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Polymers 2021, 13, 359 8 of 11 The highest value of WU was reached for the uncrystallized membrane. The thickness of this sample is comparable to commercial Nafion 212, i.e., about 50 μm. Both membranes have been obtained by recasting method; however, the preparation of the uncrystallized does not use a high-boiling solvent, in contrast to Nafion 212. In fact, as reported in the literature (e.g., Alberti [26], Gebel [25], Moore [24]), the use of high-boiling solvents such as DMF and DMSO increases the crystallinity on both preformed and casting membranes. Therefore, the uncrystallized Nafion membrane demonstrated a weaker structure. Fi- nally, the Nafion 117 oriented presented a lower density, had a higher equivalent volume, and therefore, had a greater willingness to host water [19]. H PFG NMR Investigation 1H-PFG NMR experiments allowed us to investigate the molecular dynamics of the water confined inside the prepared electrolytes, via the direct measurements of the water self-diffusion coefficients (D) [33,34]. Figure 3 shows the Arrhenius plot of the water self- diffusion coefficients measured on completely swelled membranes in the temperature range between 20 and 130 ◦C. For all the membranes, the diffusivity increases with temperature up to 80 ◦C due to thermal energy, then drops precipitously as a consequence of the evaporation of the bulk-like water fraction. In this regard, once above 80 ◦C, the D values of the various Nafion membranes become quite comparable and merge towards the same value. This is indicative that the “bound-water” fraction that remains in the ion clusters as hydration to sulfonic groups is almost invariable, regardless of the casting procedure/preparation of the polymeric film. Instead, the analysis of D in the temperature range of 20–80 ◦C allows us to clarify the relationship between proton transport properties and a membrane’s nanomorphology: Despite the highest water uptake (29 wt%), uncrystallized Nafion displays slightly lower D than both Nafion 212 and Nafion recast (w.u. 22 wt% and 24 wt%, respectively). Likely, the decreasing of the crystallinity degree induces a more branched channel structur- ing, increasing the diffusion path’s tortuosity for water, thus slowing the overall diffusivity. A Nafion membrane structure more regular with a lamellar morphology of the poly- mer chains reduces the mobility of water molecules, being limited to only two directions. Indeed, Nafion 117 and, even more, the Nafion 117 oriented, show a significant decrease of water diffusion. 3.3. Dynamic Mechanical Analysis Figure 4 shows the temperature evolution of storage modulus (E’) and dumping factor (tan δ) of all the membranes. It can be observed that the storage moduli of Nafion 212 and Nafion recast (both thermally activated before the characterization) are almost comparable, and are the highest among the investigated membranes: E’ for these membranes is ca. 200 MPa, which is almost 3.5-fold higher than the other samples in which the storage modulus is ca. 60 MPa. Figure 4. Storage modulus (a) and tan δ (b) versus temperature of PFSA membranes.PDF Image | Morphology of Nafion-Based Membranes Affects Proton Transport
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