Polymerized Ionic Liquids with Polythiophene Backbones: Self-Assembly, Thermal Properties, and Ion Conduction

Single ion conductors, based on polymerized ionic liquids (PILs) with a polythiophene backbone bearing imidazolium salts with butyl, hexyl, octyl, and decyl side groups and six different counteranions ([Br]−, [BF4]−, [ClO4]−, [PF6]−, picrate, and [B­(Ph)4]−), are synthesized and studied with respect to the thermal, structural, and ion-conductivity properties. PILs bearing the polythiophene backbone are unique as they can simultaneously conduct electronic charge and ions at nanometer length scales. In addition, the π–π stacking of the polythiophene backbones results in exceptional smectic-like order. Increasing side group length from butyl to decyl increases the room temperature conductivity by 4 orders of magnitude (internal plasticization). Anion size (anionic radii from 0.19 to 0.44 nm) affects both the structure (from smectic-like to amorphous with increasing anion radius) and the ionic conductivity. Conductivity values differ by 6 orders of magnitude by varying anion size at ambient temperature. As a result, conductivities as high as 2 × 10–3 S/cm could be obtained at high temperatures. Differences in conductivity are discussed in terms of changes in glass temperature (T g), anion size, and value of dielectric permittivity. Overall ion transport in PILs based on polythiophene backbones is controlled by the low T g, value of dielectric permittivity, smectic layering, and ion association lengths not exceeding a single smectic layer.