Glass Transitions in Polyelectrolyte Complex Materials

Polyelectrolyte complexation leverages self-assembly to form materials ranging from dense liquids to glassy solids. While liquid complex coacervates have a long history of industrial use, the potential for solid polyelectrolyte complex materials is less established. Furthermore, heuristics related to more “traditional” thermoplastics 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, charge density, hydrophobicity, and chain length, along with temperature and humidity on the properties of the resulting polyelectrolyte complex materials. We characterized a glass transition temperature-relative humidity line, demonstrating that charge density and hydrophobicity dictate humidity sensitivity while side chain mobility (i.e., T_g) determines temperature sensitivity. This glass transition was attributed to saturation of the ion pairs with water molecules, and the number of water molecules per ion pair was shown to be largely independent of copolymer chemistry. Thus, the identity of the ionizable groups dominates both the water affinity and mechanics of the materials. We also observed unexpected trends in the glass transition as a function of the degree of length match/mismatch of the polycation and polyanion, with matched-length polymers 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.