Counterion dynamics in polyurethane-carboxylate ionomers with ionic liquid counterions

Polyurethane carboxylate ionomers based on poly(ethylene glycol) (M _n = 600) with sodium and various ammonium, phosphonium and imidazolium cations are synthesized for systematic comparison of different cationic counterions. Generally, larger cations act as plasticizers, lowering T_g because of weaker Coulombic force for ion associations (acting as physical cross-links). T_g can be reduced from 47 °C to -6 °C when replacing Na⁺ with large ether-oxygen containing ammonium without changing polymer composition and the lower T_g can enhance ionic conductivity by 5 orders of magnitude. Ionic conductivity has a stronger correlation with segmental relaxation than T_g, suggesting that counterion motion is coupled to the poly(ethylene oxide) local motions. An electrode polarization model is used to quantify the conducting ion concentration and mobility. All cation mobility follows VFT behavior, whereas conducting ion concentration has an Arrhenius temperature dependence, with slope providing activation energy and intercept determining the fraction of counterions available to participate in ionic conduction. Sodium counterions are mostly trapped by pPDI-carboxylate-pPDI segments, whereas the larger counterions are less trapped. Cation species and methoxyalkyl tails were found to impact both conducting ion concentration and their mobility but T_g and α-relaxation time are the key factors for ionic conductivity at a given temperature.