Polyelectrolyte Complex Films from Blends Versus Copolymers

The release of counterions drives the efficient blending of oppositely charged polyelectrolytes when they complex. In theory, mixing like-charged macromolecules before complexation should offer a significant scope for controlling the composition and properties of polyelectrolyte complexes/coacervates. In practice, not much is known about the relative affinities between charged components and how they might compete for limited supplies of oppositely charged partners. In this work, we contrast the use of mixtures of poly­(styrene sulfonate), PSS, and poly­(methacrylic) acid, both polyanions, with copolymers of the same repeat units competing for poly­(diallyldimethylammonium), PDADMA, a polycation. Using the layer-by-layer, or multilayering, technique ensures that the polycation is always a limiting reagent. Although the composition of the thin film of a polyelectrolyte complex faithfully mirrored the solution copolymer composition, strong deviations from solution composition were observed with blends of homopolymers. There is a degree of thermodynamic control over film composition, related to the difference in free energy of formation of PDADMA/PSS versus PDADMA/poly­(methacrylic acid) repeat unit pairs.