Solutions of polymer blends in highly saline water: Salt-induced inversions of viscosity effects for poly(ethylene oxide) + poly(sodium 4-styrenesulfonate)

Abstract

Solution viscosities were measured for an uncharged polymer [poly(ethylene oxide): PEO], for a polyelectrolyte [poly(sodium 4-styrenesulfonate): PSS-Na] and for the blends of these components in water of variable salinities. The evaluation of these data with respect to the intrinsic viscosities revealed the following: for NaCl as extra salt, the intrinsic viscosities of the blends are always less than predicted by additivity; moreover the [η] values of PEO increase with rising salinity of the solvent, in contrast to that of PSS-Na which decrease. This situation leads to an inversion point of the salt effects at the salinity of approximately 2.9 mol/L. This finding becomes comprehensible by treating the salt solutions as mixed solvents. Replacing NaCl by the CaCl2 (divalent cation) as extra salt leads to the [η] values for the blends, which are larger than additive. Based on phenomenological thermodynamic consideration this behavior is attributed to an entropy driven formation of interpolymer complexes and explained in molecular terms.