What Determines the Glass Temperature and dc-Conductivity in Imidazolium-Polymerized Ionic Liquids with a Polythiophene Backbone?

Abstract

We report the results of a combined work based on density functional theory (DFT) calculations and experiments of the factors that influence the glass temperature, Tg, and the associated ion conductivity in polymerized ionic liquids bearing imidazolium salts in the side group. This study consists of four different N-alkyl side-chain lengths [with n = 4 (butyl), 6 (hexyl), 8 (octyl), and 10 (decyl)] and seven different counteranions ([Br]−, [BF4]−, [ClO4]−, [PF6]−, [Picrate]−, [TFSI]−, and [B(Ph)4]−). DFT calculations of the anion–cation complexation energies were combined with thermodynamics (differential scanning calorimetry), structural (X-ray scattering), as well as temperature- and pressure-dependent dielectric spectroscopy measurements of ion conduction. Our results show that ion conduction is facilitated by local anion jumps with a length scale on the order of the charge alteration distance. Ion complexation strongly influences the backbone dynamics and the associated Tg. A simple “stick and jump” model can account for the increased backbone mobility (reduced Tg) and the concomitant enhanced ion conductivity for anions with intermediate size. Among the different anions, [TFSI]− with its comparably large size and broad charge delocalization is only weakly coordinated with the cation. This best facilitates anion motion within the “ion paths” of the hexagonally packed cylinders and smectic morphologies.