Infrared spectroscopy of ion-induced cross-linked sulfonated poly(ether ether ketone)

The dehydration of SPEEK-[H] (protonated sulfonated poly(ether ether ketone)) converts 3-fold symmetric (C-3V) hydrated sulfonate sites to sulfonic acid sites with no local symmetry (C-1). Like Nafion-[H], SPEEK-[H] C-3V and C-1 environments afford IR group modes (C-3V,C-HF (1087 cm(-1)); C-3V,C-LF (1026 cm(-1)) and C-1,C-HF (1362 cm(-1)); C-1,C-LF (904 cm(-1))) due to the mechanical coupling of vibrational internal coordinates of an ether link with those of the sulfonate or sulfonic acid exchange site. C-3V and C-1 bands are inversely correlated during membrane hydration/dehydration. Hydrated SPEEK-[M] (M: Cu2+, Ni2+, Cd2+, Pb2+, Sr2+ or Ba2+) exhibits SPEEK-[H] C-3V bands because cation waters of hydration preclude binding to the hydrated exchange site. When cation hydration spheres are thermally stripped at high vacuum, SPEEK-[M] C-3V bands supplant SPEEK-[H] C-3V bands, inducing SPEEK cross-linking. Incomplete cross-linking is evidenced by low intensities of SPEEK-[M] C-1,C-HF bands. The 1362 cm(-1) band intensities are inversely correlated with cation hydration enthalpies. (C) 2016 Elsevier Ltd. All rights reserved.