## 3 Results and Discussion
### Moisture Uptake during the VIPS Process  
Figure 8 shows mass variation of polymer cast film during exposure to different humidity values (37%, 60% and 80% RH). In all cases, soon after the exposure started, the film mass started to increase until it reached a maximum value, and then it decreased. As the RH increased, a shift in the maximum uptake was observed, peaking at around 20, 90 and 450 min for 37%, 60% and 80% RH, respectively. The rate of moisture uptake before reaching the maximum value was also faster for higher RH. Two competing mass transfer phenomena in the cast film are expected during the exposure: (1) the water flows into the film due to the hygroscopic and miscible nature of water and DMAC; and (2) the outflow of DMAC and water from the cast film. Before reaching the maximum mass, water uptake was a predominant phenomenon, leading to an increase in mass of the cast film. The increase in absorption rate of water at higher RH was a result of a larger chemical potential at higher water  
Figure 6: Surface SEM images of membrane samples prepared at different exposure times under 60% RH, 12% \(w/w\) PVDF concentration and 500-\(\upmu\)m casting thickness. All images have the same scale bar, shown in the top-right image.  
Figure 7: Impact of exposure time on (**A**) CA (contact angle); (**B**) MPS (mean flow pore size) and (C) BP (bubble-point pore size), for membranes prepared under RH of 60% and at casting thickness of 500 \(\upmu\)m with 12% \(w/w\) PVDF concentration.  
concentration in air. After reaching the maximum value, solvent and water evaporation from the casted film prevailed over water sorption, resulting in a weight loss. The water sorption stopped when there was no chemical potential difference, which occurred when the amount of water on the cast film was large enough. The loss of mass then continued as a result of the concomitant DMAC and water loss from the film.  
The water sorption mechanism explained earlier allows the understanding of mass transfer phenomenon within the cast film. Just after the exposure started, at the very top of the film, a local exchange of water and DMAC occurred. This process continued further at deeper locations within the film creating a change in composition across the film thickness, as explained using a phenomenological model by [25]. This process would continue as long as the top film layer did not solidify as a result of phase separation. However, in the case of PVDF/DMAC/water system, during the humid air exposure, a thin film of water was observed on the top of the cast film after certain time, which was more obvious for higher RH (60% and 80%) as also found by [26] for a poly(ether-imide)/_N_-methylpyrrolidone/water system. This finding suggests that the phase inversion of the top film layer had started and completed earlier, creating a dense barrier for further exchange of DMAC and water at the deeper location. Consequently, concentration gradient from top (being the lowest PVDF concentration) to the bottom (being the highest) could not be achieved fully. This explains why the increment of surface hydrophobicity only occurred within certain period of time (2-10 min) (Figure 7). Within this time interval and until just before phase separation completed, a rapid composition change within the cast film was still possible, resulting in a change in surface properties [10]. Up to this point, it is not clear why further exposure time beyond 1 h resulted in a shift of surface morphology as shown in Figure 6.