We address complex coacervation, the liquid-liquid phase separation of a solution of oppositely charged polyelectrolyte chains into a polyelectrolyte rich complex coacervate phase and a dilute aqueous phase, based on the general premise of spontaneous formation of polycation-polyanion complexes even in the homogeneous phase. The complexes are treated as flexible chains made of dipolar segments and uniformly charged segments. Using a mean field theory that accounts for the entropy of all dissociated ions in the system, electrostatic interactions among dipolar and charged segments of complexes and uncomplexed polyelectrolytes, and polymer-solvent hydrophobicity, we have computed coacervate phase diagrams in terms of polyelectrolyte composition, added salt concentration, and temperature. For moderately hydrophobic polyelectrolytes in water at room temperature, neither hydrophobicity nor electrostatics alone is strong enough to cause phase separation, but their combined effect results in phase separation, arising from the enhancement of effective hydrophobicity by dipolar attractions. The computed phase diagrams capture key experimental observations including the suppression of complex coacervation due to increases in salt concentration, temperature, and polycation-polyanion composition asymmetry, and its promotion by increasing the chain length, and the preferential partitioning of salt into the polyelectrolyte dilute phase. We also provide new predictions such as the emergence of loops of instability with two critical points.

Poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonic acid) (PEDOT:PSS) is a material consisting of two oppositely charged polymers and is available as aqueous dispersions. Various annealing and doping methods dramatically enhance PEDOT:PSS conductivity, rendering it competitive with inorganic conductors like indium tin oxide. Yet a comprehensive understanding of PEDOT:PSS conductivity enhancement remains elusive. To unravel the chief physical interactions at play, we explore aspects of solution-state PEDOT:PSS through light scattering, X-ray scattering, rheological measurement, and the construction of a partial phase diagram. We show these features in neat water and with the addition of several conductivity enhancement agents: dimethyl sulfoxide, ethylene glycol, 1-ethyl-3-methyl­imidazolium tetrafluoroborate, or sodium chloride. We find that PEDOT:PSS solutions exist as a dispersion of charged, many-chain microgels with structural features and dynamics strongly influenced by electrostatic interactions. Two distinct phase transitions occur with high sensitivity to ionic strength: a coexistence of a dilute and a concentrated PEDOT:PSS phase and a physical gelation. Despite the rich solution properties uncovered here, we find no connection between them and the enhancement of film conductivity.

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