Effect of Copolymer Chemistry on the Humidity- and Temperature-Dependent Mechanical Properties and Structure of Polyelectrolyte Complexes

Here, we examine how temperature, humidity, and polymer chemistry affect the mechanics of polyelectrolyte complex (PEC) materials using a library of methacrylate copolymers of varying charge density and hydrophobicity to deconvolute these effects. Our results show that copolymer chemistry controls the amount of water being taken up by the PECs, thus affecting their mechanical properties. Rather than a single glass transition temperature, we characterized the glass transition temperature-relative humidity ( T g /rH g ) 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 was attributed to saturation of the PEC ion pairs with water, with the number of water molecules needed dictated by the identity of the ion pairs. Finally, while an examination of trends as a function of copolymer composition can lead to unintuitive results, the stress–strain behavior strongly correlates with water content. This study serves to bridge the critical knowledge gap to enable the processing and use of PECs for various applications and in different environments.