Improvement of polymer solubility: Influence of shear and of pressure

Abstract

Polymer solutions and liquid polymer mixtures are in their phase-separation very sensitive to shear, in contrast to low molecular weight systems. Shear-induced changes of the demixing temperature up to 100°C can be observed. The reason for this behavior lies in the ability of macromolecules to store energy in the stationary state while the system flows. Measurements with polymer solutions and with polymer blends reveal a multitude of phenomena. All experimental results can be rationalized at least semi-quantitatively on the basis of a theoretical approach using a generalized Gibbs energy of mixing containing the stored energy. Pressure induced changes in the demixing conditions for polymer solutions turn out to be rather different for systems which phase separate upon cooling or upon heating. In the former case (UCSTs) the effects are small (typically ± 1-2 K/100 bar), vary in sign and are hard to predict theoretically. In the latter case pressure practically always enhances miscibility and the effects are considerably larger (typically + 100-200K/100 bar). Theories based on the corresponding states principle yield reasonable forecasts for the behavior of such solutions.