3.1. Structure and Property of CTA Porous Substrates
Figure 1 shows the structure of CTA porous substrates prepared by TIPS, NITS, and N-TIPS methods. The CTA
TIPS porous substrate obviously shows a symmetric cross-section full of sponge-like pores, which is the typical morphology of membrane prepared by TIPS process. Herein, the CTA
TIPS substrate was prepared by immersing the CTA/DMSO2/PEG400 solution at 160 °C into a cooling bath of glycerin at 50 °C. It should be mentioned that as the solvent of CTA, DMSO2 is immiscible with glycerin, and thus the mass exchange between DMSO2 and glycerol could never happen. On the other hand, the intense heat exchange promotes the crystallization of CTA and the solid-liquid phase separation, resulting in the granular structure in the cross-section and membrane surfaces.
The NIPS process can be induced by using DI water as the cooling bath, which is miscible with DMSO2. As the CTA/DMSO2/PEG400 solution at 160 °C was immersed into DI water, the mass exchange between DMSO2 and DI water immediately took place at the surface of the liquid membrane, and a dense surface layer formed. The intruded DI water further induced the phase separation across the whole CTA/DMSO2/PEG400 solution. Finally, finger-like macropores formed inside the membrane as the typical morphology of membranes prepared by NIPS process, which were found in the cross-section of CTA
NIPS and CTA
N-TIPS substrates (
Figure 1a). The CTA
NIPS substrate completely eliminated the granular structure, while the CTA
N-TIPS substrate still retained a part of it (
Figure 1b,d). This result can be ascribed to the temperature difference between the solution and the cooling bath that determines the competitive relationship between TIPS and NIPS processes. Specifically, the CTA
NIPS substrate was prepared in the cooling bath at 95 °C, in which the driven force for the crystallization of CTA was dramatically weakened, and thus the TIPS process was inhibited. As the temperature of the DI water decreased to 50 °C, both TIPS and NIPS processes were induced and the CTA
N-TIPS substrate was obtained. It can be seen that the sponge-like sublayer is thicker and the figure-like macropores are reduced in the CTA
N-TIPS substrate compared to the CTA
NIPS substrate.
The surface structure of the porous substrate plays an essential role in the interfacial polymerization and the polyamide morphology. As shown in
Figure 1c, the CTA
TIPS substrate has a granular top surface due to the crystallization of CTA on the surface. As the NIPS process was introduced, the granule size decreases and the top surfaces of the CTA
NIPS and CTA
N-TIPS substrates become smooth. These results are further confirmed by AFM.
Figure 2 shows that the CTA
N-TIPS substrate has the smoothest top surface with an average roughness value as low as 4.46 nm, much lower than those of CTA
TIPS (17.75 nm) and CTA
NIPS (9.24 nm).
The surface pore size and the surface porosity of the CTA porous substrates were counted and shown in
Figure 3a. It is clear that the CTA
N-TIPS substrate had a surface pore size of 11.1 nm, similar to that of the CTA
TIPS substrate (11.7 nm). However, the surface porosity of CTA
N-TIPS substrate dramatically increased, which means that the NIPS process is beneficial to the surface pore formation through the mass exchange of DMSO2 and DI water. In contrast, the CTA
NIPS substrate had the largest surface pore size (18.8 nm) and highest surface porosity (5.4%). On the other hand, the overall porosity of the CTA
N-TIPS substrate was 71.9%, larger than that of the CTA
TIPS substrate (60.9%) and the CTA
NIPS substrate (67.7%) (
Figure 3b).
As indicated in
Figure 3b,c, the properties of CTA porous substrates were tested in terms of pure water flux, tensile strength, and elongation. The CTA
N-TIPS substrate displayed the largest water flux of 1124.0 L/m
2·h, which was almost two times as large as that of CTA
NIPS substrate (602.4) and forty times larger than that of the CTA
TIPS substrate (29.7). The excellent water permeability of CTA
N-TIPS substrate is mainly contributed to its large overall porosity, porous top surface, and sponge-like pore structure.
Figure 3c demonstrates the mechanical properties of CTA porous substrates. It is widely believed that high mechanical strength is one of the advantages of polymer membranes prepared using the TIPS method. Herein, the CTA
TIPS substrate showed higher tensile strength (17.4 MPa) than that of the CTA
NIPS substrate (9.8 MPa) due to its uniform pore structure and relatively low porosity. However, the elongation of CTA
TIPS substrate (13.5%) was a little lower than that of the CTA
NIPS substrate (14.6%), which is because the stacked granular structure may become the breaking point. In contrast, the CTA
N-TIPS substrate demonstrated the best tensile strength (18.2 MPa) and elongation (21.1%), which was beneficial for both the TIPS and the NIPS process.
3.2. Surface Chemical Composition and Morphology of TFC-FO Membranes
ATR-FTIR was used to characterize the surface chemical composition of TFC-FO membrane and confirm the formation of a polyamide (PA) layer on the CTA substrate. As shown in
Figure 4a, three absorption peaks at 1663 cm
−1, 1610 cm
−1, and 1542 cm
−1 were detected, which represent the stretching vibration of -C=O (amide I), the aromatic amide bond, and the stretching band of C-N (amide II), respectively [
22]. Moreover, the XPS spectra also indicated the formation of PA layer based on the signal of nitrogen element (
Figure 4b). These results reveal that the IP reaction was conducted and a PA-selective layer was formed on all kinds of CTA porous substrates. Furthermore, the surface element content of TFC-FO membranes was further calculated by the XPS result (
Table 2). According to the O/N ratio, the cross-linking degree of PA on TFC
NIPS membrane is higher than those of TFC
TIPS and TFC
N-TIPS membranes.
As shown in
Figure 5a, all the TFC-FO membranes present a “ridge-and-valley” top surface as a typical morphology of the PA layer from interfacial polymerization. Moreover, a large leaf-like structure can be observed on TFC
NIPS membrane, whereas a nodular and worm-like structure is highly obvious on TFC
N-TIPS membrane. This result means more MPD exists on the CTA
N-TIPS and CTA
NIPS substrates than on the CTA
TIPS substrate, which is due to the relatively large surface pore size and surface porosity of the former (
Figure 6). The adsorbed MPD within the porous substrates further diffuses out, reacts with the residual acyl chlorides, and forms a thick PA film [
33]. Correspondingly, the PA layer of TFC
NIPS membrane has an average thickness of 300 nm, larger than that of TFC
N-TIPS (235 nm) and TFC
TIPS membranes (207 nm). The enlarged view of the cross-section reveals that the PA layers on the loose CTA
N-TIPS and CTA
NIPS substrates have intensively nodular protuberance, and voids existed inside the whole ridge-and-valley structure (
Figure 5b). In contrast to the dense PA layer of TFC
TIPS membrane, the nodular protuberance and voids facilitate to increase the water transporting area and then enhance the water flux of TFC-FO membranes [
34,
35].
Moreover, AFM was also conducted to detect the surface morphology and the mean roughness (
Ra) of the TFC-FO membranes. As can be seen in
Figure 7, the rutted and uneven surface is obvious on TFC
TIPS membrane, whereas the small bulges homogenously lay on TFC
NIPS membrane. This result is similar to that observed under SEM, as mentioned above. Correspondingly, the
Ra values for TFC
TIPS (198 nm) are much larger than those for TFC
NIPS (127 nm), and TFC
N-TIPS (110 nm). This result suggests that CTA porous substrates with a smooth surface and high surface porosity may lead to relatively smooth PA films.
3.3. FO Performance of TFC-FO Membranes
The FO performance of the fabricated TFC-FO membranes was evaluated in both AL-FS and AL-DS by using DI water as a feed solution and 1 M NaCl as a draw solution. As shown in
Figure 8a, all the TFC-FO membranes displayed higher water fluxes in AL-DS than those in AL-FS due to the severe ICP effect in the AL-FS. Among others, TFC
N-TIPS (16.84 LMH in AL-DS and 14.89 LMH in AL-FS) and TFC
NIPS membranes (19.54 LMH in AL-DS and 16.94 LMH in AL-FS) presented relatively higher water fluxes than TFC
TIPS membrane (10.08 LMH in AL-DS and 8.41 LMH in AL-FS). Moreover, the reverse salt flux of TFC
N-TIPS membrane was much lower (4.67 gMH in AL-FS, and 10.03 gMH in AL-DS) than in TFC
N-TIPS and TFC
TIPS membranes (
Figure 8b). Therefore, TFC
N-TIPS membrane demonstrates the smallest specific salt flux (
JS/
JW ratio), indicating the best selectivity for water molecules (
Figure 8c).
The performance of TFC-FO membranes can be connected to the morphology and the surface composition of the PA layer, including the surface structure, the thickness, and the cross-linking degree [
36,
37]. Compared with TFC
TIPS membrane, the higher water flux for TFC
N-TIPS and TFC
NIPS membranes can be attributed to their intensive water-transporting area from the nodular protuberance and voids inside the PA layer, although their PA layers are thicker (
Figure 5). On the other hand, the severe reverse salt flux of TFC
TIPS membrane may be ascribed to the low cross-linking degree of the PA layer. Additionally, the water flux of TFC-FO membrane is connected to the ICP effect of CTA porous substrate [
11].
Table 3 lists the intrinsic transport parameters of TFC-FO membrane, including the pure water permeability coefficient (
A), salt permeability coefficient (
B), salt rejection (
Rs), and structural parameters (
S). The TFC
N-TIPS and TFC
NIPS membranes have much smaller
S values than TFC
TIPS membrane, which indicates a relatively low ICP effect in the former. This result can be contributed to the high porosity and low pore tortuosity of the CTA
N-TIPS and CTA
NIPS substrates, which can be indicated by the small
τ values. The pure water permeability of the FO membrane was measured in a RO mode at 5 bar, using the DI water as the recycle solution. The
A values of TFC
NIPS (1.03 L/m
2·h·bar) and TFC
N-TIPS (0.90 L/m
2·h·bar) membranes were 50~60% higher than that of TFC
TIPS (0.61 L/m
2·h·bar) membrane. Meanwhile, the TFC
N-TIPS membrane demonstrated the highest NaCl rejection (92.6%) and smallest
B/
A value (0.39 bar), which reveals that the TFC
N-TIPS membrane has excellent selectivity. In summary, the TFC
N-TIPS membrane showed the best overall performance compared with the TFC
TIPS and TFC
NIPS membrane.