Controlling Ionomer Phase Separation Through Side-Chain Engineering

Phase separation in perfluorinated ion-conducting polymer, or ionomer, membranes defines pathways for ion transport and controls overall conductivity. However, this phase-separated morphology is difficult to characterize and challenging to control based on molecular-level insights. Herein, we show two strategies to control phase-separation and domain spacing in perfluorinated ionomers, and resonant X-ray scattering is used to provide enhanced contrast and chemical sensitivity to decipher nanoscale morphology. Perfluoro ionene chain extended ionomers containing two or three ionic groups per side-chain exhibit tunable domain spacing based on side-chain length. Furthermore, the chain extended ionomers have greater long-range order and higher proton conductivity and water uptake compared to conventional perfluorinated sulfonic acid ionomers. Perfluorinated sulfonic acid iononers with photoacid dyes covalently bound to side-chain ends, which are promising for light-driven ion conduction, reveal a phase-separation length scale that increases with increasing dye content. Overall, these studies provide insights into detailed connections between polymer chemistry and phase-separated morphology to inform molecular-level design of next-generation membranes.