Synthesis of original para‐sulfonic acid aminoethylthioethylbenzenesulfonic by telomerization,and its grafting onto poly(VDF‐co‐HFP) copolymers for proton exchange membrane for fuel cell

The synthesis of a novel aromatic sulfonic acid bearing an amino function H₂N? C₂H₄? S? C₂H₄? C₆H₄? SO₃Na ( 1 ) from the radical addition of mercaptoethylamine hydrochloride onto styrene sodium sulfonate, and its subsequent grafting onto poly(vinylidene fluoride‐co‐hexafluoropropylene), poly(VDF‐co‐HFP), copolymer are presented. First, the radical telomerization, carried out under radical conditions and in water, led to various products [monoadduct ( 1 ), multiadducts, and polymers], the amounts of which depend on the experimental conditions and [mercaptan]₀/[monomer]₀ initial molar ratio (R₀). An R₀ ≥ 1 led to the monoadduct ( 1 ) only and achieved in ～85% yield. The zwitterionic isomer was obtained mainly and its chemical modification was possible to get an original aromatic sodium sulfonate containing an amino end group. A kinetic study of the telomerization was presented for R₀ < 1. Thermogravimetric analysis of the telomer showed that this compound was stable up to 200 °C. Second, the grafting of ( 1 ) onto poly(VDF‐co‐HFP) copolymer was also investigated. Such a grafting proceeded as expected by a classic mechanism of grafting of amines. Molar percentages of grafted telomer were assessed by ¹H NMR spectroscopy and by elemental analysis. Ion exchange capacity (IEC) values of the membranes were deduced from the mol % grafted telomer. Scanning electron microscopy pictures showed a good homogeneity in the cross‐section of membranes, and energy dispersive X‐ray evidenced that all SO₃Na groups of the grafted amine were changed into SO₃H after treatment with concentrated HCl. Method involving an impedance analyzer, working at increasing high frequencies was used to assess the protonic conductivities, σ. These values were lower than that of Nafion117^®, but σ increased with the IEC to 0.4 mS/cm at room temperature and 95% relative humidity. Water and methanol uptakes were also assessed, and it was shown that σ increased when water uptakes increased. Membranes started to decompose from 170 °C under air. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 121–136, 2009