Study on the mechanism of graft polymerization and sulfonation of proton exchange membranes for fuel cell

Abstract

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-grafted poly(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.