## 3 Results and Discussion  
### Chemical and Physical Characteristics of PolyHIPE Membrane  
The characteristics of the PolyHIPE membrane depend on the composition of monomers, crosslinking agent, organic/aqueous phase ratio, and the amount of surfactant used for the preparation of emulsion solution (HIPEs). In order to prepare a feasible membrane substrate, the PolyHIPEs with different monomer compositions were prepared and evaluated (Table 1). In this study, the monomers were styrene (St) and 2-ethylhexylacrylate (EHA). The use of EHA was to improve the flexibility of the substrate. The amount of crosslinking agent, DVB, was fixed at 10 wt % of organic phase, as suggested by a previous study [21]. The amount of Span 80 was 20 wt % of the organic phase, and the volume ratio of organic and aqueous phase was 2:8. All the compositions were pre-tested to have good emulsion stability during the preparation of PolyHIPE.  
Figure 1 shows the FTIR of PolyHIPE. The characteristic peaks at about 700; 1453; 2856; and 2923 cm\({}^{-1}\) are associated with the methylene group of styrene [22]. The stretching vibration of carbonyl group (C=O) of EHA is seen at 1726 cm\({}^{-1}\). The peak at 1603 cm\({}^{-1}\) is due to the skeletal deformation vibration of C-H in DVB [23]. Therefore, the PolyHIPE used in this study is a poly(styrene-co-2-ethylhexyl acrylate) copolymer crosslinked with DVB.  
The physical property of PolyHIPE substrate is affected by the styrene/EHA/DVB ratio. Table 1 shows that the hardness of membrane increases almost linearly with increasing styrene fraction. For styrene fractions higher than 60 wt %, the PolyHIPE substrates are glassy and brittle, which are not suitable to be used as a membrane. For EHA fraction of 90 wt %, the PolyHIPE substrate is very soft and too sticky to handle. With 90 wt % of styrene, the porosity is about 0.68 \(\pm\) 0.02, which is much smaller than that of initial volume fraction of aqueous phase (0.8) in the emulsion solution. This may be due to shrinkage of poly(styrene-co-2-ethylhexylacrylate) after polymerization. The specific volume of substrate decreased with increasing styrene content. The porosity for EHA fraction of 60 wt % and 90 wt % is 0.77 \(\pm\) 0.03 and 0.78 \(\pm\) 0.01, respectively, which are very close to 0.80, indicating that EHA can reduce the shrinkage of PolyHIPE substrate.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**SteHA:DVB 1** & **Visual Physical Quality** & **Shore Hardness LX-C** & **Porosity (\%)** & **Specific Volume (cm\({}^{3}\)/g)** \\ \hline
90:0:10 & hard, brittle & 85.0 \(\pm\) 2.5 & 0.68 \(\pm\) 0.02 & 3.22 \(\pm\) 0.28 \\
60:0:30:10 & hard, brittle & 65.1 \(\pm\) 3.4 & 0.71 \(\pm\) 0.01 & 3.70 \(\pm\) 0.57 \\
30:60:10 & flexible, strong & 52.6 \(\pm\) 4.5 & 0.77 \(\pm\) 0.03 & 4.35 \(\pm\) 0.35 \\
0:90:10 & elastomeric, sticky & 32.5 \(\pm\) 6.2 & 0.78 \(\pm\) 0.01 & 4.76 \(\pm\) 0.59 \\ \hline \end{tabular}  
* Monomer mixture composition expressed as wt %. Styrene (St); 2-ethylhexylacrylate (EHA); divinylbenzene (DVB).  
\end{table}
Table 1: HIPE compositions and quality of resulting PolyHIPE membrane.  
Figure 1: FT-IR spectrum of PolyHIPE membrane (30 wt % styrene (St), 60 wt % 2-ethylhexylacrylate (EHA), and 10 wt % divinylbenzene (DVB)).  
In the followed study, the PolyHIPE membrane was prepared with 20 wt % organic phase (30 wt % St, 60 wt % EHA, and 10 wt % DVB) and 80 wt % aqueous phase. The averaged Young's modulus and Shore hardness of the substrate is about 1.25 \(\pm\) 0.42 MPa and 52.6 \(\pm\) 4.5 LX-C, respectively. The flexibility evaluated according to (ASTMF 137-08) shows that the porous PolyHIPE can be bent over a cylindrical mandrel with diameter as small as 0.2 cm. With this composition, a strong and elastomeric membrane can be robustly obtained. The mechanical properties of PolyHIPE membrane are acceptable for long term separation operation, as demonstrated in the reusability study in this study (Section 3.2.3).