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This study focuses on changes in the structure of ionomer membranes, provided by the 3M Fuel Cells Component Group, as a function of the equivalent weight (EW) and the relationship between the structure and the properties of the membrane. Wide-angle X-ray diffraction results showed evidence of both non-crystalline and crystalline ordered hydrophobic regions in all the EW membranes except the 700 EW membrane. The spectral changes evident in the vibrational spectra of the 3M membranes can be associated with two major phenomena: (1) dissociation of the proton from the sulfonic acid groups even in the presence of small amounts of water; and (2) changes in the conformation or the degree of crystallinity of the poly(tetrafluoroethylene) hydrophobic domains both as a function of EW and membrane water content. All the membranes, regardless of EW, are thermally stable up to 360 °C. The wet membranes have conductivities between 7 and 20 mS/cm at 125 °C. In this condition, the conductivity values follow VTF behavior, which suggests that the proton migration occurs via proton exchange processes between delocalization bodies (DBs) that are facilitated by the dynamics of the host polymer. The conductivity along the interface between the hydrophobic and hydrophilic domains makes a larger contribution in the smaller EW membranes likely due to the existence of a greater number of interfaces in the membrane. The larger crystalline domains present in the higher EW membranes provide percolation pathways for charge migration between DBs, which reduces the probability of charge transfer along the interface. Therefore, at higher EWs although there is charge migration along the interface within the hydrophobic-hydrophilic domains, the exchange of protons between different DBs is likely the rate-limiting step of the overall conduction process.

Dissipative particle dynamics (DPD) simulations have been carried out to study the hydrated morphology of 3M perfluorosulfonic acid (PFSA) fuel cell membranes as a function of the equivalent weight (EW), molecular weight (MW), and hydration level. The 3M PFSA ionomers were modeled using typical EWs of 578, 640, and 790 g/mol, and molecular weights were varied from about 45,000 to 90,000 g/mol in order to be close to the experimental range. The morphology changes corresponding to the EW, MW, and hydration level were comparatively investigated by inspecting the water distributions, followed by quantitative analysis by radial distribution functions and Bragg spacing according to the periodicity of water domains. Compared to the morphologies of short-side-chain PFSA membrane (Wu, D.; Paddison, S. J.; Elliott, J. A. Macromolecules 2009, 42, 3358-3367), the longer side chain in 3M PFSA membrane provides more flexibility for the sulfonate-terminated side chains and generally results in the stronger aggregation of water clusters. This results in lower water uptake for higher EW, corresponding to a lower ion-exchange capacity (IEC), which is attributed experimentally to a higher crystallinity of the fluorocarbon phase, although our simulations were not able to observe the crystallites directly.

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An extensive SAXS investigation of the 3M perfluorinated sulfonic acid ionomer was performed to investigate the morphological changes that occur during and after annealing at temperatures above the T α. The effect of film thickness in the range studied, 11–45 μm, was found to be negligible. These properties were studied as a function of equivalent weight from 700 to 1100 and correlated with the water uptake as measured by dynamic vapor sorption. Isoscattering points were observed in dynamic annealing experiments of the unboiled annealed films at q = 0.023, 0.096 Å–1. On initial water uptake these films also showed isoscattering points at q = 0.024, 0.220 Å–1; q = 0.029, 0.223 Å–1; and q = 0.030, 0.211 Å–1 at 50, 80, or 95 °C, respectively, indicating a decrease in the symmetry of the scattering objects in these size regimes. Isoscattering points were absent in similar water uptake experiment for the films after boiling.