Aqueous Processing of Stoichiometric and Nonstoichiometric Materials from Complex Coacervates

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.