A series of TFC membranes were fabricated using the polyelectrolyte-modified PAN-A substrates and characterized by SEM to generate the top views with varied magnifications and cross-sectional views in Figures 7a and 7b, respectively. It is evident that delamination occurred during the IP for fabricating the TFC-A and TFC-PDADMAC. This observation is consistent with the hypothesis that a surface more hydrophilic than that of the PAN-O substrate could be unfavorable for ‘locking’ the generated IP layer; the water wettability of the surface modified by PDADMAC was slightly higher than that of the PAN-O substrate, though the deposition of PDADMAC somehow increased the hydrophobicity of the surface of the PAN-A substrate as revealed by the contact-angle characterization in Figure 5.

All the IP layers generated on the polyelectrolyte-modified PAN-A substrates were characteristic of relatively dense and smooth substructures in terms of the top views at a larger magnification. A possible explanation is that the relatively hydrophobic surface could favor the diffusion of TMC in the organic phase toward the interface of polymerization and thereby enhance the degree of crosslinking⁴³, which might limit the growth of heterogeneous substructures. On the other hand, the cross-sectional views indicate that thinner IP layers (~200 nm) were formed on the surface of the PAN-A substrates modified by PEI and PAH. This observation is consistent with the hypothesis that the film formation should be dominated by the crosslinking that yielded a relatively dense network and accelerated the IP process toward the self-limiting regime⁴⁴. In addition, the cross-sectional view for the TFC-A and TFC-PDADMAC verifies the IP-layer detachment from the substrate.

The integrity of the TFC-PEI and TFC-PAH is confirmed by the characterization results of the water permeability and NaCl rejection, which are displayed in Figures 8a and 8b, respectively. Although the TFC-PAH yielded a lower value of the water permeability compared with that for the TFC-PEI, it achieved the highest rejection of NaCl with a value of ~90%. Having a higher value of water permeability and a lower value of NaCl rejection just manifest the imperfect IP layer formed on the PAN-A substrate modified by PDADMAC. The characterization results also imply that less part of the IP layer peeled off from the surface modified by PDADMAC, since the water permeability is markedly lower than that for the PAN-A substrate.