Ion specific, odd–even glass transition temperatures and conductivities in precise network polymerized ionic liquids

Abstract

A series of crosslinked network polymerized ionic liquids (n-PILs) were synthesized with precise linker lengths, ranging from 4–12 carbons, to investigate the role of network nanostructure and electrostatic interactions on ion transport. The n-PILs contain tethered ammonium cations and either spherical BF₄ or large, non-spherical bis(trifluoromethane)sulfonimide (TFSI) mobile anions. In the TFSI networks, a pronounced odd–even effect is observed with carbon linker length where the glass transition temperature (T_g) can vary by 45 K and conductivity by almost two orders of magnitude simply by adding one carbon between the junctions. The dynamic fragility of the temperature-dependent conductivity also shows an odd–even effect. In the BF₄ networks, made from the same initial network, higher T_gs are observed with no odd–even effect. Both versions of the networks exhibit ionic aggregation peaks (via wide-angle X-ray scattering) which show monotonic variations with carbon length. These findings indicate that major odd–even T_g and conductivity effects can be observed with small variations in the chemical structure.