RESUMO
Sulfonated poly(ether ether ketone) (SPEEK) membranes were thermally treated at temperatures between 120 and 160 degrees C. Water uptake measured at different relative humidity values or by full immersion in water between 25 and 145 degrees C was found to depend very strongly on previous thermal treatment and casting solvent. Water-uptake coefficient values as low as 10-15 even upon immersion in water at 100 degrees C were obtained with membranes treated at 160 degrees C. This effect is related to cross-linking by SO2 bridges between macromolecular chains. An important role is also played by the casting solvent: among the investigated solvents, dimethylsulfoxide (DMSO) gave the best results. A chemical kinetics model is outlined that permits the estimation of the relevant kinetic parameters, especially the activation energy of the cross-linking reaction, which was found to be about 60 kJ/mol. These results are of significant importance for the improvement of proton-exchange membrane fuel cells.
RESUMO
MxOy nanometric powders (Nb2O5, In2O3, and ZrO2) have been efficiently synthesized at low temperatures with the use of simple precursors and with no acid or base catalysis or stabilizing agents. The powders have been characterized by Thermogravimetry/Differential Thermal Analysis, X-ray diffraction, and scanning electron microscopy. The oxides obtained have well-defined crystalline structures, exhibit homogeneity, and crystallite sizes ranging from 9 to 16 nm in diameter.