New fluorinated polymer- based nanocomposites via combination of sol-gel chemistry and reactive extrusion for polymer electrolyte membranes fuel cells (PEMFCs)

Proton exchange membranes designed via reactive extrusion from the in situ generation of functional silica-like particles in a new poly(vinylidene fluoride-co-hexafluoropropylene (poly(VDF-co-HFP)) copolymer are presented. These sulfonic acid-functionalized polysiloxane-based fillers were synthesized via sol-gel chemistry from 3-mercaptopropyltri(ethoxy)silane and polydimethoxysiloxane in the molten poly(VDF-co-HFP) copolymer. To process such nanocomposites materials, the reactive extrusion parameters were selected to (i) reach silanol condensation extents in the processing conditions of a conventional poly(VDF-co-HFP) copolymer and (ii) obtain a surface functionalization high enough with respect to reach a suitable protonic conductivity value of the nanocomposites films. Nevertheless, the enhancement of the proton conductivity being deeply associated with the morphologies of the nanocomposites materials (i.e. the dispersed functional silica phase needs to display the largest interface area with the polymer matrix), an interfacial agent was synthesized. This compatibilizer was either poly(VDF-co-alpha-trifluoromethacrylic acid) or poly(VDF-co-MAF) copolymers or a poly(VDF-co-HFP) copolymer grafted with maleic anhydride, denoted poly(VDF-co-HFP)-g-MA. In a second step, to obtain the required sulfonic acid-functionalized silica dispersed phase from the presence of the mercaptan functions, the resulting films were oxidized. Electrochemical properties were evaluated: for a theoretical ionic exchange capacity (IEC) of 2 meq.g(-1), experimental IEC ranged between 1.0 and 1.3 meq.g(-1) . The proton conductivity was found to reach 78 mS cm(-1) at room temperature for 100% of relative humidity. Interestingly, these values are higher than those of the same membranes processed without any compatibilizer (54 mS cm(-1)) and that of Nafion (R) NR112 (52 mS cm(-1)). This approach also allows limiting the swelling (or water uptake) of proton conductive membranes below 15 wt% while it reachs 30 wt% for Nafion (R) NR112 in similar conditions.