## 3 Results and Discussion
### Decoupling Pore Size and Surface Properties: Effect of Casting Thickness  
The three main parameters explored in Section 3.2 and the phenomenological explanation of water imbibition and water/DMAC mass transfer in Section 3.3 are important to (1) determine the properties of the resulting membrane and (2) the suitability of the membrane for MD application. In an attempt to elucidate the roles of those parameters and the underlying mechanisms to produce desired MD membranes, the impact of casting thickness was further explored. Our key hypothesis here is that the gradient in polymer concentration during the phase inversion process can be exploited by membrane thickness control, to obtain highly hydrophobic surface properties while maintaining an acceptable BP and MPS. The tests were performed under 80% RH, which offers higher chemical potential of water absorption into the cast film.  
Figure 8: Water uptakes from humid atmosphere at different relative humidity (RH) values (37%, 60% and 80%) for membranes prepared using 12% \(w/w\) PVDF solution at a casting thickness of 500 \(\upmu\)m.  
A gradual change in the surface morphology of the membrane samples was observed by extending the exposure time (Figure 9), which was similar to the one observed by varying the time parameter at RH of 60% in Figure 6. However, no obvious impact of casting thickness was observed, or at least it was not clearly visible from the surface SEM images. The change of CA with casting thickness shows a general decrease and this is clearly observed in the case of 7.5-min exposure time (Figure 10). It is also observed that the effect of casting thickness on CA and BP is more pronounced in the cases of relatively longer exposure times (5 and 7 min) in comparison to shorter ones (2 and 3.5 min). These observations may be attributed to the influx and outflux of water vapor and DMAC, respectively. At a thinner casting thickness, the resistance to mass transfer of water vapor and DMAC is relatively lower than that at a thicker casting thickness. Consequently, and due to this larger influx and outflux in the case of thinner casting thickness, the morphological change (and consequently roughness) is more pronounced, leading to higher contact angles. The consequences of lower mass transfer resistance in thinner membranes leading to larger influx and outflux of water vapor and DMAC, respectively, can explain the general decrease of BP with thickness. As more water vapor is absorbed by the cast film, more DMAC is partitioned out, leaving behind large void spaces and consequently pores. This effect is less prominent in thicker membranes, causing lower BP due to the higher mass transfer resistance to water vapor influx. In fact, these observations reconfirm the findings on the effect of increasing humidity on BP (Table 4). As was seen before, the BP increased with humidity due to the higher chemical potential between the cast solution and humid atmosphere. Therefore, by decreasing the mass transfer resistance or increasing the chemical potential (humidity), a larger BP is obtained. As a consequence of the CA and BP trends, the LEP values in Figure 10 show a slight increase in LEP with thickness and this is clearly seen for the case of the 5-min exposure time. In this case, the slight increase in LEP can potentially be attributed to the relatively larger percentage decrease in BP in comparison to the decrease in CA. As shown in Figure 10, the casting thickness played a significant role in maintaining a small BP. As the casting thickness was reduced, the incremental BP size significantly increased, which substantially diminished the LEP. Under this situation, increasing thickness offers substantial advantages since under thicker casting thickness (e.g., 1000 \(\upmu\)m) LEP could be controlled well.  
Figure 9: Surface SEM images of PVDF membranes prepared using VIPS process at different casting thicknesses and exposure times, all using 12% \(w/w\) PVDF polymer solution and under 80% RH.