## 3 Results and Discussion  
### Sulfonation  
With regard to the heterogeneous sulfonation process, the DVB-crosslinked polystyrene particles are so dense and hydrophobic that it is difficult for the hydrophilic sulfonation agent to enter the internal volume of the particles without the swelling agent. In the large-scale industrial production of polystyrene-based strongly acidic cation exchange resins, 1,2-dichloroethane (DCE) is the most widely used as the swelling agent to accelerate the reaction rate and to increase the sulfonation degree, especially when employing inexpensive concentrated sulfuric acid (usually 93-98 wt%) as the sulfonation agent. In view of the fact that China has become the world's largest production country for ion exchange resins [14], the concentrated sulfuric acid (98 wt%) and DCE are still employed to sulfonate the polystyrene/PVDF alloy particles, in order to realize the relative \"seamless combination\" with the already existing industrial base of cation exchange resins. Figure 2 investigated the effect of DCE as the swelling agent on the sulfonation efficiency, indicating that DCE was necessary for the polystyrene/PVDF alloy particles to obtain a satisfactory reaction degree, especially when the DVB content (which also represents the crosslinking degree) increased to 5% or 8%. For instance, DCE directly endowed the sulfonation degree with a more than a 100% increase (from 36% to 75%) for the 8% DVB case. In the absence of the suitable swelling agent,it was difficult for the concentrated sulfuric acid to rapidly permeate into the highly hydrophobic polystyrene-DVB beads, which led to the sulfonation mainly occurring at the outer surface rather than in the inner surface of the beads [20]. For the polystyrene/PVDF alloy particles, the hydrophobic PVDF component as the main matrix material will undoubtedly limit the concentrated sulfuric acid's penetration into the alloy particles. Fortunately, Figure 2 has shown that DEC is still an effective swelling agent, and thus, the classical manufacturing method of polystyrene-based strongly acidic cation exchange resins is also available for the preparation of the polystyrene/PVDF cation exchange membrane. Figure 3 shows the FTIR spectra of polystyrene-DVB particles (Figure 3a), PVDF powder (Figure 3b), the polystyrene/PVDF alloy particles (Figure 3c) and the polystyrene/PVDF-based cation exchange alloy resin (Figure 3d). In the spectrum for PVDF (Figure 3b), bands at 1400 and 1185 cm\({}^{-1}\) are assigned to the deformation vibration of CH\({}_{2}\) linked with CF\({}_{2}\) and the stretching vibration of CF\({}_{2}\), respectively. These two peaks were also observed in Figure 3c and Figure 3d, indicating the incorporation of PVDF both in the polystyrene/PVDF alloy particles and the cation exchange alloy resin. Similarly, the characteristic bands of polystyrene-DVB particles (Figure 3a) at 679, 1452 and 1493 cm\({}^{-1}\) are present in Figure 3c. However, the bending vibration band of the benzene ring plane at 1028 cm\({}^{-1}\) (in Figure 3a), had split into two independent peaks, respectively at 1009 and 1039 cm\({}^{-1}\) (see Figure 3d). This was believed to be affected by the symmetric stretching vibration of the S=O bond affiliated with the SO\({}_{3}\)- group [21]. Also shown in Figure 3d, the fairly broad band at about 3460 cm\({}^{-1}\) was assigned to the bending vibration of the OH group, which was probably caused by the drifting of its stretching vibration toward the low frequency direction, under the hydrogen bond interaction between the H\({}_{2}\)O molecule and the oxygen atom of the SO\({}_{3}\)- group [21]. Moreover, the aromatic C-H absorption peak was present at 1637 cm\({}^{-1}\) (Figure 3d) and would strengthen and broaden with the increase of the sulfonation degree (not shown). Furthermore, the band at 1128 cm\({}^{-1}\) should be assigned to the antisymmetric stretching vibration of the S=O bond [22].