Characterizations of optimum membrane

Figure 4.19 presents the micrographs of optimum membrane (represented as E5-3.6) and pristine membranes (represented as E0). As observed from the surface, the APTES-SiO2 particles are well-dispersed on the surface of the membrane. The introduction of APTES-SiO2 created low surface porosity which might induce a lesser membrane surface area for the adsorption of oil to take place which will subsequently promote low adsorption, particular when dealing with high concentration of oil. As shown in the figure, a typical asymmetric structure can be seen in the cross-section of both membranes which are made up of porous finger-like

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sub-layer, fully developed macrovoids at the bottom-layer and thin dense top layer. As seen, the macrovoids of the pristine membranes are larger than the optimum membrane which can be ascribed due to the increase in dope viscosity upon addition of APTES-SiO2 particles. Additionally, the wall thickness, inner diameter (ID) and outer diameter (OD) of the optimum and pristine membrane were also measured and presented in Table 4.7.

Figure 4.19: FESEM images of membrane cross-section (left) and surface (right) for (a) E0 membrane (b) E5-3.6 membrane.

Table 4.7: Wall thickness, inner and outer diameter of E5-3.6 and E0 membrane Membrane Inner skin

layer (nm) Outer skin layer (nm) ID (mm) OD (mm) E0 223.10 ± 26.91 493.3 ± 40.52 0.427 ± 0.08 0.853 ± 0.03 E5-3.6 363.6 ± 81.24 515.8 ± 57.20 0.432 ± 0.04 0.859 ± 0.05

In Figure 4.20, the 3D image of the surface topology revealed that the pristine membrane displayed a higher surface roughness (Ra) in comparison with optimum

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membrane. For pristine membrane, the surface roughness was 93.174 nm and was decreased to 74.408 nm for optimum membrane. The decrease in the roughness can be due to the surface enrichment of PES by well dispersed APTES-SiO2. The reduced surface structure could be as a result of the interaction between APTES-SiO2 particles in the casting solution and the phase inversion kinetics. The incorporation of APTES-SiO2 particles in the PES casting solution will reduce the activity of polymer and DMAc, increase thermodynamic stability, thereby decreasing the driving force for DMAc outflow of fiber precipitation in external coagulation bath and accordingly produce membrane with smoother surface roughness. This result is in tandem with observation by Vatanpour et al. (2012) who found that the mean roughness decreases with increase in TiO2 coated CNTs. This is different from the result by Dutreilh- Colas et al. (2008) who found that the roughness parameters of an hybrid silica film increases due to the presence of small drops of silica on the surface. Yang et al. (2015) also reported that the roughness of acrylate polyurethane film increases when coated with SiO2 film. High roughness might translate to high porosity (see Table 4.8) and the nano-structured architecture of membrane (Zhang and Wang, 2014). Thus, in this work, the decrease in roughness of composite membrane may be associated due to the impregnation of well dispersed APTES-SiO2 in PES matrix.

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APTES-SiO2 is well known for its hydrophilic properties when introduced in membrane matrix. Therefore, the presence of APTES-SiO2 in the dope solution is expected to result in improvement of hydrophilicity. In order to investigate the influence of APTES-SiO2 on membrane hydrophilicity, the dynamic water contact (variation of WCA with time) was measured and presented in Figure 4.21. As observed, the WCA of the pristine membrane was higher than the optimum membrane. Furthermore, the decline rate of optimum membrane was faster as compared to the pristine membrane. This signifies an improvement in hydrophilicity upon incorporation of APTES-SiO2 particles. This is similar with observation by Vatanpour et al. (2012) who found that the surface hydrophilicity increases when TiO2 coated CNTs was incorporated in the PES membrane matrix.

Figure 4.21: Dynamic contact angle of E0 and E5-3.6 membrane.

The ability of water to pass through membrane surface could also be influenced by the pore size. In order to investigate the influence of pore size on the

0 10 20 30 40 50 60 70 80 0 20 40 60 80 100 120 Ang le (º ) Time (s) E0 E5-3.6

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membrane hydrophilicity, Figure 4.22 presents the PSD curve. As observed, there is difference in slope of both membranes, an indication of a slightly different PSD upon introduction of APTES-SiO2 particles. The MPS were further calculated and presented in Table 4.8.

Table 4.8: Mean pore size and porosity (£) of E0 and E5-3.6 membranes. Membrane Mean pore

size (nm)

Porosity (£)

E0 39.98 ± 0.18 82.91 ± 3.6

E5-3.6 43.29 ± 0.11 78.27 ± 1.4

From the figure, the PSD of E0 membrane was slightly broader as compared to the optimum membrane (E5-3.6). The mean pore size was increased by ~8.3% upon introduction of 3.6 wt.% APTES-SiO2 particles and as a consequence will bring about reduction in capillary force and thereby result in decrease in water contact angle. This difference can be ascribed due to introduced particles which produced slightly bigger pore size at some certain locations.

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Figure 4.22: Pore size distribution of E0 and E5-3.6 membranes.

The porosity and mean pore size are displayed in Table 4.8 indicated that the porosity of E5-3.6 membrane was also affected upon addition of APTES-SiO2 particles in the membrane matrix, where porosity decreased from 82.91 to 78.27%. This can be as a result of an increase in the content of solid or APTES-SiO2 contents in the membrane matrix. A high APTES-SiO2 particles in the casting solution will yield smaller porosity and thus increases the mass transfer resistance through the HF membrane (Shen et al., 2014). Similar observation have been reported by Li et al. (2009) during the preparation of PES-TiO2 membrane where membrane porosity was found to decrease with increase in TiO2 contents.

To obtain useful information regarding the functional groups and molecular structures, FTIR spectroscopy was carried out. Figure 4.23 presents the FTIR spectra of the E0 and E5-3.6 membranes. The broad adsorption peak at 944 cm-1 correspond

0 10 20 30 40 50 60 70 80 0.01 0.02 0.03 0.04 0.05 0.06 0.07 F re qu ency Pore size (µm) E5-3.6 E0

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to the Si-OH stretching while the broad peak of 817 cm-1 corresponds to the symmetric vibration stretching of Si-O (Muhamad et al., 2015b; Yu et al., 2015). The presence of amino groups associated with 3-aminopropyltriethoxysilane was also observed. Two small shoulder peaks at 3347 and 3290 cm-1 corresponds to the N-H asymmetric stretching of the amine H-bonds, indicating a possible interaction of NH2 toward PES surface (Aneja et al., 2015; Wang et al., 2013). The presence of an additional new N-H vibration peak, observed at around 1668 cm−1 results from the existence of free –NH2 group (Miao et al., 2016). By going from E0 to E5-3.6 HF membrane, the strength of –OH stretching peaks at 3438 cm-1 was enhanced. These functional groups are responsible for improving surface hydrophilicity as well as fouling mitigation.

Figure 4.23: FTIR spectra of E0 membrane and E5-3.6 membrane.

400 800 1200 1600 2000 2400 2800 3200 3600 4000 T ra ns m it ta nce (%) Wavenumber (cm-1₎ NH2 NH Si O Si Si O H E5-5.6 membrane E0 membrane

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