(147a) A Comparative Study of the Hydrated Morphologies of Perfluorinated Ionomers with Distinct Protogenic Groups

The hydrated morphologies of a perfluorosulfonic acid (PFSA) ionomer and a bis[(perfluoroalkyl)sulfonyl]imide-based ionomer have been investigated through dissipative particle dynamics (DPD) simulations as a function of ionomer equivalent weight (EW) and degree of hydration. The PFSA ionomers with EWs of 589, 651 and 801 g/mol were chosen to determine the effect of EW on their hydrated morphologies. With a common hydrophobic polytetrafluoroethylene backbone, the PFSA ionomer and imide-based ionomer were further compared to investigate the effect of the different protogenic group on the morphologies.

Coarse-grained mesoscale models were constructed by dividing the hydrated ionomer into components consisting of a polytetrafluoroethylene backbone bead, ionomer specific backbone bead, terminal side chain beads and a water bead consisting of a cluster of six water molecules. Flory-Huggins χ-parameters and repulsion parameters between DPD beads were calculated based on the optimized structures of the beads. Equilibrated morphologies of the ionomers containing various water contents were then determined in a simulation box with dimension of about 32.4 nm. The hydration level (λ) was varied in both systems with water contents corresponding to 6, 12, 18 and 24 H2O per side chain.

Water contour plots revealed that as the hydration level was increased, the isolated water clusters present at the lower water contents increase in size eventually resemble channels or pores in the PFSA ionomer. While for the imide-base ionomer, the water clusters have stronger segregation and only form continuous water region at relatively high hydration level (e.g. λ = 24). This illustrate why the imide-base ionomer shows especially low conductivity at low to intermediate hydration levels and gain conductivity closed to other PFSA ionomers again at a specific higher hydration level. The simulations of the PFSA at different EWs suggest that the high EW ionomers exhibit significantly greater dispersion of water regions that the low EW ionomers. The high EW ionomers stimulate the segregation of hydrophobic and hydrophilic phases in the PFSA membranes. Finally, water-water particle radial distribution functions (RDFs) were evaluated for all of the PFSA ionomers and the average size of water clusters estimated.