# Preparation and Chemical Protective Clothing Application of PVDF Based Sodium Sulfonate Membrane
## 4 Discussion  
Nafion and polystyrene sulfonic acid (PSSA) are the state-of-the-art membrane materials for CPC. However, Nafion is too expensive for large scale applications, and PSSA does not possess sufficient mechanical properties. In this study, PVDF-\(g\)-SSS membranes were prepared using a convenient, low cost, and one-pot method. A series of PVDF-\(g\)-SSS membranes with different SSS grafting ratios were synthesized (M1 to M6). The successful grafting of SSS onto modified PVDF was clearly demonstrated by FT-IR. SEM images clearly show the morphologies of membranes were uniform, dense, and nonporous. This indicates that the selectivity of the membrane is not due to aperture screening but a dissolution-diffusion mechanism. Figure 8: Vapor permeation for water, DMMP, and CEPS for M1–M6. Figure 9: Selectivities of water over DMMP and CEPS for M1–M6. IEC measurement suggests a high conversion rate of SSS monomer (>70%). The WU and LSR were found to increase with IEC in a nonlinear manner. For M5 and M6 with IEC of 1.95 mmol/g and 2.29 mmol/g, an obvious leap in WU and LSR was observed. Mechanical properties tests suggest that the membrane flexibility is significantly improved by grafting modification, but tensile strength degrades obviously when the IEC exceeds 2.29 mmol/g (M6). The results show that the mechanical properties and dimensional stability of membrane materials with IEC lower than 2 mmol g\({}^{-1}\) are controlled within an acceptable range, which is due to the cross-linking structure of membranes. Moderate cross-linking can limit the movement of the chain segments and the swelling of the membrane [37; 38]. The contact angle measurement shows that hydrophilicity could be improved by increasing the content of the hydrophilic sodium sulfonate group. The increase of hydrophilic sodium sulfonate groups increases the surface energy of the material and enhances the water wettability of the membrane. Furthermore, water wettability is crucial for sweat permeation, thus directly influencing the comfort of CPC. The WVTR test shows that the water vapor transfer rate of membranes increase with increasing IEC. From M3 to M4, the WVTR shows an obvious leap, which may be due to the formation of a continuous water transport channel at high SSS content. As well known, due to electrostatic interaction, the branch chains contained hydrophilic sulfonic acid group will gather together to form ion clusters. When being hydrated, ion clusters will swell and interconnect to form hydrophilic channels, thus accelerating the transfer of water. The increase of the sulfonic acid group content is beneficial to the increase of the number and size of ion clusters, thus accelerating the formation of water channels [39; 40; 41]. Membranes with IEC \(>\) 1.7 mmol g\({}^{-1}\) shows WVTR above 2000 g m\({}^{-2}\) day\({}^{-1}\), which can fulfill the requirement of CPC application. DMMP vapor permeation (VP) test shows the VP of DMMP and CEPS decrease with IEC values, which is opposite to that of water vapor permeation. As a result, high water/DMMP selectivity >150 could be achieved at the IEC of 2.29 mmol/g, and high water/CEPS selectivity >3000 could be also achieved at 2.29 mmol/g. This excellent selectivity at high IEC values may be explained by the formation of an interconnected SSS phase, which allows the transport of water and blocks the transport of DMMP. Vapor permeation occurred via two processes: (a) vapors sorption process and (b) vapors diffusion process [42]. The SSS groups in PVDF-_g_-SSS can form hydrated ionic regions, i.e., a water channel, which can lead to good absorption ability to water molecules. On the other hand, the hydrophobic CWA simulants would be repelled by the water channels, resulting in low absorption. In the diffusion process, water mainly diffuses through the hydrophilic phase, while DMMP and CEPS mainly diffuse through the hydrophobic phase [10].