## 3 Results and Discussion
It is speculated that the high proportion of the soft monomer leads to the insufficient mechanical strength and thus results in partial pore collapse and closure [20; 33], which will be elaborated in the following discussion. As shown in the pore size distribution curves (Figure 4), RAFT-2 contains a large amount of smaller mesopores with a size about 1-2 nm, which does not appear in FRP-2, indicating remarkable difference in microstructure for materials prepared by RAFT polymerization and conventional free radical polymerization [34]. The stress-strain curves of synthesized polyHIPE membranes are shown in Figure 5 with results summarized in Table 2. As can be seen, the tensile strength, elongation at break, and modulus of toughness of RAFT-2 are obviously higher than those of FRP-2. Specifically, the tensile strength enhances from 266.8 \(\pm\) 7.05 kPa to 347 \(\pm\) 4.31 kPa, and elongation at break increases from 17.36 \(\pm\) 2.40% to 24.00 \(\pm\) 3.26%. Correspondingly, the M\({}_{\rm T}\) of RAFT-2 reaches to 47.60 \(\pm\) 7.88 kJ\(\cdot\)m\({}^{-3}\), which is about twice that of FRP-2. Figure 3: SEM images of polyHIPE membranes prepared by FRP and RAFT polymerization: (**a**) RAFT-2, (**b**) FRP-2. Figure 2: The optical images of polyHIPE membranes prepared by FRP and RAFT polymerization. The difference in the structure and properties of polyHIPE membranes prepared by RAFT polymerization and conventional free radical polymerization are intimately related to the formation of polyHIPE membranes. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Sample** & **P\({}_{\text{A}}\) (\%)** & **P\({}_{\text{T}}\) (\%)** & **P\({}_{\text{C}}\) (\%)** & **BET** & **E\({}^{\text{a}}\) (MPa)** & **\(\sigma\)\({}^{\text{b}}\) (kPa)** & **\(\epsilon_{\text{f}}\)\({}^{\text{c}}\) (\%)** & **M\({}_{\text{T}}\)\({}^{\text{d}}\)** \\  & & & & & & & & \\ \hline RAFT-2 & 67.21 & 75.69 & 88.80 & 3.42 & 2.04 \(\pm\) 0.34 & 347 \(\pm\) 4.31 & 24.00 \(\pm\) 3.26 & 47.60 \(\pm\) 7.88 \\ FRP-2 & 68.71 & 75.99 & 90.42 & 3.25 & 1.63 \(\pm\) 0.76 & 266.8 \(\pm\) 7.05 & 17.36 \(\pm\) 2.40 & 23.86 \(\pm\) 8.71 \\ \hline \end{tabular}  
\({}^{\text{a}}\) Young’s modulus, \({}^{\text{b}}\) ultimate tensile strength, \({}^{\text{c}}\) elongation at break, \({}^{\text{d}}\) modulus of toughness.  
\end{table} 
Table 2: The structure and mechanical properties of polyHIPE membranes prepared by FRP and RAFT polymerization. Figure 4: The pore size distribution curves of polyHIPE membranes prepared by FRP and RAFT polymerization. Figure 5: The stress–strain curves of polyHIPE membranes prepared by FRP and RAFT polymerization. to the kinetic characteristics of the polymerization process. In conventional free radical polymerization, the radicals formed by the decomposition of the initiator rapidly grow into polymer chains with high molecular weight, and then terminate in a very short time. Due to the low concentration of polymer chains, cyclization and intramolecular crosslinking reactions are prone to form microgels. As the reaction proceeds, the microgels form a network structure through intermolecular crosslinking [35], as illustrated in Figure 6a. The crosslinking network formed by microgels is heterogeneous and contains a lot of structural defects, which is unfavorable to the mechanical properties. By contrast, all of the polymer chain are generated at the beginning and grow simultaneously during RAFT polymerization. Therefore, a large number of the oligomer living chains tend to form a uniform intermolecular crosslinking network with less structural defects, as illustrated in Figure 6b, which helps to improve the mechanical properties [36]. Although RAFT polymerization effectively improves the toughness of polyHIPE membranes, the DSC results show that the Tg of RAFT-2 is about \(-6.7\)\({}^{\circ}\)C (Figure 7), which is lower than test temperature (i.e., 15 \({}^{\circ}\)C), indicating that RAFT-2 is in a rubbery state, so Young's modulus and the tensile strength of RAFT-2 are still low.