Counterion motions and thermal ordering effects in perfluorosulfonate ionomers probed by solid-state NMR

Perfluorosulfonate ionomers (PFSIs) neutralized by tetraalkylammonium ions have been investigated using variable temperature ¹H and ¹³C solid-state NMR (ssNMR) spectroscopy to probe the thermally induced properties of tetraalkylammonium ions at temperatures near the α-relaxations. Tetramethylammonium (TMA⁺), tetraethylammonium (TEA⁺), tetrapropylammonium (TPA⁺) and tetrabutylammonium (TBA⁺) ions have been incorporated in our study for the systematic control of ionic interactions within PFSIs according to the chain-length of ammonium ions. ¹³C static ssNMR results show that bulkier TPA⁺ and TBA⁺ ions undergo molecular tumbling motions near or above the α-relaxation temperature of the corresponding PFSI ionomers, with jumping rates of κ ≈ 1 kHz. Moreover, the results of ¹H spin-lattice (T₁) relaxation time measurements suggest that smaller TMA⁺ ions in the PFSI exhibit a thermally induced ordering effect as the sample temperature approaches the α-relaxation temperature of TMA⁺-PFSI. This ordering phenomenon is also supported by the results from small-angle X-ray scattering.