Associative Phase Separation in Single-Step Polyelectrolyte Complex Coatings

Polyelectrolytes can be used to make water-based, recyclable, and robust coatings, making use of their strong electrostatic interactions. They are conventionally applied in a layer-by-layer fashion, which is labor-intensive and thus expensive. Recently, polyelectrolyte complexes have also been applied in a single step, where both polycation and polyanion are simultaneously applied to the surface. To avoid complexation before application, a volatile base was added to deprotonate amines on the polycation. Only after evaporation of the base, charges are introduced on the polycation, initiating complexation. However, the associative phase separation in these polyelectrolyte coatings is poorly understood. Here, we study the associative phase separation in poly(ethylene imine):poly(acrylic acid) coatings using confocal fluorescence microscopy. We find that the initial polyelectrolyte concentration has a profound impact on the macroscale homogeneity during and after drying. Using fluorescently labeled polyelectrolytes, we show that during coating drying, a continuous polyelectrolyte complex matrix forms, with polyelectrolyte-poor inclusions. We explain these observations by connecting the changes in composition in the coating film to the underlying phase diagram of the polyelectrolyte mixture. We anticipate that the increased understanding of the associative phase separation described in this study will significantly aid the development of homogeneous functional polyelectrolyte coatings.