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Polymers 2021, 13, 359 4 of 11 remove all the organic impurities, (3) 1M H2SO4 at 80 ◦C for 1 h to remove any metallic impurities, (4) 0.001 M ethylenediaminetetraacetic acid (EDTA) solution at RT for 1 day to remove all the paramagnetic contaminants (such as the copper contained in the Nafion commercial solution), (5) 2 M HCl at 80 ◦C for 2 h, and (6) EDTA. After each acidic treatment, the membrane was rinsed three times in boiling deionized H2O for 10 min to remove any trace of the acids. The thickness of the dry membranes was about 50 μm. 2.5. Water Uptake (WU), λ and nc The samples were kept in liquid water at 25 ◦C for 24 h in a hermetically-sealed Teflon vessel. The membranes were carefully wiped off and the mass was determined (mwet). They were then dried over P2O5 for 3 days and weighed (mdry). The WU was calculated according to Equation (1): mwet − mdry WU = m The hydration number λ, i.e., the number of water molecules per SO3H group, was calculated by Equation (2): WU λ= IEC×M(H2O) ×10 (2) where the ion exchange capacity (IEC) of Nafion 1100 is 0.909 meq. g−1 and M is the molar mass of water. The uncertainty is estimated to be about ± 0.5. dry ×100 (1) The λ values were converted into nc values by the Equation (3): nc = 100 (3) λ−6 This equation is valid for λ ≥ 10 [5,20]. 2.6. 1H NMR Spectroscopy NMR The NMR measurements were performed with a Bruker AVANCE 300 Wide Bore NMR spectrometer working at 300 MHz on 1H, and equipped with a Diff30 Z-diffusion 30 G/cm/A multinuclear probe with exchangeable RF inserts (Bruker, Milan, Italy). The self-diffusion coefficients (D) of water were determined by pulsed field gradient stimulated-echo (PFG-STE) technique [29]. The sequence consists of three 90◦ rf pulses (π/2-τ1-π/2-τm-π/2) and two gradient pulses applied after the first and the third RF pulse. At time 2τ1 + τm, the echo was found. The FT echo decays were analyzed by means of the relevant Stejskal–Tanner expression Equation (4): I = I0 e−βD (4) where I and I0 represent the intensity/area of a selected resonance peak with and without gradients, respectively, D the self-diffusion coefficient, and β the field gradient parameter. Following the usual notation, the magnetic field pulses had amplitude g, duration d, and time delay ∆. Accordingly, the field gradient parameter can be defined by Equation (5): 2 δ β= (γgδ) ∆−3 (5) The used experimental parameters were: δ = 0.8 ms, time delay ∆ = 8 ms, and the gradient amplitude varied from 100 to 900 G cm−1. Based on the very low standard deviation of the fitting curve and repeatability of the measurements, the uncertainties of the self-diffusion measurements were approximately 3%. The NMR samples were prepared according to the procedure described in detail elsewhere [30]. The self-diffusion coefficientsPDF Image | Morphology of Nafion-Based Membranes Affects Proton Transport
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