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With the rise of green engineering, there is an increasing need to manufacture materials without relying on organic solvents. Using all-aqueous approaches mitigates the industrial safety and environmental concerns that are associated with volatile organic compounds, while enabling scalable and sustainable fabrication processes. Water-insoluble polyelectrolyte complexes (PECs) arise due to the electrostatic attraction between oppositely charged polyelectrolytes in solution. Notably, when salt is present, these rigid or glassy PECs can be transformed into malleable and liquid states, enabling researchers to process solid materials from the previously deemed unprocessable. The liquid PEC phase, also known as a polyelectrolyte complex coacervate, arises through liquid–liquid phase separation and offers a tunable viscosity to match the needs of the processing method. These coacervates exhibit adjustable rheological properties by varying parameters, including temperature, salt type, ionic strength, polymer ratio, and molecular weight. This tunability makes them attractive for applications ranging from coatings and adhesives to biomedical delivery systems. Notably, the transition between liquid and solid PECs is reversible, as removing salt ions restores the physical cross-links. Additionally, PECs exhibit exceptional stability in various organic solvents and solutions with extreme pH values, without requiring chemical cross-linking. However, the aqueous processing strategies and reversibility of PECs have yet to be fully explored. In this Account, we primarily focus on the well-studied PEC system composed of the strong polyelectrolytes poly(sodium 4-styrenesulfonate) and poly(diallyldimethylammonium chloride). First, we describe how salt concentration is a crucial parameter that enables the aqueous processing of coacervates via electrospinning, spin coating, bar casting, and 3D printing into fibers, coatings, membranes, and 3D structures. We also discuss the impact that processing conditions, like drying and quenching, have on the properties of solid materials, such as their porosity and mechanical strength. Next, we highlight reports that explore how the solubility mismatch between polyelectrolyte pairs and salt ions result in solid and liquid PECs that are nonstoichiometric, thereby exhibiting an overcompensation phenomenon or nonstoichiometry. How the mechanical behavior of a material changes as a function of temperature, i.e., their thermomechanical properties, as well as membrane separation performance are notably influenced by nonstoichiometry, even when the degree of nonstoichiometry is minimal. Interestingly, we are starting to see research reports in the literature on how post-treatment methods, including salt and heat annealing, previously applied to polyelectrolyte multilayer films, offer some transferability to bar-casted separation membranes, which warrants further research. We conclude with a forward-looking discussion that highlights the potential opportunities and challenges related to the future implementation of PEC-based materials. 

Polymeric membranes fabricated via the nonsolvent-induced phase separation process rely heavily on toxic aprotic organic solvents, like N-methyl-pyrrolidine (NMP) and dimethylformamide. We suggest that the “saloplastic” nature of polyelectrolyte complexes (PECs) makes them an excellent candidate for fabricating next-generation water purification membranes that use a more sustainable aqueous phase separation process. In this study, we investigate how the properties of PECs and their interactions with salt can form pore-containing membranes from the strong polyelectrolytes poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC) in the presence of potassium bromide (KBr). How the single-phase polymer-rich (coacervate) dope solution and coagulation bath impacted the formation, morphology, and pure water permeance (PWP) of the membranes was systematically evaluated by using scanning electron microscopy and dead-end filtration tests. The impact of a salt annealing post-treatment process was also tested; these treated PEC membranes exhibited a PWP of 6.2 L m–2 h–1 bar–1 and a dye removal of 91.7% and 80.5% for methyl orange and methylene blue, respectively, which are on par with commercial poly(ether sulfone) nanofiltration membranes. For the first time, we have demonstrated the ability of the PEC membranes to repel Escherichia coli bacteria under static conditions. Our fundamental study of polyelectrolyte membrane pore-forming mechanisms and separation performance could help drive the future development of sustainable materials for membrane-based separations.