## 3 Results and Discussion
### Improvement of Membrane Properties  
#### 3.4.1 Pore Size Distribution and Porosity  
Figure 5 shows the pore size distribution of membranes before and after the heat treatment. The as-spun membrane has an average pore diameter of 1.3 \(\upmu\)m and a much wider range (from 0.8 \(\upmu\)m to 2.2 \(\upmu\)m) than the other samples. The heat-pressed membrane (HP) has the smallest average pore size (1.1 \(\upmu\)m) and a much smaller range of the pore diameter compared with the neat membranes because the fibers were deformed and widened under high pressure at 150 \({}^{\circ}\)C [24]. Annealing the HP membranes for one day can further narrowed the pore size distribution, although the mean pore diameter increased slightly from 1.1 \(\upmu\)m to 1.2 \(\upmu\)m. The increase in the average pore size may be due to the further decrease in the membrane thickness from 39 \(\upmu\)m to 37 \(\upmu\)m, which might be caused by the release of internal stress generated during heat-pressing; the other possible cause could be the supplementary merging of the crystalline phase of the PVDF which was converted from the amorphous phase during annealing [30]. The results were consistent with the study of [PERSON] et al., as it was found that an increase in the annealing temperature would lead to an increase in the average pore size of the membranes [35]. Further annealing for another day led to a greater decrease in the range of the pore diameter and a sharp peak appeared, although the average mean pore size was further slightly increased to 1.3 \(\upmu\)m (which was same as that of the neat membrane). An increased average pore diameter led to an increase in the flux performance [36], justifying that MD applied with the A2 and A3 membranes had the highest flux performance compared to the other samples. In the literature, the pore size distribution of commercial MD membranes generally lies between 0.2 \(\upmu\)m and 1 \(\upmu\)m as the increase in the maximum pore size greatly decreases the LEP, leading to serious wetting problems [3,6]. Narrowing the range of the pore diameter greatly improves the wetting resistance as the LEPs of A2  
Figure 4: XRD (X-ray diffraction) patterns (20 spectra) of as-spun and thermal-treated electrospun membranes.  
and A3 were much higher than that of the neat membrane. The other factor regarding the improvement of the wetting resistance was the evolution of the crystalline structure in the micro-view, which was fully discussed in last two sections. The heat-pressing process turned the amorphous state of the PVDF into crystalline phases, which had much higher thermal and mechanical stability [30]; the annealing process helped some of the \(\beta\)- and amorphous-phase PVDF get converted into the \(\alpha\) phase, which had higher hydrophobicity owing to its non-polar structure. Both factors contributed to A2's high wetting resistance in the MD process even though it had the highest average pore size (but a narrower pore size distribution). A high average pore size led to a high flux performance, improving the permeation efficiency of the MD. A3 had a nearly identical pore diameter distribution as A2; therefore, annealing for two days was sufficient for the improvement of the pore diameter distributions.  
The porosities of the neat and thermal-treated membranes did not have significant differences. The porosities of the neat, HP, A1, A2, A3 membranes were 91%, 86%, 86%, 87% and 87%, respectively, somehow following the trend of the contact angle. However, their differences were too minimal to affect the MD flux performance.