New semi-IPN PEMFC membranes composed of crosslinked fluorinated copolymer bearing triazole groups and sPEEK for operation at low relative humidity

The synthesis, processing, crosslinking, and characterization of original proton conducting membranes are presented. First, fluorinated terpolymers were obtained from the radical terpolymerization of chlorotrifluoroethylene (CTFE), 2-chloroethyl vinyl ether (CEVE) and glycerine carbonate vinyl ether (GCVE) followed by two chemical modifications into poly(IEVE-alt-CTFE)x-g-1H-1,2,4-triazole-3-thiol-co-(GCVE-alt-CTFE)y terpolymer that bear dangling cyclocarbonate and triazole functions (where IEVE stands for 2-iodoethyl vinyl ether). The successful grafting (overall yields >80%) of these terpolymers was monitored by 1H and 19F NMR spectroscopy and by thermal analyses (DSC and TGA) and did not shown any opening of the cyclocarbonate ring. Such fluorofunctional terpolymers were involved in the preparation of blend membranes with sulfonated PEEK (sPEEK), followed by the crosslinking in the presence of two telechelic diamines of different chain lengths via cyclocarbonate/amine reaction. The characterizations of these resulting membranes (thickness = 35-50 μm) in term of thermal stability (>200 °C under air), water uptake (as low as <20%), conductivity, and mechanical properties (strain-stress relationships and by dynamical mechanical analysis) were comprehensively investigated to understand the influence of the nature of the diamines, the crosslinking, and the presence of azole group on membrane microstructure. Proton conductivities of crosslinked blend membranes comprising such fluorinated polymer components functionalized with triazole at 140 °C and at low relative humidity (<25%) reached 4.3 mS cm−1 for a blend membrane containing 60%-wt of s-PEEK and 40%-wt of poly(CTFE-alt-IEVE)82%-g-1H-1,2,4-triazole-3-thiol86%-co-(CTFE-alt-GCVE)18% terpolymer. They displayed glass transition temperatures, Young modulus and tensile strengths up to 223 °C, 4 MPa and 6%, respectively.