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.