Relationships between Water Domain Morphology and Diffusivity in Hydrated Ion-conducting Polymers

There is a need for new fluorine-free polymers for proton and anion-exchange membranes in various electrochemical devices including fuel cells. Achieving high conductivity in hydrated, ion-conducting polymers typically requires the hydrophobic backbone of the polymers to be nano-phase separated from the hydrophilic domains so that the ions (protons or hydroxide ions) have well-defined water domains for efficient transport. The ion and water transport thus depends strongly on the hydration level and on the detailed morphology of the hydrophilic domains. Molecular dynamics (MD) simulations are well-suited to understanding the molecular-level relations between morphology and dynamics. I will discuss our recent simulations of both anionic and cationic all-hydrocarbon polymers that conduct protons and hydroxide ions, respectively.  Comparisons will be made to experiments on the same systems, especially X-ray scattering and conductivity. For both classes of polymers, the morphologies of the hydrophilic domains can be characterized through their channel width distributions, surface areas, fractal dimensions, and signatures in simulated X-ray scattering intensities. Water and ion diffusivities calculated from the MD simulations are consistent with changes in the nanoscale morphology with changing water content.  Interestingly, the water content is a larger determinant of water diffusivity than the ion exchange capacity (IEC) of the polymers.