Role of Semi-Crystalline Morphology on Proton Conduction Pathways in a Precise Sulfonic Acid Polyethylene

Linear polyethylenes with precisely periodic functional groups (precise polyethylenes) have been shown to exhibit unusually well-defined semi-crystalline morphologies containing layers of functional groups. These layers may be useful for protonic, ionic or molecular transport. Recently, a precise polyethylene was synthesized with sulfonic acid groups bonded to every 21$^{\mathrm{st}}$ carbon. We have studied the proton dynamics in this polymer using broadband dielectric spectroscopy at various hydration levels and thermal histories to evaluate the utility of these self-assembled pathways for improving transport. To understand the effect of structure on proton dynamics, detailed characterization has been carried out including X-ray scattering and atomistic molecular dynamics simulations. Comparable proton conductivities are measured in the semi-crystalline and amorphous states, showing that the greatly reduced chain mobility in the semi-crystalline state does not lead to a commensurate reduction in conductivity. This suggests that the proton pathways within the crystallites play a role in enhancing transport.