## 3 Design Consideration on Radiation-Grafted AEM
### Selection of Polymer Backbone, Irradiation Dose and Dose Rate
[95] have studied the alkaline stability of radiation-grafted LDPE-based AEM at temperatures above 60 \({}^{\circ}\)C. The LDPE-based AEMs were subjected to a 4.5 MeV electron beam irradiation at a dose of 100 kGy in air and subsequently grafted with vinylbenzyl chloride (VBC) and aminated with trimethylamine (TMA), N-methylpyrrolidine (MPY) and N-methylpiperidine (MPIP) head groups, respectively. They found that TMA-type LDPE-based AEM has the highest Cl\({}^{-}\) ionic conductivity (100 mS cm\({}^{-1}\) at 80 \({}^{\circ}\)C) and lowest alkaline degradation compared to the other two types. LDPE-g-VBC-TMA fabricated via the simultaneous irradiation technique using \(\gamma\)-ray in N\({}_{2}\) saturated solution at a dose rate of 2 kGy h\({}^{-1}\) and a total radiation dose of 20 kGy by [PERSON] et al. [69; 96] also showed a similar ionic conductivity of 90 mS cm\({}^{-1}\) at 50 \({}^{\circ}\)C.  
While LDPE-based AEM has superior ionic conductivity and fast water transport kinetics, it is still prone to environmental stress cracking, particularly in the long-term operation of fuel cells and electrolysis cells. Limited by the membrane thickness for a fuel cell and electrolysis cell, [PERSON] et al. [63] have proposed to replace the LDPE-based AEM with high-density polyethylene-based AEM (HDPE-based AEM) to enhance the mechanical properties of AEM. They have produced HDPE-based AEM using a 4.5 MeV dynamic continuous electron beam at a high dose rate electron beam of 100 kGy in air. The HDPE-based AEM was then grafted with VBC saturated in N\({}_{2}\) for 4 h at 50 \({}^{\circ}\)C. This HDPE RG-AEM showed an enhancement in tensile properties (52% higher than that of LDPE), fuel cell performance (voltage degradation up to 7%) and operational stability (400 h of continues operation), yet with a comparable ionic conductivity (-214 mS cm\({}^{-1}\)), ionic exchange capacity (-2.44 mmol g\({}^{-1}\)) and water uptake (-155%) properties to LDPE-based AEM, which is contributed to by the changes in nanomorphology that improved the water transport mechanism after radiation grafting. Apart from HPDE, ultra-high molecular weight polyethylene (UHMWPE) is another class of low-cost and extremely tough polyethylene that can be used as the base polymer. [PERSON] et al. [67] grafted VBC onto UHMWPE powder with \(\gamma\)-ray, which is subsequently compressed into thin AEM membrane and quaternized with TMA (UHMWPE-g-VBC-TMA). The highest ionic conductivity obtained in this investigation was 47.5 mS cm\({}^{-1}\) at 90 \({}^{\circ}\)C, which was much lower than that of LDPE-based and HDPE-based AEMs discussed above. However, its ionic exchange capacity was notably constant (-0.58 meq mol g\({}^{-1}\)), even after 120 h of submersion in 10 M NaOH at 60 \({}^{\circ}\)C. Later, [PERSON] et al. [68] quaternized UHMWPE-g-VBC with modified guanidine, 1,1,3,3-tetramethyl-2-n-butylguanidine (UHMWPE-g-VBC-TMBG). The IEC values increased with the total irradiation dose, with the highest IEC obtained for this membrane being 0.56 meq g\({}^{-1}\) with a total irradiation dose of 25 kGy. This trend agreed with other studies. However, they also observed that the maximum ionic conductivity was achieved at 27.7 mS cm\({}^{-1}\) at 90 \({}^{\circ}\)C with a total irradiation dose of 5 kGy. Beyond this point of irradiation dose, the ionic conductivity of the membrane decreased, which may be attributed to the excessive phase separation that disrupts the connectivity of ion conduction channels, resulting in higher resistivity. [PERSON] et al. [64] grafted VBC onto various sources of polyethylene, consisting of casting membranes of both low-density polyethylene (cast-LDPE) and linear low-density polyethylene (cast-LLDPE) from pellets supplied by Sigma Aldrich, and commercial membrane films from British Polyethene Industri plc (BPI-LDPE), VMR International (VMR-LDPE), ENTEK microporous (ENTEK-UHMWPE), using \(\gamma\)-ray with a dose rate of 2 kGy h\({}^{-1}\), and functionalized with TMA.