Ion transport in solid polymer electrolytes with helical secondary structure

The transport of ions in polymers has primarily focused on synthetic backbones which adopt random coil conformations. Peptides can form secondary structures including helices due to intramolecular hydrogen bonding which possess a macrodipole along the backbone. Homopolypeptides with degree of polymerization spanning N = 50-925 were synthesized with tethered ammonium cations and mobile anions to determine if helical backbones impact conductivity in the solid state. For N = 50, a random coil is observed to be less conductive than a helical polymer with otherwise identical monomer chemistry. Increasing N leads to increases in both conductivity and dielectric constant of the helical systems attributed to the growing macrodipole. The glass transition is negligible affected and not responsible for conductivity trends. Helical backbones exhibited high thermal stability, while random coils underwent a reorganization upon heating to 150 C and an increase in glass transition temperature. Peptides with tethered ethylene oxide chains and added salt were also investigated and compared to random coil analogues, showing distinct salt dependences on glass transitions, conductivity, and ion aggregation.