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.