Developing Design Rules for Polyelectrolyte Complex Materials: Role of Polyelectrolyte Length, Charged Group, and Backbone

Polyelectrolyte complexation is an entropically driven, associative phase separation that has been leveraged to produce aqueously processed plastics known as polyelectrolyte complexes (PECs). Previously, we showed that their affinity to water and their chain mobility are important aspects to consider when designing PEC materials. To establish a more complete picture of influencing parameters, we examined the effect of polymer chemistry, specifically chain length and the side chain and backbone chemistry, on both the phase behavior and mechanical properties of homopolymer PECs. We combined compositional studies of PEC phase behavior with analyses of PEC dynamics and mechanics to understand how these aspects of polymer chemistry affect material performance. We observed that the identity of the ionizable groups heavily affected ion solvation, where PECs with lower water affinities had higher glass transition humidities and were generally more brittle, compared to PECs with higher water affinities. In contrast, backbone chemistry affected chain mobility, allowing acryloyl chemistries to have lower glass transition humidities compared to methacryloyl. Finally, chain length effects depended on the degree of match/mismatch of the polymer’s lengths, with matched PEC systems having higher glass transition humidities than mismatched. Comparisons of the phase behavior and glass transitions revealed that side chain and backbone chemistry effects are universal across different mediums, while length effects are medium specific. These results establish fundamental structure–property relationships for the rational design of functional PEC materials.