Viscosity of nafion oligomers as a function of hydration and counterion type: A molecular dynamics study

The design of fuel cells and lithium ion batteries is constrained, in part, by mechanical creep and perforation of the polymer electrolyte, a process that is poorly understood at the molecular level. The mechanical stiffness (quantified as shear viscosity) and structure of a widely used polymer electrolyte, Nafion, are studied in the limit of a low solvent volume fraction (≤26% v/v H₂O) using molecular dynamics simulations. The viscosity is shown to increase by up to 4 orders of magnitude in response to changes in composition representing as little as 2 wt % of system. Two types of compositional changes are considered, changes in solvent volume fraction and counterion type. A system with a counterion X^v+ for every v Nafion monomers and y water molecules is denoted as (RSO₃)_vX·(H₂O)_y. The following trend is observed in viscosity: (RSO₃)₂Ca > RSO₃Na > RSO₃H·(H₂O)₃ > RSO₃H · RSO₃H·(H₂O)₁₀. This trend correlates with changes in the strength of the SO₃^-/X^v+/SO₃^- cross-links and the size of the cross-link networks. Counterion type is shown to strongly influence the morphology. The simulations are able to reproduce some important experimental trends without crystalline domains or high-MW effects like entanglements, providing a simplified understanding of the mechanical properties of Nafion.