Fundamentals and Design Rules of Polyelectrolyte Complex Materials: A Comprehensive Review

Polyelectrolyte complexes (PECs) are a versatile class of emerging materials whose structure and properties derive from the associative phase separation of oppositely charged polyelectrolytes. Their behavior is highly sensitive to environmental factors, including temperature, salt concentration, solvent quality, humidity, and the intrinsic chemistry of the polyelectrolytes, such as charge density and patterning, hydrophobicity, and backbone chemistry. These dependencies generate distinct design rules for PECs that are different from those associated with traditional neutral polymers. There are also further differences for PECs saturated in aqueous media and those allowed to equilibrate and dry in air, highlighting the need for a unified set of design rules that connects molecular parameters to macroscopic properties such as viscoelasticity, phase behavior, and water uptake. In this review, we summarize recent progress in understanding the thermodynamic and mechanical principles governing PECs, while identifying unifying trends across systems of varying composition and environmental conditions. By consolidating design rules across disparate chemistries and circumstances, PECs are poised to evolve from empirically tuned systems into a broadly applicable platform for functional and structural materials.