The Effect of Charge Density on Polyelectrolyte Complexation

Mixing oppositely charged polyelectrolytes (PEs) in solution generally induces associative phase separation into a polymer-lean supernatant phase and a polymer-rich polyelectrolyte complex (PEC) phase. In nature, PECs are essential components of membraneless organelles in cells and are thought by some to have played a role in the origins of life. They also have promising applications in therapeutic delivery and materials science, including hydrogels, membranes, electrospun fibers, and saloplastics. Each of these applications is enabled by the specific phase behavior, structure, and viscoelastic properties of PECs under a given set of conditions. Therefore, understanding and controlling PEC phase behavior and structure is key to enabling bottom-up, rational material design. This thesis explores the impact of charge density (f ) on the phase behavior and structure of polyelectrolyte complexes (PECs). A unique synthetic platform was developed to produce PECs from homologous polyelectrolytes with a near-ideally random distribution of charged and neutral comonomers. This platform enabled the investigation of phase behavior across a broad range of charge densities. Unique structural features, such as positional charge correlations in PECs, which diminished with the addition of salt, were revealed via small-angle neutron scattering (SANS). This work also highlights the distinct phase behavior and structure of asymmetrically charged PECs. Additionally, coexisting complex phases (multiphase PECs) were explored through mixtures of charge density-mismatched polyelectrolytes (three or more species) with high chemical and structural resemblance. Experimental results were compared with molecular dynamics simulations and theoretical predictions, demonstrating excellent agreement. Overall, this work seeks to systematically understand how charge density (f) affects polyelectrolyte complexation, providing insights for the rational design of PEC based materials.