## 3 Results and Discussion  
The introduction of polar monomers into the HIPE is conducive to Ostwald ripening, resulting in emulsion instability and thus affects the structure and properties of polyHIPEs [29]. Previous studies have shown that Ostwald ripening can be effectively inhibited by introducing electrolyte [30] and a composite emulsifier [31] composed of nonionic and anionic emulsifiers. In this paper, a composite emulsifier (Span80/DDBSS equal to 9/2 in molar ratio) and an electrolyte (0.12 mol\(\cdot\)L\({}^{-1}\) CaCl\({}_{2}\)) were applied to enhance the emulsion stability. The amount of composite emulsifier was 30 wt.% of the total monomers. The synthesized recipes and the corresponding emulsion stability are summarized in Table 1. It can be seen that emulsions with a BA content less than 75 mol% remain stable at 25 \({}^{\circ}\)C for more than 15 days, indicating superior stability. However, when the BA content reaches 92 mol%, the emulsion becomes unstable after one day, which is attributed to serious Ostwald ripening. The stable HIPEs were poured into the mold followed by thermally initiated polymerization to prepare polyHIPE membranes. The effect of conventional free radical polymerization (FRP) and RAFT polymerization on the structure and properties of polyHIPE membranes are compared while maintaining the molar ratio of BA/St/DVB equal to 9/2/1. As shown in Figure 2, the size of the samples is basically the same and the appearance is white, indicating that there is an obvious phase separation structure inside the samples. As presented in Figure 3, two kinds of polyHIPE membranes both exhibit an interconnecting open-cellular structure. Herein, the pore connectivity rate (P\({}_{\text{C}}\)) is defined to characterize the open-cellular extent inside polyHIPE membranes, which can be calculated by Equation (2):  
\[\text{P}_{\text{C}}=\text{P}_{\text{A}}/\text{P}_{\text{T}} \tag{2}\]  
where P\({}_{\text{A}}\) is the actual porosity measured by a mercury porosimeter and P\({}_{\text{T}}\) is the theoretical porosity calculated by Equation (3):  
\[\text{P}_{\text{T}}=\frac{\text{V}_{\text{H}_{2}\text{O}}}{\text{V}_{\text{H} _{2}\text{O}}+\text{V}_{\text{St}}+\text{V}_{\text{DVB}}+\text{V}_{\text{BA} }}=\frac{\text{m}_{\text{H}_{2}\text{O}}/\rho_{\text{H}_{2}\text{O}}}{\text{m }_{\text{H}_{2}\text{O}}/\rho_{\text{H}_{2}\text{O}}+\text{m}_{\text{Si}}/ \rho_{\text{Si}}+\text{m}_{\text{V}\text{BN}}/\rho_{\text{VBN}}+\text{m}_{ \text{NA}}/\rho_{\text{BA}}} \tag{3}\]  
where the V\({}_{\text{H}_{2}\text{O}}\) is the volume of water phase, V\({}_{\text{St}},\text{V}_{\text{DVB}},\text{V}_{\text{BA}}\) are the volumes of different ingredients, m\({}_{\text{St}},\text{m}_{\text{DVB}},\text{m}_{\text{BA}}\) are the actual weights of different ingredients, and \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**BA/St/DVB a**} & **RAFT b** & **Emulsifier (30 wt.\%)** & **Electrolyte (mol\(\cdot\)L\({}^{-1}\))** & **Initiator b** & & **Emulsion** \\  & & **(mol\%)** & **(mol\%)** & **(mol\(\cdot\)L\({}^{-1}\))** & **(mol\%)** & **O/W c** & **Stability** \\ \cline{3-8}  & & **Span80/DDBSS** & **CaCl\({}_{2}\)** & **AIBN** & & **(days)** \\ \hline RAFT-1 & 11/0/1 & 0.4 & & & & & \(>\)1 \\ \hline FRP-2 & 9/2/1 & — & & & & & \(>\)15 \\ RAFT-2 & 9/2/1 & 0.4 & & & & \(>\)15 \\ RAFT-3 & 7/4/1 & 0.4 & 9/2 & 0.12 & 0.67 & 1/3 & \(>\)15 \\ RAFT-4 & 5/6/1 & 0.4 & & & & \(>\)15 \\ RAFT-5 & 3/8/1 & 0.4 & & & & \(>\)15 \\ RAFT-6 & 7/2/3 & 0.4 & & & & \(>\)15 \\ \hline \hline \end{tabular}  
* molar ratio, b molar ratio of monomers, c volume ratio.  
\end{table} 
Table 1: The recipe and stability of HIPEs. The results of mercury porosimeter are summarized in Table 2. As can be seen, the specific surface area and pore connectivity of RAFT-2 and FRP-2 are basically consistent. However, the Pc of RAFT-2 and FRP-2 is merely around 90%.