## 3 Results and Discussion  
The C layers in SWE membranes should have high chemical resistance to harsh alkaline attacks (e.g., 31 wt % NaOH), particularly at 90 \({}^{\circ}\)C which is a commonly used SWE operation temperature. The selection of materials for the C layer is, however, very limited, since most polymeric materials are easily chemically decomposed under these tough conditions. One of feasible solutions is to select crosslinked polymeric networks where carboxylic acid groups exist. In the design of this polymeric network, AA was expected to form the polymer main chains containing carboxylic acid groups, while DVB was introduced as a crosslinker to induce the interconnected polymeric networks. BIE and AIBN were used to initiate the free radical polymerization via photolysis [17] and thermolysis [18] under the UV irradiation where heat is generated, respectively. The formation of crosslinked polymeric networks as shown in Figure 1 was confirmed on the basis of the solubility test of AA-DVB membrane conducted in representative polar aprotic solvents (i.e., N-methyl-2-pyrrolidone, dimethylacetamide, dimethylsulfoxide, dimethylformamide) at 60 \({}^{\circ}\)C for one day. The AA-DVB membrane was completely insoluble in all the polar aprotic solvents. It indicates that its polymer chains are successfully interconnected each other to make crosslinked polymeric networks. The carboxylic acid groups in the AA-DVB may also experience inter- and/or intramolecular dimerization [19; 20] which induce physical crosslinking obtained as a result from hydrogen bonding between oxygen atoms in the carbonyl (C=O) groups and hydrogen atoms in the hydroxyl (-OH) groups as shown in Figure 1. The dimerization of the carboxylic acid groups is expected to be formed during the AA-DVB synthesis, and not to be easily broken even after 0.5 M NaCl treatment, because the interaction of carboxylic acid groups to alkali metal cations is much weaker than that of sulfonic acid groups, so carboxylic acid groups in the proton form (i.e., -COO-H+) would not be converted into those in the sodium form (i.e., -COO-Na+). Moreover, The AA-DVB membrane did not experience any weight change even when exposed to 31 wt % NaOH at 90 \({}^{\circ}\)C for one day. It means that the resulting AA-DVB layer is able to show high chemical resistance to the alkaline attack, which would be applied in the cathode side during a practical SWE operation, when used as the C layer in NR212/AA-DVB membranes. Figure 4 shows how to control the C layer thickness in the double-layered NR212/AA-DVB membranes. The theoretical thickness of the C layer was calculated on the basis of the density of free-standing AA-DVB membrane in the dry state (see Table 1) under an assumption that a certain content of AA-DVB in the coating solution was used without any losses during the spray coating. On the other hand, the actual C layer thickness in the dry state was obtained by subtracting the NR212 thickness from the whole thickness values of NR212/AA-DVB membranes which was accomplished when the same AA-DVB contents were coated on the NR212 membrane. All the C layer thickness values linearly increase as a function of AA-DVB contents. There are, however, some gaps between the actual and theoretical thickness values. It may be related with the losses of AA-DVB coating solution during the spray coating. The targeted C layer thickness can be accomplished using these relationships. In membrane appearance, NR212 used as S layer is transparent and colorless as shown in the edge of NR212/AA-DVB_2 membrane coupon in Figure 4. Meanwhile, AA-DVB layer on NR212 layer maintains its transparency, but show yellowish color. The yellowish color becomes darker as the C layer thickness gets thicker from 2 to 45 \(\upmu\)m. The density values of NR212/AA-DVB membranes in Table 1 decrease as the thickness of AA-DVB layer increases, regardless of which humidity condition was applied for their density evaluation. The density trend is associated with a relatively low density of AA-DVB in comparison to NR212.