## 9 Application of Derivatives of Styrene in Membrane of Fuel Cells
### Polystyrene-\(b\)-polybutadiene-\(b\)-polystyrene (SBS)  
SBS is another type of thermoplastic polymer derived from styrene, known for its flexibility and good durability. Despite these qualities, SBS is not directly used in fuel cells due to its low conductivity and chemical resistance. To enhance its properties, researchers functionalize it with hydrophilic groups. As an example, [PERSON] and his colleague scientists [120] prepared an AEM membrane by using a crosslinking and post-functionalization method. Crosslinking and polymerization are achieved by application of UV rays which opens the rings available on an SBS polymer and develop a connection with bromoalkyl groups [Figure 5a]. Crosslinking reduces excessive water uptake and swelling, maintaining mechanical integrity while ensuring sufficient hydration for ion transport. These broomalkyl groups are converted into quaternary ammonium (QA) groups through a reaction with trimethylamine (TMA). The study optimizes IEC to balance high ion conductivity with mechanical stability and controlled swelling, achieving values typically in the range of 1.5-2.5 meq. g\({}^{-1}\). Higher IEC enhances conductivity by increasing the number of ion-exchangeable groups but can also lead to excessive water uptake and swelling. The dual-functional epoxide groups in the polymer enable simultaneous quartermation (for high IEC) and crosslinking (to mitigate swelling and improve mechanical strength). As a result, the modified membrane contains hydrophilic side chains and PB blocks which provide support to mechanical strength. The modified SBS-based membrane contains long hydrophilic chains that enhance ionic clustering and create ionic channels for improved ion transport [Figure 5b,c]. The results show that with tune parameters of modified membrane, the hydroxide conductivity increases more than 100 mS cm\({}^{-1}\) when tested at a temperature of 80 \({}^{\circ}\)C. Moreover, it exhibits better toughness, mechanical strength, and high alkaline stability in 1 M NaOH solution at 80 \({}^{\circ}\)C for 500 h.  
Figure 5: (**a**) Schematic diagram of preparation of SBS, h-SBS-m, e-SBS-m; (**b**) approximated IEC of SEBS-based membrane; (**c**) graph between IEC vs. water uptake value of SEBS-based membrane [120].  
[PERSON] and his colleagues [121] prepared hydrogenated h-SBS triblock copolymer-based membranes by a series of chemical reactions including a thiol-ene click reaction containing polybutadiene with 6-mercapto-1-hexanol. Afterward, the remaining bonds were hydrogenated by a bromining reaction of alcohol groups. In the end, a substitution of TMA was carried out for complete formation of copolymer. The amount of functional groups contain 50% carbon single bonds and double bonds are controlled by controlling the thiol-ene click reaction. H-SBS polymer membrane modified with TMA and 24% functionalized groups exhibits an IEC and -OH ion conductivity of 1.83 meq. g\({}^{-1}\) and 93 mS cm\({}^{-1}\), respectively, when tested at a temperature of 80 \({}^{\circ}\)C. The results show that the hydroxide ion conductivity and IEC remain stable when modified membrane is preserved under a 1 M NAOH solution at a temperature of 80 \({}^{\circ}\)C for 4 weeks. Moreover, the modified SBS-TMA membrane provides a strong mechanical strength, less swelling under water and humid conditions, a minimum water uptake, and stable ion conductivity when tested at 80 \({}^{\circ}\)C temperature. These properties prove that the modified SBS-TMA membrane with additional functional groups are a good candidate for the application of AEMS as compared to rigid SBS polymer membrane.