A series of composite membranes based on sulfonated polyether ether ketone with embedded powdered heteropolycompounds was prepared and their electrochemical and thermal properties were studied. An increase in degree of sulfonation as well as introduction of these fillers resulted in increased T g and enhanced membrane hydrophilicity, bringing about a substantial gain in proton conductivity. The conductivity of the composite membranes exceeded 10 −2 S/cm at room temperature and reached values of about 10 −1 S/cm above 100 • C.

Synthesis and characterization of sulfonated poly(ether ether ketone) for proton exchange membranes Xing, P.; Robertson, Gilles; Guiver, Michael; Mikhailenko, S.; Wang, K.; Kaliaguine, S. AbstractSeries of sulfonated poly(ether ether ketone)s (SPEEKs) were prepared by sulfonation of commercial Victrex ® and Gatone ® PEEK for a comparative study of proton exchange membranes (PEM) intended for fuel cell applications. The degree of sulfonation (DS) of the sulfonated PEEK was determined from deuterated dimethyl sulfoxide (DMSO-d 6 ) solution of the purified polymers using 1 H NMR methods. The second method using a solvent suppression technique, in which DS results were obtained directly from 1 H NMR spectra of SPEEK dissolved in sulfuric acid (non-deuterated) reaction medium was evaluated. The variation between the two methods was determined. The room temperature sulfonation of PEEK, monitored directly by second 1 H NMR method, proceeded rapidly initially, reaching DS ∼ 0.8 within 1 week, but progressed slowly thereafter. A maximum DS of 1.0 was determined after 1 month at ambient temperature (∼22• C). The thermal properties of SPEEK were characterized by means of DSC and TGA. The mass averaged molecular weights M w of both Victrex ® and Gatone ® PEEK were estimated from intrinsic viscosities measured in sulfuric acid solutions. It was verified that higher temperature (55• C) did not induce any apparent chain degradation of Victrex ® (or Gatone ® ) PEEK by M w tests. The water uptake and swelling properties of prepared films were studied and the proton conductivities at different temperatures were measured. The conductivities of the SPEEKs were found to increase with increasing DS and temperatures. The effect of film casting solvents on the conductivities is also discussed.

A series of sulfonated poly(aryl ether ketone) (SPAEK-6F) copolymers were prepared by aromatic nucleophilic polycondensation of hexafluoroisopropylidene diphenol with 5,5′-carbonylbis(2-fluorobenzenesulfonate) and 4,4′-difluorobenzophenone (DFBP) at various molar ratios. The synthesized SPAEK-6F polymers possessed high molecular weights revealed by their high viscosity, and tough and flexible membranes were obtained by casting from DMAc solution. The sulfonate or sulfonic acid content (SC) of the polymers, expressed as a number per repeat unit of polymer, was determined by 1 H NMR spectroscopy and was in good agreement with the initial monomer ratios. The membranes exhibited increased water uptake and swelling with increasing SC and temperature. SPAEK-6F membranes with SC ranging from 0.6 to 1.14 maintained adequate mechanical strength after immersion in water at 80°C for 24 h. Fenton's reagent test revealed that the SPAEK-6F membranes had good stability to oxidation. The proton conductivities of the SPAEK-6F films increased with SC and temperature, reaching values above 3 × 10 -2 S/cm at 80°C for SC g 0.75. The tensile test indicated that the SPAEK-6F membranes with SC 0.75, 0.98, and 1.14 are tough and strong at ambient conditions. Consequently, these materials proved to be promising as proton exchange membranes and may be potentially useful for application in fuel cells operated at medium temperatures.

Comparative studies of membranes prepared using different solvents, have shown that the casting solvent plays a significant role, affecting their proton conductivity and mechanical strength. It has been found that using DMF strongly decreases the membrane conductivity in comparison with other solvents studied. The 1 H NMR results yield an insight into the mechanism of this effect, evidencing the formation of the strong hydrogen bonding of sulfonic acid groups with DMF. This can explain the large discordances of more than an order of magnitude existing between the conductivity results for sulfonated polyetheretherketone (PEEK) in some previous studies and in this work. It is also found that residual sulphuric acid, which is very difficult to eliminate from highly sulfonated polyetheretherketone (SPEEK), also affects its conductivity and under high temperature treatment, enters into reaction with both DMF and N,N-dimethylacetamide (DMAc), causing their degradation. As discussed in the present contribution, the conductivity measurement technique may also be a reason for discrepancy in the reported conductivity characteristics of SPEEK.

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