literature. (ii) Under very special conditions the above-mentioned extension breaks away from the main heterogeneous region so that a closed miscibility gap is created; this prediction has so far not been verified experimentally.