Dynamics of ionic liquid–polymer gel membranes—Insight from NMR relaxometry for [BMIM][BF4]–PVDF-HFP systems

1H spin-lattice relaxation experiments have been performed for ionic liquid-polymer gel membranes, including 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) with different proportions. The experiments have been performed in a broad range of resonance frequencies (from about 5 Hz to 40 MHz) vs temperature and complemented with analogous studies for [BMIM][BF4] in bulk as a reference. A model of the relaxation processes in the membranes has been proposed. The model includes two relaxation contributions. One of them corresponds to the concept of restricted, two-dimensional translation diffusion with a residence lifetime, while the second one has the form characteristic of polymers (mathematically similar to the limiting behavior of two-dimensional translation diffusion with a very long residence lifetime). The extensive dataset has been consistently interpreted in terms of the model, revealing two dynamical processes on the time scales of 10-7 s (for the second relaxation contribution) and 10-9 s (for the first one). The relationship of these relaxation contributions to the motion of the polymer or ionic liquid-polymer complexes and to the translation diffusion of BMIM cations in the matrix has been discussed.