Polymer solutions in supercritical fluids: Their flow behavior compared to that in normal solvents

Abstract

The dissimilar flow behavior of polymer solutions in super critical fluids, as compared with that in normal solvents, are primarily resulting from the great differences in the free volume of the pure solvents. The present analysis compares the generalized intrinsic viscosities {η} and the average shear overlap parameters for the two types of solvent. {η} depends on the polymer concentration c; it starts from the intrinsic viscosity [η] at infinite dilution and ends at the intrinsic bulkiness in the limit of the pure polymer. The overlap parameter states the average number of polymer molecules that flow together under given conditions. With regard to these parameters, super critical fluids differ from normal solvents in the following manner: [η] is much less, the intrinsic bulkiness much larger and {η} (c) runs through a deep minimum; in addition the cross-over concentration, separating the solvent dominated from the polymer dominated flow regime, is much higher. Another fundamental dissimilarity is the change in the enthalpy of mixing with increasing c from exothermic to endothermic. The above features explain, why super critical fluids have numerous advantages (like viscosities that remain very low even at high polymer concentrations) over conventional solvents and why they are used for the modification and synthesis of high-molecular substances.