Abstract
The dehydration of SPEEK-[H] (protonated sulfonated poly(ether ether ketone)) converts 3-fold symmetric (C3V) hydrated sulfonate sites to sulfonic acid sites with no local symmetry (C1). Like Nafion-[H], SPEEK-[H] C3V and C1 environments afford IR group modes (C3V,HF (1087 cm−1); C3V,LF (1026 cm−1) and C1,HF (1362 cm−1); C1,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. C3V and C1 bands are inversely correlated during membrane hydration/dehydration. Hydrated SPEEK-[M] (M: Cu2+, Ni2+, Cd2+, Pb2+, Sr2+ or Ba2+) exhibits SPEEK-[H] C3V 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] C3V bands supplant SPEEK-[H] C3V bands, inducing SPEEK cross-linking. Incomplete cross-linking is evidenced by low intensities of SPEEK-[M] C1,HF bands. The 1362 cm−1 band intensities are inversely correlated with cation hydration enthalpies.
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