Ionic Conductivity in Polyfluorene-Based Diblock Copolymers Comprising Nanodomains of a Polymerized Ionic Liquid and a Solid Polymer Electrolyte Doped with LiTFSI

Diblock copolymer electrolytes based on a π-conjugated polyfluorene (PF) backbone were synthesized comprising nanodomains of a polymerized ionic liquid (PIL) and of a solid polymer electrolyte (SPE). The former consists of a single-ion conductor based on an imidazolium alkyl chain with a [Br]− counteranion grafted on the PF backbone. The latter consists of short ethylene oxide (EO) chains, grafted on the PF backbone and further doped with LiTFSI. The two nanophases support ionic conductivity, whereas the rigid PF backbone provides the required mechanical stability. In the absence of LiTFSI, ionic conductivity in the PIL nanophase is low and exhibits an Arrhenius temperature dependence. LiTFSI substitution enhances ionic conductivity by about 3 orders of magnitude and further changes to a Vogel–Fulcher–Tammann temperature dependence. However, at ambient temperature, ionic conductivity is lower than in the corresponding PEO/LiTFSI electrolytes. X-ray studies and thermal analysis revealed that the conjugated backbone imparts liquid-crystalline order that can be fine-tuned through the EO side group length. Ionic conductivity measurements performed as a function of pressure identified local jumps of [Li]+ and [Br]− ions in the respective SPE/PIL nanophases as responsible for the ionic conductivity. Between the two ions, it is [Li]+ that has the major contribution to the ionic conductivity. The current results provide designing rules for new copolymers that comprise two different ionic nanodomains (PIL and SPE) and a conjugated backbone that can further support electronic conduction.