Synthesis and characterization of new fluorinated poly(ether imide) copolymers with controlled degree of sulfonation for proton exchange membranes

A series of random sulfonated polyimide (SPI) with controlled sulfonation degrees were synthesized by one step high temperature polycondensation reaction using a commercially available sulfonated diamine, 4,4′-diaminostilbene-2,2′-disulfonic acid (DSDSA), 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA), and a fluorine containing quadri diamine (QA). Transparent, flexible and mechanically strong thin films were prepared by the solution–evaporation method using dimethylacetamide (DMAc). The structures of the resulting SPIs were confirmed by FTIR and 1H NMR spectroscopy. The polymers showed the onset of decomposition at around 290–300 °C for the ionic moieties and above 570 °C for the polymer backbone. The membranes exhibited good proton conductivity at higher sulfonation content which was found in the range of Nafion® 117 and were also comparable to many typical proton exchange membranes. A clear microphase separated structure was observed under transmission electron microscopy (TEM) which indicated the formation of hydrophilic and hydrophobic domain throughout the membrane.