## 3 Results and Discussion
### Surface Morphology  
SEM was used to analyze the surface morphology of the prepared membranes, as shown in Figure 3. The untreated PVDF membrane (H-1) (Figure 3a) exhibited a relatively smoother surface with disordered porous structures. In contrast, the LEP-H1 membrane (Figure 3b) displayed a rougher surface with more pronounced porosity due to the plasma etching process. [59; 60]. Figure 4b shows that the mean pore size of the LEP-H1 increased slightly to 0.574 um compared with 0.503 um for the H-1 (Figure 4a), suggesting that plasma treatment alters membrane pore morphology [61; 62]. The values of the surface roughness (_Ra_) increased considerably from 558 nm (Figure 3g) for the H-1 to 1046 nm (Figure 3h) for the LEP-H1, further evidencing the impact of plasma treatment.  
The cross-section of the AM-NaA/PVDF membrane with sample H-6 (Figure 3f) revealed that the pores remained unblocked by the hydrogel, with a total membrane thickness of 95.23 \(\pm\) 5.14 um. The surface of sample H-6 (Figure 3e) exhibited a more porous and uniformly structured morphology compared to H-3 and H-4 (Figure 3c,d), respectively, consisting of a sponge-like skin layer, with an average pore size of 0.549 um (Figure 4c) and _Ra_ of 770 nm (Figure 3i). The morphology uniformity can be attributed to the hydrophilic surface of LEP-H1, which facilitated the adhesion of the AM-NaA solution.  
For the copolymer P(AM-NaA), the reactivity ratios of AM and NaA are reported as 1.326-1.003 and 0.228-0.295, respectively [63]. It can be predicted that the distribution of the two monomer units on the copolymer chain is relatively uniform, and their content is related to the monomer feed ratio, which is more obvious in Figure 3c-e. In sample H-3, with a 1:1 AM/NaA mass ratio, the relatively balanced incorporation of both monomers likely contributed to a certain uniform pore size distribution due to a consistent chain composition. In contrast, for sample H-4, since the content of the AM monomer in the copolymer is larger than that of NaA, the hydrogen bonding between the copolymer chain segments is stronger, but due to the coexistence of -COO\({}^{-}\), the electrostatic repulsion between the chain segments cannot be ignored. However, the presence of -COO\({}^{-}\) groups from NaA  
Figure 2: (**a**) Grafting yield (GR%) and (**b**) water contact angle (WCA) of H-6 as a function of reaction time.  
introduced electrostatic repulsion, counteracting the hydrogen bonding and resulting in a slightly larger and less uniform pore size. Thus, the microstructure of the copolymer hydrogel is governed by the balance between hydrogen bonding and electrostatic repulsion, influencing polymer chain packing, as schematically depicted in Figure S2.  
The EDS spectra of the LEP-H1 (Figure 5(a3,a6)) demonstrated a uniform oxygen distribution on the membrane surface, suggesting that the plasma process effectively introduced oxygen-containing functional groups on the membrane surface. Furthermore, the EDS spectrum of the H-6 indicated that the elements F, O, C, and Na are uniformly distributed across the membrane surface (Figure 5(b1-b6)). The appearance of Na elements further verified that acrylate was successfully loaded on the PVDF membrane surface.  
Figure 3: Morphology analysis of (**a**) SEM of H-1 (10,000\(\times\)), (**b**) SEM of LEP-H1 (10,000\(\times\)), (**c**) SEM of H-3 (5000\(\times\)), (**d**) SEM of H-4 (5000\(\times\)), (**e**) SEM of H-6 (5000\(\times\)), (**f**) cross-sectional SEM of H-6 (2000\(\times\)), and (**g**–i) AFM images of H-1, LEP-H1, and H-6, respectively.  
Figure 4: Pore size and pore size distribution (PSD) of (**a**) H-1, (**b**) LEP-H1, and (**c**) H-6.  
Figure 5: Elemental mapping of (**a1**–**a5**) LEP-H1 and (**b1**–**b5**) H-6, and the elemental distribution of (**a6**) LEP-H1 and (**b6**) H-6.