## 3 Results and Discussion
### Increased Crystallinity and Appearance of a Phase after Annealing  
FT-IR spectra of the \(\alpha\) and \(\beta\) phases of PVDF in the PVDF-HFP copolymer were investigated comprehensively. It was found that the bands in terms of the \(\beta\) phase of PVDF appeared at 840 cm\({}^{-1}\) and 1278 cm\({}^{-1}\)[29; 30] in all the as-spun and thermal-treated membranes, as shown in Figure 3; the bands related to the \(\alpha\) phase of PVDF appeared at 615 cm\({}^{-1}\), 765 cm\({}^{-1}\), 795 cm\({}^{-1}\), 975 cm\({}^{-1}\) and 1212 cm\({}^{-1}\)[29; 30; 31], and they were only found in the annealed membranes. The phase of PVDF in the neat membrane was basically \(\beta\), as the peak representing the \(\alpha\) phase rarely appeared. Two factors contributed to the as-spun membranes mainly consisting of the \(\beta\) phase: (1) the existence of the HFP copolymer in the polymer chains [29]; (2) stretching and pulling of the electrospun fiber during the whipping process in electrospinning [31; 32]. After the neat membrane was heat-pressed, the band at 840 cm\({}^{-1}\) representing the \(\beta\) phase increased from 53% to 63.6%, while the band in terms of the \(\alpha\) phase did not appear after the heat-press treatment. The increase in the transmittance of \(\beta\) phase bands indicated the increase in the crystallinity of the membrane as the amorphous phase of the PVDF was converted into the \(\beta\) phase due to the mechanical deformation, caused by the pressure  
Figure 2: Comparison of flux performance between post-treated and as-spun electrospun membranes and commercial ones.  
applied on the membranes during heat-pressing [25; 26]. [PERSON] et al. found that annealing PVDF above 80\({}^{\circ}\) led to an increase in both the total crystallinity and the \(\alpha\) phase PVDF percentage [29; 33]. [PERSON] et al. also pointed out that annealing of the membrane resulted in the transformation of some regions of the PVDF phase from \(\beta\) to \(\alpha\)[29]. In this study, bands in terms of the \(\alpha\) phase started to appear at 615 cm\({}^{-1}\), 765 cm\({}^{-1}\), and 975 cm\({}^{-1}\) after one day of annealing (A1), and were more obvious at 615 cm\({}^{-1}\), 765 cm\({}^{-1}\), 795 cm\({}^{-1}\), 975 cm\({}^{-1}\) and 1212 cm\({}^{-1}\) after two and three days of annealing (A2 and A3), while the transmittance at the 840 cm\({}^{-1}\) band decreased back to 62.1% which is same value as in the neat membrane. It is worth noting that the membrane annealed for three days (A3) had nearly identical FT-IR spectra results as the one annealed for two days (A2), which means two days of annealing could be long enough for sufficient state conversion. The appearance of the \(\alpha\) phase PVDF in the membrane is favorable for the MD process owing to its non-polar properties because it leads to a decrease in the dipolar interaction between the water molecules and the membrane [34]. The existence of the \(\alpha\) phase PVDF can increase the liquid entry pressure and, hence, the wetting resistance, contributing to better long-term MD performance. [PERSON] et al. stated that annealing could also affect the microstructure evolution [35], which benefits the long-term MD operation as well. By increasing the crystallinity and releasing the internal stresses caused by the heat-pressing, the thermal stability of the membrane could be improved, preventing the LEP from dropping rapidly owing to distortion at a high feed temperature.