The cross-sectional views in Figures 3b provide further evidence for the delamination of the TFC membrane fabricated using the PAN-A substrate. These observations are consistent with the work by Alsvik et al .11, arguing that the hydrophilicity of the substrate could result in unfavorable interactions between the generated IP layer and substrate. It is also interesting to note that the PAN-H substrate yielded an IP layer with thicker substructures (~500 nm) in comparison with those (~300 nm) resulting from the PAN-O substrate. The shrinkage of the surface pores might increase the resistance for the diffusion of MPD in the aqueous phase and thereby favor the development of the polymerization toward the aqueous phase, which is similar to the case of a lower concentration of MPD (i.e., limited diffusion on the aqueous side) in the work by Chai and Krantz41. When the evolution of the coupled diffusion and reaction processes was spatially dominated by the porous surface of the substrate, it was reasonable to expect that the polymerization should be perturbed in a destabilizing way to yield more periodic patterns, such as the extended ‘leaves’40.
The experimental results for the water permeability and salt rejection of NaCl are displayed in Figures 4a and 4b, respectively. The integrity of the IP layers formed on the PAN-O and PAN-H substrates is confirmed by comparing the water permeability for the TFC membranes with different PAN substrates; that is, the water permeabilities for the TFC-O and TFC-H are significantly decreased, whereas the one for the TFC-A is almost equal to that for the substrate as shown in Figure 2. On the other hand, it is not surprising to note that the salt rejection of NaCl for the TFC membrane with the PAN-A substrate is much lower than those for the TFC membranes with the PAN-O and PAN-H substrates, since the active layer peeled off on the surface of the PAN-A substrate. Moreover, the TFC-H has a lower water permeability compared with that for the TFC-O substrate, whereas the corresponding rejection of NaCl is slightly increased. This is consistent with the SEM characterization showing an IP layer with thicker substructures for the TFC membrane with the PAN-H substrate, thereby offering additional evidence for the integrity of the TFC-O and TFC-H.