The odd-even effect is a well-known phenomenon where various physical properties (e.g. density, melting points) exhibit non-monotonic changes between odd/even structural units and are frequently rationalized in terms of differences in molecular packing ability. Recently, odd-even effects were reported for glass transition temperatures (Tg) in ionic liquid networks of divalent ammonium cations and trivalent citrate anions. Such odd-even Tg effects are less commonly reported and their physical origin is still unclear. Here we report an odd-even Tg effect in amorphous polymerized ionic liquid networks (Figure 1) whose magnitude depends on the specific ions. Initially, the network contains fixed ammonium groups and a mobile BF<sub>4</sub> counterion which shows a monotonic decrease in Tg with increasing length of the linker between network junctions. Upon ion exchange to mobile bis(trifluoromethane sulfonamide) (TFSI) anions which exhibits weaker electrostatic interactions with ammonium, the Tg drops substantially and shows an odd-even effect with linker length. These networks are ion exchanged from the exact same precursor material ostensibly ruling out network topology as a cause of the effect. Wide angle x-ray scattering of the TFSI networks reveals the typical peaks associated with the ionic aggregate formation; however, the scattering peaks exhibit monotonic intensity variations while Tg exhibits a pronounced odd-even effect in the TFSI networks. Thus, in these systems, the scattering data is not a reliable indicator of dynamics. Conductivity measurements of TFSI systems indicate that the networks with odd linker length all have higher conductivities (&gt; 1 order of magnitude) than the even linker lengths providing an important insight into the design of ionic polymers for energy storage applications.

Ion specific odd-even effects glass transitions in polymerized ionic liquid networks

256th National Meeting (2018)

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