Abstract
Original synthesis and appropriate characterizations of novel fluorinated copolymers bearing phosphonic acid side functions for novel membranes for further fuel cell applications are presented. These copolymers were achieved by chemical modifications of various poly(CTFE-alt-CEVE) and poly[(CTFE-alt-IEVE)-co-(CTFE-EVE)] terpolymers (where CTFE, CEVE and EVE stand for chlorotrifluoroethylene, 2-iodo ethylvinyl ether and ethylvinyl ether) via arbuzov reaction. The hydrolysis of the phosphonate moieties into phosphonic acid groups was carried out quantitatively in the presence of bromotrimethylsilane. The Ionic exchange capacities (IECs) values were determined by potentiometric titration from 2.9 to 6.8 meq/g. The thermal and electrochemical properties of the resulting membranes (processed by casting) were investigated. Whatever the phosphonic acid content, the main degradation started from 250°C, showing a high thermo-oxidative stability of these copolymers. At 25°C and 100 % relative humidity, the level of conductivity was found in the range of 0.02 to 20 mS/cm and highly dependant on the IEC. Finally, moderate to good conductivity values (about 0.25 mS/cm) at higher temperature (120 °C) and lower RH (25 %) were observed, which demonstrated a limited dependence towards both temperature and RH.