Abstract
Electro-responsive metallopolymers can possess highly specific and tunable ion interactions, and have been explored extensively as electrode materials for ion-selective separations. However, there remains a limited understanding of the role of solvation and polymer-solvent interactions in ion binding and selectivity. The elucidation of ion-solvent-polymer interactions, in combination with the rational design of tailored copolymers, can lead to new pathways for modulating ion selectivity and morphology. Here, we present thermo-electrochemical-responsive copolymer electrodes of N-isopropylacrylamide (NIPAM) and ferrocenylpropyl methacrylamide (FPMAm) with tunable polymer-solvent interactions through copolymer ratio, temperature, and electrochemical potential. As compared to the homopolymer PFPMAm, the P(NIPAM(0.9)-co-FPMAm(0.1)) copolymer ingressed 2 orders of magnitude more water molecules per doping ion when electrochemically oxidized, as measured by electrochemical quartz crystal microbalance. P(NIPAM(0.9)-co-FPMAm(0.1)) exhibited a unique thermo-electrochemically reversible response and swelled up to 83% after electrochemical oxidation, then deswelled below its original size upon raising the temperature from 20 to 40 °C, as measured through spectroscopic ellipsometry. Reduced P(NIPAM(0.9)-co-FPMAm(0.1)) had an inhomogeneous depth profile, with layers of low solvation. In contrast, oxidized P(NIPAM(0.9)-co-FPMAm(0.1)) displayed a more uniform and highly solvated depth profile, as measured through neutron reflectometry. P(NIPAM(0.9)-co-FPMAm(0.1)) and PFPMAm showed almost a fivefold difference in selectivity for target ions, evidence that polymer hydrophilicity plays a key role in determining ion partitioning between solvent and the polymer interface. Our work points to new macromolecular engineering strategies for tuning ion selectivity in stimuli-responsive materials.