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Polymers 2021, 13, 1150 7 of 14 Figure 4. Dependence of the weight uptake (W) and molar uptake (λ) of a solvent of the Li-Nafion membrane vs. mixing temperature at heating and cooling mode for the following solvents: ethylene carbonate (EC) (a); sulfolane (SL) (b); EC/SL with ω(SL) = 0.2 (c); EC/SL with ω(SL) = 0.7 (d). The effect of temperature on equilibrium water content is well known for the proto- nated form of Nafion. Nevertheless, for H+ Nafion, the increase in W with temperature is much less pronounced (see, for example, data for Nafion 115 in Ref. [33]; for Nafion 212 and Nafion XL, Ref. [34]): within the same temperature range of 40 to 80 ◦C, the water uptake increased by less than 10% and W did not exceed 40%. Such a dramatic difference in behaviour upon swelling suggests significant variations in the molecular mechanisms of solvent absorption between the protonated Nafion–water and lithiated Nafion–EC/SL systems. To the best of our knowledge, the present work is the first to report the strong influence of temperature on the aprotic solvent uptake for lithiated Nafion and observe the heating-cooling hysteresis. 3.4. Thermal Analysis and Wide-Angle X-ray Diffraction of “Dry” Membranes Despite having a large number of bulky side chains (Figure 1a), Nafion membranes are known to have significant levels of crystallinity [1,25,35]. This crystallinity is one of the most critical properties of Nafion, providing both its structural integrity and mechan- ical stability even at high levels of swelling [1]. As shown in Starkweather’s paper [35], the three-dimensional hexagonal structure of Nafion is similar to that of PTFE at ele- vated temperatures. Compared with PTFE, the X-ray diffraction peaks of Nafion are broadened and shifted toward smaller angles, indicating that crystals are smaller and less perfect [35,36]. It has been proposed that the lamellar morphology of crystallites is randomly distributed over the amorphous Nafion matrix [35,37,38]. For extruded Nafion membranes, the relative degree of crystallinity of about 7–17% is lower than that of PTFE (55–75%) [1]; the difference can be attributed to the effect of side chains and ionic groups hindering crystallisation [1,25,35]. In the present work, wide-angle X-ray diffraction was carried out as described in Section 2.5 for “dry” samples of Nafion 115 membrane in its H+ (as received) and Li+ forms in order to reveal the influence of the lithiation procedure (Section 2.1) on the degree of crystallinity and structural characteristics. Diffraction patterns of the samples in the 2θ 1PDF Image | Li-Nafion Membrane Plasticised with Ethylene Carbonate Sulfolane
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