Counter Anion Type Influences the Glass Transition Temperature of Polyelectrolyte Complexes

Salt acts as a plasticizer in polyelectrolyte complexes (PECs), which impacts the physical, thermal, and mechanical properties, thus having implications in applications, such as drug delivery, energy storage, and smart coatings. Added salt disrupts polycation-polyanion intrinsic ion pairs, lowering a hydrated PEC's glass transition temperature (T_g). However, the relative influence of counterion type on the PEC's T_g is not well understood. Here, the effect of anion type (NaCl, NaBr, NaNO₃, and NaI) on the T_g of solid-like, hydrated PECs composed of poly(diallydimethylammonium) (PDADMA)-poly(styrenesulfonate) (PSS) is investigated. With increasing the chaotropic nature of the salt anion, the T_g decreases. The relative differences are attributed to the doping level, the amount of bound water, the mobility of water molecules within the PECs, and the strength of interactions between the PEs. For all studied salt concentrations and salt types, the T_g followed the scaling of -1/T_g ≈ ln([IP]/[H₂O]), in which [IP]/[H₂O] is the ratio of intrinsic pairs to water. The scaling estimates that about 7 to 17% of the intrinsic ion pairs should be weakened for the PEC to partake in a glass transition. Put together, this study highlights that the T_g in PECs is impacted by the salt anion, but the mechanism of the glass transition remains unchanged.