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Polymers 2021, 13, 359 9 of 11 This outcome confirms that the thermal activation procedure is crucial to achieving tough and mechanically stable polymeric film. As noted in Table 2, the Nafion 212 and recast PEMs show a high value of nc, which, according to Alberti et al. [5], is related to a greater mechanical stability. It is worth noting that the storage modulus of uncrystallized Nafion starts decreasing at 70 ◦C, whereas the other membranes show a high E’ until 100 ◦C. As described in the experimental methods, the Nafion uncrystallized was prepared in absence of high-boiling solvents. This reduces the overall crystallinity of the membrane and clearly has a detrimental effect on its thermal resistance. The dumping factor plots showed in Figure 4b further elucidates the relationship between thermal resistance and membranes architecture. The tan δ profile of each Nafion membrane is characterized by a single peak in the high temperature region that is typically related to the α transition (Tg) of the ionic clusters [35,36]. In the case of solution-cast membranes, the Tg of uncrystallized Nafion is shifted to lower temperatures (i.e., 128 ◦C) in comparison with Nafion 212 and Nafion recast, which show the α transition at ~145 ◦C. The outcome, which is clearly amenable to the lower crystallinity of the uncrystallized Nafion, de facto limits the practical application of this PEM in high-temperature fuel cell devices, which has a temperature target fixed by DOE of 120 ◦C. Turning our attention to the extruded membranes, the higher Tg of Nafion 117 oriented compared to the parental Nafion 117 (i.e., 139 ◦C vs. 132 ◦C, respectively) is compatible with a reduced flexibility of the polymer chains after strict orientation of its ionic clusters, which need more energy to move, leading to a higher relaxation temperature. 4. Conclusions This study extends our knowledge of the morphology of Nafion membranes from previous INCA method results. In particular, it has been argued that the decay of the through-plane conductivity in Nafion-oriented membranes is the result of the formation of “oriented-ribbon”-type morphologies and poor connections between ion clusters, which macroscopically is accompanied by a consistent decrease in density. In this study, both in-plane and through-plane conductivities were investigated using a wide range of tem- peratures (20–120 ◦C). There was evidence of a strong anisotropy between σTP and σIP in the case of extruded membranes, which increases from 20% for the Nafion 117 commercial membrane to almost 106% in the Nafion 117-oriented membrane. In this last sample, the lamellar-like morphology significantly reduced the water diffusivity because it was restricted in only two directions. Regarding the uncrystallized Nafion, in previous studies, we observed a strong lower- ing of nc/T plots due to either an important decreasing in crystallinity, or more likely due to a decomposition of the hydrogen bonds between adjacent ribbons. The high water uptake ability of this membrane, however, is not accompanied by a corresponding high conductiv- ity. On the contrary, both the lower proton conductivity and water diffusion, compared to Nafion 212 and Nafion recast, suggest that the decreased degree of crystallinity induces more branched channel structuring, increasing tortuosity and slowing down the overall transport properties. In addition, the β-transition shifts at 20 ◦C lower than the other solution-cast membranes, which, along with having a lower storage modulus, definitely limits the practical application of uncrystallized Nafion. On the other hand, materials with a high degree of crystallinity induced by the use of high-boiling solvents, such as DMF and DMSO, have optimal structural characteristics for use in medium/high-temperature fuel cells. Author Contributions: Conceptualization, R.N. and I.N.; methodology, R.N., I.N. and M.L.D.V.; software, C.S. and E.L.; Writing-Original Draft Preparation, E.L. and R.N.; Writing-Review & Editing, C.S. and I.N.; validation, R.N. and E.L. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding.PDF Image | Morphology of Nafion-Based Membranes Affects Proton Transport
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