“…Polyelectrolyte complexes (PECs) are dense, water-swollen, polymer-rich phases spontaneously formed upon mixing oppositely charged polyelectrolytes. − Since polyelectrolyte complexation is an associative phase separation process based primarily on charge–charge interactions, PECs can incorporate a variety of organic, inorganic, and biologic-charged molecules, making them attractive candidates for applications in nucleic acid delivery, underwater adhesives, and nanoscale reactors. − However, the rational design of PECs remains difficult, as their properties depend on a complex interplay between environmental factors such as solvent mixture, salinity, pH, and temperature in addition to the variety of supramolecular interactions between the oppositely charged polyelectrolytes. , Recently, there has been a surge in interest toward using structural features of polyelectrolytes, such as length, charge density, and charge blockiness, to alter the stability, rheology, and structure of PEC materials. − Structure–property relationships centered on polyelectrolyte design are attractive because they can provide robust, chemically agnostic design principles for PECs and provide a path toward tailored PEC design within applications where environmental conditions are beyond control . Polyelectrolyte charge density is one of the most promising parameters recently investigated, as it has been shown to consistently alter the rheology and stability of PECs. , …”