## 3 Results and Discussion
Figure 5 shows FT-IR spectra of NR212/AA-DVB_2 to confirm the synthesis of the C layer on one surface of NR212. In the FT-IR spectra of NR212, a sharp peak associated with the asymmetric stretching vibration of -SO3- groups is observed at 1060 cm\({}^{-1}\). After AA-DVB C layer formation, the peak is weakened owing to the limited penetration depth of FT-IR beam [21]. The NR212/AA-DVB_2 spectra has characteristic peaks assigned to C=C stretch in the aromatic rings of DVB, and -CH stretch in the main chain of AA-DVB layer [15; 22]. NR212/AA-DVB_2 also exhibits a strong C=O vibration peak at around 1700 cm\({}^{-1}\). This is attributable to carboxylic acid groups which come from  
It is necessary to reveal the effect of the C layer on the transport behavior of Na\({}^{+}\) ions through NR212/AA-DVB membranes for a desirable SWE membrane design. Figure 6 shows Na\({}^{+}\) ion conductivity of NR212/AA-DVB membranes at the elevated temperatures. In all the membranes including Flemion(r) and free-standing membranes (i.e., NR212 and AA-DVB) used as references, Na\({}^{+}\) ion conduction becomes fast, as the temperature increases, owing to the enhanced diffusion of Na\({}^{+}\) ions and the relatively fast molecular motion of their polymer chains. Among them, AA-DVB membrane exhibit the lowest Na\({}^{+}\) ion conductivity. On the other hand, NR212 exhibits the highest Na\({}^{+}\) conductivity, because of its well-defined hydrophilic-hydrophobic microphase-separated morphology derived from its side chains containing highly acidic sulfonic acid groups at their terminal ends and polytetrafluoroethylene main chains with extremely high hydrophobic character [23; 24; 25]. The Na\({}^{+}\) conductivity values of AA-DVB membrane are at least 10 time lower than those of NR212 with the identical thickness of 50 \(\upmu\)m. It may be due to the combined effect of the difference in acidity between carboxylic acid in AA-DVB and sulfonic acid groups in NR212, and restricted chain mobility associated with crosslinked AA-DVB matrix. The fairly reduced water uptake of AA-DVB in Table 1 also seems to negatively affect its Na\({}^{+}\) ion conduction. Note that Na\({}^{+}\) ions transport in the hydrated state via the formation of ionic complexes with water molecules. For example, the average hydration number of Na\({}^{+}\) ion, which is defined as the number of water molecules strongly bound to a Na\({}^{+}\) ion, is 4.6 in the temperature range from 47 to 95 \({}^{\circ}\)C [26]. Accordingly, the increase in C layer thickness causes the Na\({}^{+}\) ion conduction through the double-layered membranes to be reduced. Even the reduced Na\({}^{+}\) conductivity values of NR212/AA-DVB membranes are superior to those of Flemion(r). Another important membrane parameter to determine electrochemical SWE characteristics is OH\({}^{-}\) ion permeability, which is a measure of how fast OH\({}^{-}\) ions can be diffused across the test membranes in the fully hydrated state where SWE operation is carried out. Figure 7 shows the changes of OH\({}^{-}\) concentration and pH value in 0.01 M HCl solution filled in chamber B of the two-chamber diffusion cell as a function of times. Here the chamber B simulates the anolyte reservoir filled with the identical acidic solution in a SWE cell, and NaCl as the anodic reactant is excluded to electrochemically detect only OH\({}^{-}\) ions permeated through the membranes. The initial pH value of the HCl solution is 2 at \(t=0\). When 31 wt % NaOH solution in the volume same to that of the HCl solution is introduced in the chamber A, OH\({}^{-}\) ions begin to diffuse through the membranes. Consequently, the OH\({}^{-}\) ion concentration in the acidic solution at the chamber B increases, irrespective of which membrane sample is used. The rate at which the OH\({}^{-}\) concentration increases (i.e., concentration gradient of OH\({}^{-}\) ions), becomes retarded, as the C layer thickness increases.