## 2 Materials and Methods
### Characterization  
Contact angle, which was generally used to indicate hydrophobicity of membrane surface, was measured by Theta Lite 100 (Attenson) (Biolin Scientific, Paramus, NJ, USA) with sessile drop method [24; 26]. Then 5-8 \(\upmu\)L of water droplet was placed onto the membrane surface for analysis. A mounted motion camera was applied to capture the images at a rate of 12 frames per second. Contact angle could be obtained by analyzing the recorded video with aid of the Attenson software. An average of three values was used as contact angle data for each sample.  
As-spun, heat-pressed and annealed membrane samples was measured with a lab-made setup for their liquid entry pressure [9; 24]. A gas supply was connected to a hollow stainless plate by a tube, with a digital gauge standing as the intermediate between them. On the top of the stainless plate, a stainless cylinder container was fully filled with distilled water while a plastic plug stuck in its bottom. The samples were firmly fixed on the top of cylinder by a stainless cap with a lock catch. The nitrogen gas was steadily released to increase the pressure by 5 kPa per 30 s, until the first bubble sign appeared, which was recorded as LEP. Each sample was tested in triplicate and average data was recorded.  
Fourier-transform infrared (FT-IR) spectroscopy (Varian 2000, Agilent, Santa Clara, CA, USA) was used to investigate the PVDF-HFP phases of the virgin membrane and thermally-treated membranes. Each spectrum was acquired with signal averaging 32 scans at a resolution of 8 cm\({}^{-1}\), in transfer mode by pressing the sample with KBr to a pellet.  
\begin{table}
\begin{tabular}{c l c} \hline \hline
**Sample Name** & **Description** & **Membrane Thickness (\(\upmu\)m)** \\ \hline Neat & As-spun electrospun PVDF-HFP (polyvinylidene fluoride-\(\alpha\)-hexafluoropropylene) membranes & 51 \\ HP & Neat membrane heat-pressed at 150 \({}^{\circ}\)C under 6.5 kPa for 24 h & 39 \\ A1 & HP membranes annealed for 1 day at 120 \({}^{\circ}\)C & 37 \\ A2 & HP membranes annealed for 2 days at 120 \({}^{\circ}\)C & 34 \\ A3 & HP membranes annealed for 3 days at 120 \({}^{\circ}\)C & 34 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Sample names and thicknesses of the membranes prepared in the present study.  
X-ray diffraction (XRD) (Siemens D5000, Siemens, Karlsruhe, Germany) was carried out over Bragg angles ranging from 10\({}^{\circ}\) to 30\({}^{\circ}\) (Cu K\(\alpha\), \(\lambda\) = 1.54059 A).  
The pore size and pore size distribution of the fabricated neat and post-treated electrospun membranes were measured by capillary flow porometry (CFP-1200-AEXL, Porous materials Inc., Ithaca, NY, USA). All samples were firstly wetted by Galwick (a wetting liquid with a low surface tension of 15.9 dynes/cm) and tested under the pre-set conditions. Then the dried samples were applied with N\({}_{2}\) gas to determine the gas permeability under same conditions. The final average pore size and its distribution were automatically calculated with both data sets of wet and dry tests by the specific software.  
Membrane porosity was calculated by using gravimetric method, where the volume of the membrane pores was divided by the total volume of the whole membrane. Ethanol (Univar 1170 from Ajax Finechem Pty Ltd., Sydney, Australia) was used to completely wet the membranes. The weight (\(w_{1}\), \(g\)) of wetted membrane was measured after the residual ethanol on the surface was removed. Then the membrane samples were dried after being left still in the open air for 15 min and then weighed (\(w_{2}\), \(g\)). The porosity of the electrospun membranes could be calculated with the following equation:  
\[\epsilon_{m}=\frac{(\mathbf{w}_{1}-\mathbf{w}_{2})/\rho_{e}}{(\mathbf{w}_{1}-\mathbf{w}_{2})/ \rho_{e}+\mathbf{w}_{2}/\rho_{p}}\]  
where \(\rho_{e}\) is the density of the ethanol (g/m\({}^{3}\)) and \(\rho_{p}\) is the density of the PVDF-HFP (g/m\({}^{3}\)) [16; 24].