Design Rules for the Processing and Use of Polyelectrolyte Complex Materials

Polyelectrolyte complexation leverages self-assembly in water to form materials that can range from polymer-dense liquids to glassy solids. While the polymer-dense liquid complexes have a long history of industrial use, the potential for such polyelectrolyte complex solid materials is less established, and traditional heuristics related to polymer chemistry and properties do not always seem to apply. To address this gap, we have leveraged compositional and dynamic mechanical analysis to examine the effects of polymer chemistry such as backbone chemistry, ionizable group, charge density, and hydrophobicity, as well as chain length, along with temperature and humidity on the properties of the resulting material properties. Rather than a single glass transition temperature, we characterized the glass transition temperature-relative humidity line, demonstrating that charge density and hydrophobicity dictated humidity sensitivity while side chain mobility (i.e., T_g) dictated temperature sensitivity. The origin of this glass transition in PECs was attributed to saturation of the ion pairs with water molecules, with the number of water molecules needed being independent of copolymer chemistry. Similarly, the identity of the charged side chain heavily affected the water affinity and mechanics of the materials, depending on ion solvation. Interestingly, the effect of length depended on the degree of length match/mismatch of the polymers, with matched ones having higher glass transition humidities than mismatched. This study serves as the basis for developing design rules to enable the processing and use of polyelectrolyte complexes for various applications and in different environments.