## 3 Results and Discussion
### Thermal Stability  
The thermal stability of promising membranes from Table 4 was measured by thermos gravimetric analysis (TGA). Figure 7 shows TGA curves for the non-woven PET, GDV15-TMA, GDV15-TPA, GDV45-TMA, GDV45-TEA, GDV75-TMA, and GDV75-DABCO which were recorded under nitrogen gas from 30 \({}^{\circ}\)C to 900 \({}^{\circ}\)C. Weight ratio of polymer network to non-woven PET was nearly 1.3:1, although the composition of monomers was varied. Non-woven PET presented a single-step decomposition and endured up to 370 \({}^{\circ}\)C due to its strong chemical structure. Non-woven PET, which is coated by polymer exhibited two step decomposition. The slight mass loss below 120 \({}^{\circ}\)C was due to evaporation of water absorbed in the GDV15-TMA, GDV15-TPA, GDV45-TMA, GDV45-TEA, GDV75-TMA, and GDV75-DABCO membranes. The mass loss in quaternary ammonium (TMA, TEA and TPA) occurred in the 200-280 \({}^{\circ}\)C range. The mass loss in the 280-470 \({}^{\circ}\)C range was due to the degradation of polymer backbone (GDV15-TMA, GDV15-TPA, GDV45-TMA, GDV45-TEA and GDV75-TMA). Membranes which had quaternary ammonium (TMA, TEA and TPA) showed weight loss at mostly the same stage. However, mass loss in the diammonium (DABCO) was observed in the 130-250 \({}^{\circ}\)C range. The mass loss in the 250-450 \({}^{\circ}\)C range was due to the degradation of polymer backbone (GDV75-DABCO). Membranes quaremized by DABCO were relatively unstable compare to those quaremized by TMA, TEA, and TPA. All membranes listed in Table 4 showed thermal stability, with 5% mass loss in the 150-370 \({}^{\circ}\)C range. Ammonium functional groups (TMA, TEA, and TPA) are stable up to 200 \({}^{\circ}\)C, whereas diammonium functional group (DABCO) is stable up to 130 \({}^{\circ}\)C [31]. Therefore, these functional groups collapse when membranes are exposed at over 200 \({}^{\circ}\)C and at 130 \({}^{\circ}\)C over a long time, respectively. GDV45-TMA exhibited the highest thermal stability, but GDV15-TPA and GDV75-DABCO showed the lowest thermal stability among the membranes. Therefore, TMA functional group with low GMA content displayed the strongest thermal stability compared to other functional groups.  
amount of the C-Cl bond. However, two forms of DABCO mentioned above coexist in the quaternized membrane. The bis-quaternized DABCO membrane, which serves as cross-linker was more chemically unstable than the mono-quaternized DABCO membrane [36]. Mono-quaternized DABCO are then destroyed after bis-quaternized DABCO collapse because hydroxide can approach positively charged DABCO when bis-quaternized DABCO disappears. It was known that bonding VBC with DABCO showed resistance in alkaline condition due to steric hindrance. However, bonding epoxy with DABCO easily degraded in alkali environment when epoxy of GMA was relatively high in the membrane. It indicates that combination of epoxy with DABCO is easily exposed to hydroxide ion attack compared to the combination of VBC with DABCO. In the end, IEC and conductivity decline in GDV75-DABCO membrane. Chain structure functional groups (TMA, TEA, and TPA) are more desirable than heterocyclic structure functional groups (DABCO) for long exposure in hydroxide solution. Still, further investigation of the nucleophilic attack mechanism on each of the membranes are needed. According to alkaline resistance results, GDV15-TPA and GDV75-TMA membranes have the highest tolerance to alkaline solutions among membranes. Therefore, tracking the progress of alkaline stability at high temperatures (over 60 \({}^{\circ}\)C) is required in further work.