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α peak [44]. However, very little is known about thermal transitions in Li-Nafion; differ- Polymers 2021, 13, 1150 ential scanning calorimetry (DSC) measurements observed relaxations at 81 °C [49] and 212 ± 18 °C [1]. In the present work, DSC measurements were carried out to analyse the thermal be- haviour of lithiated Nafion compared with its protonated form. Figure 6 provides the DSC 9 of 14 curves for the “dry” samples of Nafion 115 in H+ and Li+ forms. The region near –100 °C at the γ transition was not analysed due to instrumental effects. As can be seen from Fig- ure 6, the DSC curve of H+ Nafion demonstrates thermal effects typical for a protonated Rmefesm.[b1r,a4n1e–,4i8n]c).ludMinogretohveewr,etlhl-edetrfainesditieonndotethmeprmeriactuefrfesctdoofcnroytstalwlinaeysmfealltlinwgiathti2n74th°Ce, inwdhiiccahteids elinmtirtesl;ythceoynsairseteanltsowitnhfluGeienrckeed’sbryesruesltisdu[2a5l]wanatdero(usreeW, AfoXr Dexdamatapl(es,ewe oSerkctiofn A3l.m2)e.idaandKawano[43]).ThemeltingpointofH+NafiondeterminedbyDSCwasabout 275◦CFo[2r5L].i-INtiasfsiiogn,ifithceanDtSthCatctuhrevmeeshltoinwgsinaNtrafinosnitiocncunresaorv7e7ra°Cv,erwyhbircohadistcelmospertaotuthreat ◦+◦ rraenpgoertbeedlobwy2th7e5 gCro[u2p5]o.fTChheraenfgocrhe,utnheInrmstiatlutreanofsiAtiopnpslieindHCheNmaifistornya[4b9o]v,eal2o0n0g wCiathrea usually associated with crystalline melting [43,46–48]. The replacement of mobile protons diffused double effect at 198 and 218 °C (Figure 6). However, the available data did not by alkali metal cations increases both α and β relaxation temperatures by over 100 ◦C, allow us to identify the double peak at 198 and 218 °C reliably. The latter effect’s temper- while the γ relaxation temperature is around 100 ◦C [1]. It should be noted that for some ature is close to the value reported in the literature for the α relaxation in lithiated PFSA ionic forms of Nafion, the β relaxation occurs as a shoulder on the low-temperature side [1]. However, it seems more likely that the effect at 218 °C is a weak endothermic peak of the α peak [44]. However, very little is known about thermal transitions in Li-Nafion; related to crystallite melting, while the effect at 198 °C may be attributed to α relaxation. differential scanning calorimetry (DSC) measurements observed relaxations at 81 ◦C [49] Although a double effect also observed by Ozerin et al. [46] for the protonated form of and 212 ± 18 ◦C [1]. PFSA at 227 and 239 °C was attributed by the authors to crystalline melting, the reasons In the present work, DSC measurements were carried out to analyse the thermal for its splitting were not discussed in the cited work. DSC curves for Na, K, Rb, and Cs behaviour of lithiated Nafion compared with its protonated form. Figure 6 provides the Nafion salts presented by Almeida and Kawano [43] exhibit a very similar diffused double DSC curves for the “dry” samples of Nafion 115 in H+ and Li+ forms. The region near effect, with the higher value attributed by the authors to crystallite melting. Considering –100 ◦C at the γ transition was not analysed due to instrumental effects. As can be seen from the literature data, we tend to attribute the transition at 218 °C to crystal melting. A likely Figure 6, the DSC curve of H+ Nafion demonstrates thermal effects typical for a protonated explanation for the remarkable decrease in the temperature from 274 °C (protonated form) membrane, including the well-defined endothermic effect of crystalline melting at 274 ◦C, to 218 °C (Li-Nafion) (Figure 6) is a reduction in the average size of crystallites resulting which is entirely consistent with Gierke’s results [25] and our WAXD data (see Section 3.2). from the lithiation procedure. ++ FFigiguurere66. .Dififefererenntitaiallssccaannnininggccaalolorrimimeetrtryy(D(DSSCC))ccuurvrveessfoforrththee““ddryry””ssaampplelessooffNaafifoionn11155ininHH and Li+ forms. and Li+ forms. For Li-Nafion, the DSC curve shows a transition near 77 ◦C, which is close to that reported by the group of Changchun Institute of Applied Chemistry [49], along with a diffused double effect at 198 and 218 ◦C (Figure 6). However, the available data did not allow us to identify the double peak at 198 and 218 ◦C reliably. The latter effect’s temperature is close to the value reported in the literature for the α relaxation in lithiated PFSA [1]. However, it seems more likely that the effect at 218 ◦C is a weak endothermic peak related to crystallite melting, while the effect at 198 ◦C may be attributed to α relaxation. Although a double effect also observed by Ozerin et al. [46] for the protonated form of PFSA at 227 and 239 ◦C was attributed by the authors to crystalline melting, the reasons for its splitting were not discussed in the cited work. DSC curves for Na, K, Rb, and Cs Nafion salts presented by Almeida and Kawano [43] exhibit a very similar diffused double effect, with the higher value attributed by the authors to crystallite melting. Considering the literature data, we tend to attribute the transition at 218 ◦C to crystal melting. A likely explanation for the remarkable decrease in the temperature from 274 ◦C (protonated form) to 218 ◦C (Li-Nafion) (Figure 6) is a reduction in the average size of crystallites resulting from the lithiation procedure. 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 behaviourPDF Image | Li-Nafion Membrane Plasticised with Ethylene Carbonate Sulfolane
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