## 3 Design Consideration on Radiation-Grafted AEM
### Selection of Polymer Backbone, Irradiation Dose and Dose Rate  
There are two categories of polymer backbone used in radiation-induced graft polymerization: fluorinated (or partially fluorinated) polymers and nonfluorinated polymers. Fluorinated polymers such as ethylene tetrafluoroethylene (ETFE) and Nafion(r)have been extensively used as polymer backbones owing to their favorable chemical and thermal stability. For instance, [PERSON] et al. [43] grafted VBC onto poly(ethylene-co-tetrafluoroethylene) (ETFE) using \(\gamma\)-ray in argon with a total irradiation dose of 90 kGy, and functionalized with DABCO, DCX and TMA at various degrees of grafting (D.O.G). The highest ionic conductivity achieved was 74 mS cm\({}^{-1}\) at 80 \({}^{\circ}\)C. [PERSON] [83] studied the effect of the spacer groups on the alkaline stability of polymer. Butyl-spacer styrene (labelled as C4-AEM) monomer and commercial VBC (labelled as C1-AEM) were grafted onto ETFE with an electron beam in air and aminated with MPY, respectively. They claimed that the IEC loss of C4-AEM after ageing in aqueous KOH (1 mol dm\({}^{-3}\)) at 80 \({}^{\circ}\)C for 28 days was 13%, approximately half of that of C1-AEM, which had an IEC loss of 27%. The degradation of C4-AEM was predominantly contributed to by the loss of Cl\({}^{-}\) anions, but the N atoms were still retained on the polymer. [PERSON] et al. [84] grafted VBC onto both polyvinylidene fluoride (PVDF) and fluorinated ethylene propylene (FEP), respectively. It was found that FEP-g-VBC membranes were thermally stable over a temperature range of 60 to 100 \({}^{\circ}\)C, with approximately an 18% drop in IEC at 60 \({}^{\circ}\)C over a period of 119 days and possessed good ionic conductivity at room temperature (reported at 0.01-0.02 S cm\({}^{-1}\)). Meanwhile, the PVDF-g-VBC membrane was unstable because the PVDF backbone degraded upon amation and alkaline treatment. [PERSON] et al. [70] have also grafted VBC onto ETFE membrane (ETFE-g-VBC) for AEMFC applications, and the membrane has demonstrated an IEC value of 1.24 med g\({}^{-1}\) with a considerably high water uptake of 155.4%. Despite the fact that high water uptake was reported, it may not necessarily be detrimental to the AEMFC performance, because water is of pivotal importance for ion conduction. The performance of ETFE-g-VBC was promising, with a peak power density of 240 mW cm\({}^{-2}\) in comparison to 180 mW cm\({}^{-2}\) of the benchmark MEA under identical conditions. The high water uptake of AEMs produced via radiation grafting was also confirmed by [PERSON] et al. [85]. The reaction of radiation grafting promotes a high degree of grafting (D.O.G) on the membrane, which in turn facilitates a greater percentage of water uptake. [PERSON] et al. [65] have attempted to improve the alkaline stability of fluorinated polymer backbone by preventing the degradation of the imidazolium group caused by \(\beta\)-elimination and ring-opening hydrolysis reactions. They have grafted both hydrophobic styrene (St) and imidazolium containing methyl group (2-methyl-\(N\)-vinylimidazole (MNVIm)) onto ETFE (MNVIm/St-AEM) in argon atmosphere at room temperature, using a \({}^{60}\)Co \(\gamma\)-ray source with a total irradiation dose of 80 kGy. The MNVIm/St-AEM was converted into terpolymergrafted membrane (MNVIm/En/St-AEM) upon treatment with 1 M KOH at 80 \({}^{\circ}\)C, where an ethene monomer (En) was introduced between St monomer and MNVIm monomer that is bonded to the ETFE. The ionic conductivity of MNVIm/En/St-AEM reduced drastically from 251 mS cm\({}^{-1}\) (MNVIm/St-AEM) to 53 mS cm\({}^{-1}\) but maintaining a similar swelling ratio of 23%. The alkaline stability of MNVIm/En/St-AEM was tested in 1 M KOH at 80 \({}^{\circ}\)C and its ionic conductivity was recorded.