Sulfonated copolyimides containing trifluoromethyl and phosphine oxide moieties: Synergistic effect towards proton exchange membrane properties

This paper reports the synthesis and characterization of a series of new semi-fluorinated sulfonated copolyimides (co-SPI) using a new trifluoromethyl and phosphine oxide containing diamine monomer, namely bis[4-(4'-aminophenoxy)-3-trifluoromethyl phenyl] phenylphosphine oxide (DFPPO). The copolymers were prepared by one pot polycondensation reaction of a mixture of two diamine monomers, namely 4,4'-diaminostilbene-2,2'-disulfonic acid (DSDSA) and DFPPO with the equimolar amount of 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA). All these copolymers were soluble in several organic solvents and size exclusion chromatography indicated the formation of high molar masses. The membranes prepared from these copolymers by solution casting route showed good mechanical properties. The polymers were characterized by FTIR and NMR spectroscopic techniques that confirmed the polymer repeat unit structures. The degree of sulfonation of the copolymers was determined from H-1 NMR signal intensities. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) images of the membranes showed phase separated morphology. In general, all the copolyimides showed high proton conductivity and reasonably high oxidative and hydrolytic stability. The co-SPI membrane DFPNH-90 (90% degree of sulfonation) with IECw = 2.86 mequiv g(-1) showed proton conductivity as high as 237 mS cm(-1) at 80 degrees C.