PHASE-SEPARATION IN AQUEOUS MIXTURES OF SIMILARLY CHARGED POLYELECTROLYTES

The demixing ('incompatibility') of mixtures of two similarly charged water-soluble polyelectrolytes in water (PE1/PE2/S mixtures) was investigated experimentally, and the phase diagrams were compared with those typical of mixtures of two uncharged polymers and of one uncharged and one charged polymer in a common solvent (P1/P2/S and P/PE/S mixtures, respectively). The general features of the PE1/PE2/S phase diagrams and their dependences on the degree of polymerization were similar to those of P1/P2/S mixtures, but different from those of P/PE/S mixtures. The sensitivity to added salt of PE1/PE2/S systems was intermediate between those of P1/P2/S and P/PE/S systems. Qualitative explanations of the differences and similarities between the three types of mixtures are given in terms of the entropy of mixing of the counterions, and the influence of electrostatic interactions on the probability of segment-segment contacts in polyelectrolyte solutions.