Poly (styrenesulfonic acid)-grafted poly(ethylene-co-tetrafluoroethylene) polymer electrolyte membrane (ETFE-PEM) applied for fuel cells was prepared by radiation induced-grafting using gamma-ray from 60Co source based on three steps (i) irradiation, (ii) polystyrene-graftedpoly(ethylene-alt-tetrafluoroethylene) (PS-g-ETFE), and (iii) sulfonation (ETFE-PEM). Mechanism of grafting and sulfonation of ETFE-PEM with grafting degree of 22% and sulfonation degree of 93% was revealed by solid 13C nuclear magnetic resonance (solid 13C NMR), Fourier transform infrared spectroscopy (FT-IR), and field emission scanning electron microscopy (FE-SEM). The obtained results indicated that the styrene was grafted on the matrix of ETFE polymer by the π bond-breaking reaction which could be attributed to free radicals and formed the polystyrene chain at the surfaces of crystalline phases. The styrene migrated deep into the membrane due to the concentration gradient. The grafting reactions occurred at sites of C-H and C-F in the ETFE backbone but dominantly at the C-F sites while the sulfonation took place at the para position of polystyrene. The signatures related to side (secondary) reactions and by-products were not observed in the spectra indicating that the graft polymerization and sulfonation are well-controlled.
The water states of poly(styrene sulfonic acid) (PSSA) grafted poly(ethylene-co-tetrafluoroethylene) copolymers (ETFE-PEMs) with grafting degree (GD)=8.8-30.5% are investigated under ambient conditions by using FT-IR analysis. Detailed analysis of the FT-IR spectra of ETFE-PEMs allows the observation of each component of water in the two broad bands of 1500-2000 cm-1 and 2400-3800 cm-1. These components include protonated water (H3O+...(H2O)n), H-bonded water to sulfonic acid groups ((SO3)...(H3O+)(H2O)n), H-bonded water to other water molecules ((H2O)...(H2O)), and non-H-bonded water (HOH...F). Of these water forms, ((SO3)...(H3O+)(H2O)n) is predominantly represented. Free water (non-H-bonded water) is assumed to be trapped in the holes of the ETFE backbone and is thus difficult to remove at 50oC over a 24 h period. The total peak areas of the water molecules significantly increase with GD indicating that the ETFE-PEM with higher GD possesses higher water content. Moreover, the increase of the total peak area with GD via two steps reveals an interesting relationship between the hierarchical structures and the capacity of water content under ambient conditions. The observed water content at ambient conditions can significantly affect the conductance of ETFE-PEMs under conditions of low relative humidity such as during fuel cell operation or measurement procedures of specific characterisations or testing methods.
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