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.