2.1 Materials

Commercial flat sheet N66 membrane (SKU: NY013001) was obtained from Sterlitech Co. (WA, USA). Aqueous (MPD) and organic (TMC) monomers were purchased from Sigma-Aldrich, Malaysia. Hexane was procured from Merck. All chemicals were used as received without further purification. Deionised water and distilled water were used as the nonsolvents for TFC preparation.

2.2 Preparation of TFC membranes

Support membranes were dried in a vacuum oven at 60 °C for 10 min before PA thin-film deposition process. The support membranes were exposed with 1–2 wt % aqueous phase for 2–5 min at 25 °C (Table 1). After removing excess amine solution using an air blower, the membranes were immersed in a 0.1 wt % organic solution containing TMC phase solution for the IP reaction for 1–2 min. Fabricated TFC membranes were immersed in hexane solution and then in distilled water to wash away excess acyl chloride and amine solutions, respectively. The membranes were dried at 70 °C for 10 min in a circulation oven. Then, the fabricated TFC membranes underwent posttreatment by immersing in methanol for a couple of minutes.

2.3 Scanning electron microscopy

The surface and cross section views of pristine and fabricated membranes were examined using a scanning electron microscope (Carl Zeiss EVO50) attached with an EDX unit. Cross sections of the TFC membranes were prepared by fracturing the membranes using liquid nitrogen. All samples were dried in a vacuum oven and then coated with a conductive layer, Au (400 Å) of sputtered gold before photographing.

2.4 Fourier-transform infrared spectroscopy

The formation of the TFC layer by IP reaction on the support membrane was confirmed by FTIR spectrometry (Nicolet Avatar 370 DTGS). All the TFC membrane samples were ground to make membrane pellets under hydraulic pressure. The FTIR wavelengths were recorded in the range of 4000–500 cm⁻¹.

2.5 Contact angle

Hydrophilicity of the fabricated TFC membranes was evaluated by using a contact angle instrument known as JY-82 video contact. A droplet of 5 μl of distilled water was deposited on the surface of membranes for 10 s at room temperature. Angles were measured in five replicates at different sites of each membrane to minimise error.

2.6 Swelling measurements

Membranes were dried in an oven at 120 °C overnight before being preweighed. Membrane swelling was measured by immersing dried membranes in isopropanol mixtures of 90, 80 and 60 wt % for 24 h at room temperature. The swollen samples were taken out and wiped with a tissue paper, and the film was weighed. The degree of swelling is defined in Eq. 1 [13]:

3.1 SEM and EDX studies

Fig. 1(a) i and 1(a) ii demonstrate the surface and cross-sectional SEM images of the pristine. The pore distribution of the pristine membrane was distinguished as a uniform structure. The uniform structure and small pore size average of 1.15 μm offer a high surface area for IP process, making the membrane an appropriate substrate for this study [14].

Fig. 1(b–d) present (i) surface and (ii) cross-sectional SEM images of the pristine and TFC membranes prepared under various immersion times in 2.0 wt % MPD (2, 3 and 5 min) and 0.1 wt % TMC solution (1 and 2 min). The thin-film thickness increased linearly with increasing contact time in MPD solution. Longer contact time between MPD and TMC enhanced the availability of MPD monomer on the surface. The degree of cross-linking reaction was also improved because there was more interaction between MPD with TMC [4]. Thus, this leads to thicker and denser TFC active layer being developed on the pristine membrane as seen from the increase in thickness from 40.0 to 56.3 μm. The polymer substrate pores must be effectively wetted during contact with the aqueous MPD solution. Impurities defect on the TFC membrane could occur if the membrane is not properly wetted in the amine solutions because of the low MPD concentration.

The polyamide thin layer successfully deposited on substrate revealed in the SEM images when the pores seem to be covered and individual pores no longer can be identified. Tsuru et al. [15] mentioned that the TFC forms a ridge-and-valley structure, and this is favourable for enhancing the permeation area. More importantly, the TFC produced must be thin and have a strong cross-linked PA layer [16]. When the immersion time in the aqueous MPD solution was increased from 2 to 3 min, more MPD species were deposited on the surface of the membranes. Eventually, during reaction, more organic monomer can react with MPD and subsequently increase the thickness of the membranes. As the membrane produced becomes thicker, the mass-transfer coefficient also escalates, which attributes to increasing selectivity of the membrane.

Fig. 2 shows the main elements of the N66 membrane and produced TFC membranes as seen from the EDX spectrum. Carbon (C) and oxygen (O) are the main elements in the N66 and TFC membranes. The highest C peak can be observed in the control group. After the PA TFC deposition, the C content of the processed sample gradually declined. Meanwhile, O content increased markedly. This result corresponds well with the results obtained by Al-Hobaib et al. [17]. The percentage of C was decreased (O increased) as the thickness of membrane increased, which confirms that TFC layers are present on the top of the membrane.

3.2 Fourier-transform infrared spectroscopy studies

Fig. 3 shows the comparison between the FTIR spectra for pristine and TFC membranes (TFC1, TFC2 and TFC3). These absorption wavelengths indicate the existing nitro compound which consists of nitro functional groups inside the fabricated TFC membranes. These peaks were observed in all TFC membranes but not in the pristine membrane. The absorption band of C-N stretching at 1537–1540 cm⁻¹ represents the amine group [18]. However, these peaks cannot distinguish between the aliphatic ring owned by a pristine membrane or fully aromatic polyamide group of TFC membranes. Aromatic breathing ring, C6H6 peaks, was identified at the 1609–1610 cm⁻¹ where hydrogen bond of C–O and N–H stretching of amide is identified [19]. In the case of TFC3, the amine solutions may diffuse back into TMC solutions because of the longer exposure time with the TMC solution. At the wavelength of 3300 cm⁻¹, a strong carboxylic acid displayed which indicate a strong intensity within the pristine with the TFC3 sample. These bonds are stronger than the rest of TFC membranes where they were presented by a weak O–H stretching [20]. The peak at 1250 cm⁻¹ is attributed to the MPD introduced onto the substrate, which does not exist in the pristine membrane. More TFC formation can be observed at the peaks of 768 and 1450 cm⁻¹. The peak at 768 cm⁻¹ is attributed to the C–Cl stretching, while the peak at 1450 cm⁻¹ indicates CO stretching and O–H bending of carboxylic acid.

3.3 Contact angle measurement studies

Table 2 shows that as the immersion time in the MPD solution increased, the contact angle measurement of the TFC membranes also increased. All the membranes in this study showed a contact angle below 90°, which indicates that all the membranes are hydrophilic. Hydrophilic membranes are more selective toward water molecules rather than the solvent [21]. N66 membrane showed higher contact angle than TFC1, TFC2 and TFC3 membranes. The contact angle for pristine membrane is 60°, which is consistent with the standard contact angle of commercial membranes reported by Huang and McCutcheon [22]. This is because when the N66 substrate was coated with the TFC, the membrane surface became smoother and the pores were covered. This also indicates that the amount of amide groups formed on the membrane surface is getting higher. The hydrophilicity of the fabricated membrane not only depends on the immersion time but also the reaction time. The hydrophilicity enhancement of the TFC membranes is due to the pendant carboxylic acid groups inside the main chain. As the IP reaction time increased, the TFC membranes thickened, which contributed to the dense TFC especially for TFC2 and TFC3. This also explains the decrease in contact angle values. This result is in accordance with the results obtained by Gorgojo et al. [23]. A hydrophilic substrate is expected to hold MPD in the pores, and eventually some of the MPD may be trapped inside the substrate pores, which will reduce the interfacial reaction rate [24].

3.4 Effects of feed composition on membrane swelling

To study the sorption characteristics of N66 and TFC membranes, the degree of swelling of all membranes was plotted with respect to different mass percentage of water in the feed solution at room temperature. Sorption is important in PV mechanism to understand the penetration of molecules resulting from the driving force of gradient in chemical potential [25]. Chemical gradient is also expressed using partial pressure differences of the molar flux [26]. As shown in Fig. 4, the degree of swelling increased for all membrane samples with the increasing mass percentage of water in the solution. This trend is similar with those of previous studies by Thorat et al. [27] and Sajjan et al. [13] because of the strong interaction between water molecules and the membranes, which is consistent with their hydrophilic properties. The pristine membrane showed a significant degree of swelling compared with TFC membranes. This indicates that the pristine membrane has higher affinity towards water than other membranes, which can be correlated to the membrane structure. The microstructures cause the water molecules to penetrate into the membrane when the membrane is exposed for certain duration and the liquid is trapped inside the membrane's pores, which leads to a higher degree of swelling than that of the TFC membranes [28].