Ion Motion in Anion and Proton-Conducting Triblock Copolymers

To learn more about anion transport in functionalized polymers and improve anion exchange membrane (AEM) performance, we have directly compared the properties of proton exchange membranes (PEMs) and AEMs. A single batch of poly(hexyl methacrylate)-b-poly(styrene)-b-poly(hexyl methacrylate), PHMA-b-PS-b-PHMA, triblock copolymer was synthesized and postfunctionalized in the PS block with sulfonate or quaternary ammonium moieties. The conductivity, water uptake, and morphology of low, middle, and high IEC anionic and cationic PHMA-b-PS-b-PHMA triblock copolymers were investigated, and the metrics of ion diffusivity and membrane sensitivity to relative humidity are introduced. We observed that these block copolymer AEMs and PEMs had similar values for the ratio of calculated ion diffusion coefficient to the mobile species dilute ion diffusivity (D/D-0) as a function of hydration. This observation points to similar morphological barriers to transport and similar hydration dependence in each material and provides a basis for understanding ion transport in AEMs compared to dilute solution. As the ion concentration in the functionalized block was increased, the sensitivity of conductivity to relative humidity decreased due to closer proximity of the ionic groups. These guidelines for AEM design will help to more deeply understand the similarities and differences between different types of water-absorbing, ion-conducting membranes.