Effects of the polyelectrolyte deposition on the IP-film
formation
It has been demonstrated that increasing the hydrophilicity of the
substrate could result in the delamination of the TFC membrane
fabricated via IP. However, it was tempting to employ a hydrophilic
substrate for fabricating a TFC membrane via IP owing to the potential
advantages for improving the efficiency during an FO process. Therefore,
the polyelectrolyte deposition was exploited to modify the surface
wettability. Three polycations, including PDADMAC, PEI, and PAH, were
employed to tune the surface of the PAN-A substrate (schematically shown
in Figure S2), which was negatively charged owing to the carboxyl
groups.
The contact-angle measurement results for the polyelectrolyte-modified
PAN-A substrates are shown and compared in Figure 5. In particular, the
red dashed line denotes the average contact angle for the PAN-A
substrate, while the gray dashed line is for the PAN-O substrate. It
clearly indicates that the deposition of all the polycations
significantly decreased the degree of the surface hydrophilicity for the
PAN-A substrate. The PAN-A substrate modified by PDCDMAC yielded the
smallest contact angle, which was greater than that for the unmodified
PAN-A substrate. On the other hand, the degrees of the surface
hydrophobicity for the PAN-A substrates modified by PEI and PAH were
even higher than that for the PAN-O substrate. Especially, the PAN-A
substrate modified by PAH yielded the largest contact angle greater than
90º. Moreover, the substantial increase in the surface hydrophobicity
indicates that most of the nonpolar segments should be exposed to the
liquid phase while ‘preventing’ the polar groups (i.e., the amine
groups) from meeting with the TMC in the organic phase. It is reasonable
to assume that the effects of the functional groups introduced by the
polyelectrolytes could be ignored to focus on the variation in the
hydrophobicity.