## Results  
### Effect of the Binder Concentration on the Thermal Evolution and Structure of Membranes  
STA data obtained for PAA powder and powder with a binder in the range of 20-370 \({}^{\circ}\)C are shown in Figure 1.  
Two endothermal effects at 57.1 and 126.3 \({}^{\circ}\)C, and one exothermal effect at 369.1 \({}^{\circ}\)C, are presented on the DTA curve of PAA powder without binder (100 PAA) in Figure 1a. All of them are followed by mass losses according to the TG curves, as shown in Figure 1b. The endothermal effects can be attributed to adsorbed and structural water losses of polyantimonic acid, respectively. At temperatures higher than 350 \({}^{\circ}\)C, the reduction of Sb\({}_{2}\)O\({}_{5}\) takes place and is accompanied by a profound exothermic effect on the DTA curve and by a considerable mass loss. Binder addition affects mostly the initial part of the curve (up to 100 \({}^{\circ}\)C). The first endothermal effect shifts from 57.1 to 100.5 \({}^{\circ}\)C. Additionally, on the DTA curves obtained for 90 PAA and 80 PAA samples, small but rather distinct endothermal effects are seen at \(\sim\)190 and 232 \({}^{\circ}\)C. Their presence is likely due to  
Figure 1: (**a**) DTA curves and (**b**) TG curves of 100 PAA, 90 PAA and 80 PAA powders. the removal of fluoroplastic from the sample surface. An additional endothermic effect with the maximum at \(\sim\)300 \({}^{\circ}\)C is present in the curve for the 80 PAA sample. This may be attributed to fluoroplastic binder melting and the beginning of polymer degradation. At temperatures higher than 303 \({}^{\circ}\)C, the extended exothermic effect, related to binder burnout, is seen. The types and temperature effects are summarized in Table 1, together with mass losses according to TG data. As can be seen from the comparison of STA and TG data, the mass losses of pure PAA in the region of 20-350 \({}^{\circ}\)C can be attributed to water losses. This assumption is in accordance with data in the literature on PAA thermolysis [39]. Based on mass losses of up to 350 \({}^{\circ}\)C, the composition of PAA after synthesis was calculated to be Sb\({}_{2}\)O\({}_{5}\):2.92H\({}_{2}\)O. Roughly three water molecules are attributed per each Sb\({}_{2}\)O\({}_{5}\) in the PAA structure. Based on experimental STA data and data from [39], the decomposition of hydrated antimony oxide takes place accordingly to a two-step schematic reaction sequence, described by Equations (4) and (5):  
\[(2.82\mathrm{H}_{2}\mathrm{O})\mathrm{Sb}_{2}\mathrm{O}_{5}\ 0.1\mathrm{H}_{2} \mathrm{O}\ \rightarrow\ (2.82\mathrm{H}_{2}\mathrm{O})\mathrm{Sb}_{2}\mathrm{O}_{5}+0.1 \mathrm{H}_{2}\mathrm{O},\ \mathrm{T}=25\text{--}100^{\circ}\mathrm{C} \tag{4}\]  
\[\mathrm{Sb}_{2}\mathrm{O}_{5}\ 2.82\mathrm{H}_{2}\mathrm{O}\ \rightarrow\ \mathrm{Sb}_{2}\mathrm{O}_{5}+2.82\mathrm{H}_{2} \mathrm{O},\ \mathrm{T}=100\text{--}250^{\circ}\mathrm{C} \tag{5}\]  
Then, PAA powder and fluoroplastic binder were compacted into the bulk membranes. The relative densities of 90 PAA and 80 PAA membranes were calculated using mixture rule as 101.6% and 101.4%. Crystallographic density was taken for the calculation. The theoretical density was calculated using crystallographic density of Sb\({}_{2}\)O\({}_{5}\):3H\({}_{2}\)O from XRD data (PDF card 01-084-0303), whereas the fluoroplastic density data were obtained experimentally by a pycnometry technique. For both samples, the relative density slightly exceeded 100%. The differences observed may be due to changes in polymer density during compaction upon the membrane compaction. Literature data on fluoroplastic powder density varied significantly, from 1.6-2.2 g/cm\({}^{3}\)[39; 40; 41]; the fluoroplastic could also contain some co-polymer, such as polystyrene or polyethylene, in the different ratios. Therefore, it can be stated that within the determined experimental relative density error, the chosen compaction conditions are suitable for obtaining fully dense membranes.