## 3 Results and Discussion
### Membrane Morphologies in NIPS vs. VIPS
Starting with a NIPS process (path D), spontaneous demixing occurs immediately once the polymer solution gets in contact with the non-solvent in the coagulation bath [18; 19; 20]. In this case, the rate of non-solvent intrusion into the casted polymer film is much higher than the rate of solvent extrusion, leading to highly porous membrane as discussed in detail by [PERSON] and [PERSON] (2011). As the compositional pathway moves from the initial polymer solution composition to the final membrane composition, it enters the two-phase region where it splits into polymer-rich (pint 1, Figure 2) and lean (point 2, Figure 2) phases connected by a tie-line. As the final membrane composition is approached, both solvent and non-solvent eventually evaporate during the drying period and the membrane becomes a single-phase polymer solid. At the final membrane composition, the two phases are at thermodynamic equilibrium. A polymer-rich phase with composition S is marked in Figure 2 as well as a polymer-lean phase with composition L. Point L represents the composition of the voids or pores of the membrane which contain water which eventually evaporate during the drying stage. At the other extreme, when a VIPS process is applied without the final coagulation bath step, delayed demixing leads the compositional pathway to follow path A (Figure 2) [21]. In this case, the rate of non-solvent intrusion into the casted polymer film is much slower than the rate of solvent extrusion. This path represents the case of lowest RH, where the composition of the polymer solution never crosses the binodal curve and maintains a single phase. As solvent and non-solvent exchange occurs at a rate much slower than in the NIPS case, there are three different mass-transfer processes occurring simultaneously; water vapor is absorbed into the polymer solution while DMAC and water evaporate, leaving the polymer solution. The former absorption rate of water is relatively much higher than the evaporation as will be discussed later (Section 3.3). The driving force for the water absorption in this case is the chemical potential difference between the atmosphere surrounding the polymer solution and the polymer solution itself. Over time, the difference between these two chemical potentials decreases, causing the rate of water vapor absorption to change as the membrane goes through the compositional pathway A. The decrease in chemical potential is depicted in the compositional pathway of A by the increasing slope of the path, representing a decreasing intake of water and a sharper movement towards the PVDF-rich phase. As the composition crosses the crystallization line, the polymer solution solidifies, forming crystal nuclei and eventually forming a membrane with crystalline structures (evidence of this is discussed later, Figure 6). Compositional paths B-F and C-E (Figure 2) represent a combination of  
Figure 1: Top surface scanning electron microscope (SEM) images of membranes prepared using NIPS (non-solvent-induced phase separation) and VIPS (vapor-induced phase separation) processes with 12% \(w/w\) PVDF (Polyvinylidene fluoride) concentration and cast thickness of 500 \(\upmu\)m (10 \(\upmu\)m and 2 \(\upmu\)m scale bares for all top and bottom row SEM images, respectively). a complete NIPS (pathway D) and a complete VIPS (pathway A) process [21; 22]. As the RH increases, the compositional path moves from pathway A towards D, with B-F and C-E representing two RH cases with B-F having a lower RH than C-E. As the RH increases, the chemical potential difference between the polymer solution and the atmosphere surrounding it increases as well, leading to a higher rate of absorption of water (as can be seen later in Figure 8). Pathway B-F starts in a similar way to path A, but with a higher RH. As the polymer solution loses DMAC and gains water it enters the meta-stable region [21]. Once there, the casted film is immersed in the coagulation bath.