Theoretical and experimental infrared spectra of hydrated and dehydrated sulfonated poly(ether ether ketone)

•Time-dependent FT-IR spectra of SPEEK are taken during dehydration.•IR bands are assigned using DFT-calculated normal mode animations.•Dissociated -SO3− group modes are termed C3V modes: Associated -SO3H as C1.•C3v and C1 modes are observed at all states of hydration.•Data supports a morphology with branched hydrophilic and dead-end aqueous regions.

Time-dependent FT-IR spectra of sulfonated poly(ether ether ketone) during dehydration show diminishing 1081 cm−1 and 1023 cm−1 band intensities concurrent with the emergence and shifting of bands at 1362 cm−1 and 898 cm−1. Animations of density functional theory calculated normal modes enable assignment of the 1081 cm−1 and 1023 cm−1 bands as group modes that include a sulfonate exchange site with C3v local symmetry, while the 1362 cm−1 and 898 cm−1 bands are assigned as group modes that include an associated sulfonic acid with no local symmetry (C1). In contrast to analogously assigned Nafion group mode bands, the SPEEK C3v and C1 bands coexist throughout the entire dehydration-hydration cycle, suggesting the presence of associated and dissociated exchange sites in SPEEK at all states-of-hydration. This supports a morphological model for SPEEK featuring branched hydrophilic domains and dead-end aqueous confines.

Graphical abstractDownload full-size image