Cross‐Linking of Sulfonated Poly(ether ether ketone) by Thermal Treatment: How Does the Reaction Occur?

Abstract

The cross-link reaction between macromolecular chains of sulfonated poly(ether ether ketone) (SPEEK) by thermal treatment above 150 degrees C in presence of dimethylsulfoxide (DMSO) is investigated by various techniques, including elemental analysis, acid-base titration, infrared spectroscopy, water uptake (WU) measurements, and thermogravimetry. The conditions of thermal treatment are analyzed. The cross-linking reaction occurs in at least three more or less activated and deactivated positions with different activation energies, leading to different time dependencies of the cross-link reaction. The role of residual solvent DMSO is studied particularly: the cross-linking depends significantly on the amount of solvent in the membranes. Implications on WU and thermal stability are of particular importance for the development of high performance proton-conducting membranes

Thermal Stability of Sulfonated Poly(Ether Ether Ketone) Films: on the Role of Protodesulfonation

“…Previous studies on SPEEK decomposition only reported SO 2 release in a unimodal peak between 200 and 400 8C (a temperature shift of some tens of degrees could be the result of differences in heating rate), although not all studies include an evolved-gas analysis up to a temperature of 500 8C. [13,15,17,27] Scheme 1 summarizes the two possible routes for SO x loss from the material: protodesulfonation or ipso substitution. If a sulfone (R-SO 2 -R) bridge is formed by ipso substitution, this sulfone bridge can in turn decompose at higher temperatures, yielding the release of a second SO x species.…”

“…[28] Upon ipso substitution, SO 2 release would be expected. It has to be noted that the ipso substitution reaction was found to depend strongly on the presence of traces of solvent: it was found to occur in the presence of DMSO, [13,15] but not with NMP, DMAc, or DMF. [29] A third possible reaction, H-substitution, results in cross-linking as well, but will not cause SO 2 release.…”

“…[10,11] Additionally, during membrane preparation high temperature treatments are suggested to remove residual high boiling point solvents after membrane formation, [12] bring the material above its glass transition temperature to remove its thermal history, [4] or to perform temperature-promoted cross-linking. [13,14] Whereas, temperatures up to 200 8C are not uncommon in these procedures, [15] they can have detrimental effects on the integrity of SPEEK. [16] Generally, thermal changes to SPEEK are reported to occur via three separate processes: [17,18] the removal of absorbed water and solvent (T ¼ 50-150 8C); temperaturepromoted cross-linking, annealing and/or removal of the sulfonic acid group (T ¼ 150-400 8C); and backbone decomposition (T > 400 8C).…”

“…Membrane performance studies and thermogravimetric analysis on bulk material, in which the sulfonate proton was exchanged by a sodium ion, showed indications of an enhanced thermal stability. [12,15,16,20] Nonetheless, the effect of exchanging the sulfonate proton for a different counter-ion on the thermal processing has hitherto not been fully appreciated and studied systematically for SPEEK films.…”

Comparative study of the cation permeability of protonic, anionic and ampholytic membranes

“…After cooling to room temperature, the resulting membranes were peeled off and thermally treated at 180 °C for 3, 10, and 24 h. The mechanism of cross-linking was studied and described in references 30,31 .…”