Synthesis and Characterizations of Novel Proton-Conducting Fluoropolymer Electrolyte Membranes Based on Poly(vinylidene fluoride-ter-hexafluoropropylene-ter-α-trifluoromethacrylic acid) Terpolymers Grafted by Aryl Sulfonic Acids†
Résumé
The synthesis and the characterization of new polymer electrolyte membranes made of fluorinated copolymers based on vinylidene fluoride (VDF) and hexafluoropropylene (HFP) and grafted by aryl sulfonic acids are presented. They were obtained in a three-step process. First, the conventional batch radical terpolymerization of α-trifluoromethacrylic acid (TFMAA), VDF and HFP, initiated by 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane led to original fluorinated functional terpolymers bearing carboxylic acid side groups in fair to good yields (>55%). The microstructure and the thermal properties of these macromolecules were studied. Interestingly, poly[(VDF-alt-TFMAA)-co-HFP)] random terpolymers that contained alternated microblock structures based on VDF and TFMAA units separated by one HFP unit were evidenced by 19F nuclear magnetic resonance (NMR) spectroscopy. That technique also enabled us to assess the termonomer contents. Average molecular weights, glass transition temperatures, and decomposition temperatures (under air), determined by size exclusion chromatography (SEC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA), ranged between 10 000 and 21 400 g mol−1, from−27 to −18 °C and from 140 to 275 °C, respectively. Indeed, the higher the TFMAA content, the lower the thermostability of the terpolymer that arised from decarboxylation. This degradation could be overcome by the reduction of the carboxylic acid dangling functions into hydroxyl ones. The third step concerned an etherification (Mitsunobu) reaction of such resulting primary hydroxyl groups with 4-sulfonic acid phenol. Both these reactions did not affect the contents of fluorinated termonomeric units as evidenced by 1H, 19F, and 13C NMR characterization. The microstructures, physicochemical, and thermal properties of the grafted materials were evaluated by NMR and infrared spectroscopies, SEC and DSC, and TGA. Membranes incorporating these functional fluoropolymers were processed by casting, and their preliminary electrochemical properties (ionic exchange capacity, proton conductivity, and swelling rates that reached 1.2 mequiv mol−1, 9 mS cm−1, and 58%, respectively) were studied, discussed, and compared to those of Nafion and to other fluorinated aromatic membranes of different architectures.