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Polymers 2021, 13, 1150 3.5. Thermal Analysis and Wide-Angle X-Ray Diffraction of Swollen Li-Nafion Samples It is known that the morphology of an ionomer re-organises as the dry membrane absorbs a solvent; these structural changes following swelling can be observed by DSC and WAXD measurements. Therefore, the influence of a solvent on the thermal behaviour of lithiated Nafion was studied step-by-step during a gradual increase in solvent uptake. Ethylene carbonate was used as a solvent in these measurements. A weighted portion of EC was added to a piece of membrane to obtain a sample with a specified solvent uptake; Ethylene carbonate was used as a solvent in these measurements. A weighted portion of EC sation. An Li-Nafion sample saturated with EC at 40 °C was prepared as described in was added to a piece of membrane to obtain a sample with a specified solvent uptake; after Section 2.3. ◦ that, samples were stored for 24 h at the mixing temperature of 40 C for homogenisation. Figure 7 demonstrates DSC curves for Li-Nafion samples having different EC con- An Li-Nafion sample saturated with EC at 40 ◦C was prepared as described in Section 2.3. tents. It can be seen that the introduction of a relatively small quantity of EC (W = 19%) Figure 7 demonstrates DSC curves for Li-Nafion samples having different EC contents. led to an abrupt decrease in the glass transition temperature from 77 °C of a dry membrane 10 of 14 of lithiated Nafion was studied step-by-step during a gradual increase in solvent uptake. after that, samples were stored for 24 h at the mixing temperature of 40 °C for homogeni- It can be seen that the introduction of a relatively small quantity of EC (W = 19%) led to an to −2 °C without phase separation. With an increase in W to 32%, the glass transition tem- abrupt decrease in the glass transition temperature from 77 ◦C of a dry membrane to −2 ◦C perature shifted even more to the negative region to reach –39 °C. Here, it is likely that without phase separation. With an increase in W to 32%, the glass transition temperature saturation with EC (W = 132%) shifts the glass transition temperature of the membrane to shifted even more to the negative region to reach −39 ◦C. Here, it is likely that saturation below –80 °C (not observed in this work), while the DSC curve shows only the endother- with EC (W = 132%) shifts the glass transition temperature of the membrane to below mic effect of the first-order phase transition at 36.5 °C associated with the melting of eth- −80 ◦C (not observed in this work), while the DSC curve shows only the endothermic ylene carbonate released from the membrane upon cooling to 25 °C (Figure 7). effect of the first-order phase transition at 36.5 ◦C associated with the melting of ethylene carbonate released from the membrane upon cooling to 25 ◦C (Figure 7). Figure 7. DSC curves for Li-Nafion with different contents of EC (40 ◦C/min). Figure 7. DSC curves for Li-Nafion with different contents of EC (40 °C/min). The obtained results indicate that even pure EC, which is solid at room temperature, The obtained results indicate that even pure EC, which is solid at room temperature, represents a very effective plasticiser for Li-Nafion. The destruction of ionic crosslinking represents a very effective plasticiser for Li-Nafion. The destruction of ionic crosslinking due to the solvation of lithium cations by EC and charge screening releases segmental due to the solvation of lithium cations by EC and charge screening releases segmental mobility of perfluorinated Nafion backbones. The structural rearrangements in the swollen mobility of perfluorinated Nafion backbones. The structural rearrangements in the swol- membrane result from the decrease in Tg to very low values. len membrane result from the decrease in Tg to very low values. The WAXD results fully support the assumption of structural changes. For the study, The WAXD results fully support the assumption of structural changes. For the study, we selected samples saturated with a mixed solvent EC/SL with ω(SL) = 0.2 at various we selected samples saturated with a mixed so◦lvent EC/SL with ω(SL) = 0.2 at various mixing temperatures: 25, 40, 60, and 80 C. (Mixtures with ω(SL) = 0.2 were used for mixing temperatures: 25, 40, 60, and 80 °C. (Mixtures with ω(SL) = 0.2 were used for mem- ◦ membrane swelling instead of pure EC due to being entirely homogeneous at 25 C brane swelling instead of pure EC due to being entirely homogeneous at 25 °C according according to the EC/SL phase diagram [31]). Figure 8 shows diffractograms of swollen to the EC/SL phase diagram [31]). Figure 8 shows diffractograms of swollen membranes membranes having a diverse thermal history in comparison with a dry membrane. The having a diverse thermal history in comparison with a dry membrane. The appearance of appearance of a new shoulder on the side of large angles at the most intense peak (100) can a new shoulder on the side of large angles at the most intense peak (100) can be clearly seen. be clearly seen. Figure 9 shows an example of the decomposition into components of the measured profile in the region 2θ = 7–33◦. As can be seen from the figure, the saturation with the plasticiser causes the appearance of a new broad peak (new amorphous halo) having a maximum at 2θ = 19.1–19.4◦, which was absent in the dry membrane WAXD profile (right shoulder in Figure 8). The intensity of this peak noticeably increases with mixing temperature (and, accordingly, with solvent uptake; see Figure 4). Simultaneously, the intensity of the amorphous halo, which is identical to that appearing in the diffraction pattern of a “dry” membrane (Figure 5), decreases significantly. The WAXD techniquePDF Image | Li-Nafion Membrane Plasticised with Ethylene Carbonate Sulfolane
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