Synthesis of Proton-conducting Electrolytes Based on Poly(vinylidene fluoride-co-hexafluoropropylene) via Atom Transfer Radical Polymerization

The preparation of poly(vinylidene fluofide-co-hexafluoropropylene) (PVDF-HFP) grafted poly (styrene sulfonic acid) (PVDF-HFP-g-PSSA) copolymer as proton-conducting electrolytes by atom transfer radical polymerization of styrene sulfonic acid at the secondary halogenated sites of PVDF-HFP was demonstrated. The structure of the PVDF-HFP-g-PSSA copolymers was verified by Fourier transform infrared spectra, proton nuclear magnetic resonance spectra and X-ray photoelectron spectroscopy. The PVDF-HFP-g-PSSA copolymer membranes showed ion exchange capacity values ranging from 0.045 to 0.272 mEq g(-1), the water uptake varied from 13.7 to 26.8 wt.% and the proton conductivities varying from 1.85 x 10(-4) to 9.8 x 10(-4) S cm(-1), all of which could be modulated by control of the polymerization time. All the membranes exhibited decomposition temperature up to around 350 degrees C as revealed by thermogravimetric analysis. The incorporation of poly(styrene sulfonic acid) into PVDF-HFP chains resulted in melting at higher temperatures. The scanning electron microscopy observation indicated that the density of the ionic pathways increased with ionic content, which explained why the ionic conductivity rose to relatively high values as polymerization time increasing.