## 3 Results and Discussion
### Structural Characterization of the Membranes by XPS Analysis  
X-ray photoelectron spectroscopy (XPS) was used to detect the changes in surface chemistry resulting from plasma treatment and hydrogel coating by identifying new functional groups. Figure 7 presents the XPS survey spectra of H-1, LEP-H1, and H-6. The analysis of major peaks corresponding to fluorine (F1s), carbon (C1s), and oxygen (O1s) serves as a key indicator of the chemical composition and structural properties of the polymer. As expected, H-1 exhibited dominant F1s and C1s peaks, along with a low atomic ratio of O1s, reflecting the intrinsic composition of the polymer.  
Figure 6: FT-IR characterization of the H-1, LEP-H1, and the H-6.  
Figure 8 focuses on the XPS core-level spectra of the C1s and O1s regions, which provide further details about the chemical modifications across the different membrane states. In panel (a), the C1s spectrum for H-1 shows peaks at 290.72 eV, 286.22 eV, and 284.61 eV, corresponding to the CF\({}_{2}\), CH\({}_{2}\), and CH groups, respectively, which are consistent with its inherent chemical structure [65]. Panel (b) presents the C1s spectrum for LEP-H1, where the CF\({}_{2}\) peak at 290.03 eV is less intense (27.06%) compared to H-1, suggesting a reduction in fluorine content. The CH\({}_{2}\) peak at 286.57 eV (34.38%) shifts slightly, and its intensity decreases, indicating structural modifications. A new peak at 286.70 eV (22.76%) is attributed to carbonyl (C=O) groups, signifying that the LEP treatment introduces oxidation or functionalization at the PVDF surface. Additionally, a C=O peak at 289.30 eV (6.46%) further supports this observation. In panel (c), the C1s spectrum for H-6 shows a reduced CF\({}_{2}\) peak (290.97 eV, 16.62%) compared to H-1 and LEP-H1, suggesting a reduction in fluorine content in the modified PVDF. The CH\({}_{2}\) peak at 284.86 eV (49.57%) becomes significantly more intense, reflecting a higher concentration of CH\({}_{2}\) groups on the surface after the AM-NaA treatment. The appearance of C=O peaks at 288.39 eV (16.91%) indicates surface oxidation and functionalization during the AM-NaA treatment.  
The presence of small oxygen peaks in the untreated PVDF membrane (panel (d)) (such as C=O at 532.33 eV and C-O at 533.66 eV) could be attributed to surface contamination or adsorbed oxygen from the environment. Panel (e) shows the O1s spectrum for LEP-H1, where the peak at 532.23 eV (90.99%) is more pronounced, signifying a higher degree of surface oxidation and the presence of more carbonyl groups, consistent with the changes observed in the C1s spectrum. Panel (f) presents the O1s spectrum for H-6, where the peak at 531.27 eV (70.08%) corresponds to oxygen in carbonyl (C=O) groups, further confirming surface oxidation. The binding energy at 536.02 eV confirms the presence of -OH groups in the prepared AM-NaA/PVDF membrane.  
XPS analysis of the H-6 membrane also reveals a nitrogen content of 2.72%, attributed to amide groups in the hydrogel, as detected in the N 1s XPS spectrum (Figure S3). Additionally, a significant sodium content (10.87%) was detected in the Na 1s spectrum (Figure S3), corresponding to sodium acrylate units, where sodium exists as a counterion to the carboxylate groups in the copolymer structure.  
Figure 7: XPS survey spectra of H-1, LEP-H1, and H-6.  
The atomic compositions and O/C ratios, as summarized in Table 2, demonstrate the progressive chemical modifications of the PVDF membranes. H-1 shows a low O/C ratio of 0.077, reflecting its inherently hydrophobic nature. Plasma treatment increased the O/C ratio to 0.122, indicating the successful introduction of oxygen-containing functional groups. The O/C ratio further rose significantly to 0.427, highlighting substantial surface functionalization and enhanced hydrophilicity due to the hydrogel coating.  
Figure 8: XPS core-level spectra of H-1, LEP-H1, and H-6. The panels on the left display the C1s spectra, while those on the right depict the O1s spectra.