Synthesis, phase structure and ion conductivity of poly(p-phenylene) functionalized with lithium trifluoromethanesulfonimide and tetra(ethylene oxide) side chains

Rigid-rod polymers tethered with delocalized anions and flexible ion conductive side chains present a synthetic pathway towards thin, single-ion conducting electrolyte membranes with low bulk resistance and high mechanical stability. In the present work we have synthesized poly(p-phenylene) (PpP) copolymers carrying lithium trifluoromethanesulfonimide groups and tetra(ethylene oxide) side chains through Ni-mediated Yamamoto polymerization of two p-dichlorobenzene derivatives. This gave PpPs where each phenylene ring carried one of the two functional groups and the composition of the copolymers could easily be controlled by adjusting the monomer feed ratio. Results from calorimetry and X-ray scattering measurements indicated the formation of molecular composites, i.e. bicontinuous morphologies with the assembly of layered stacks of the PpP backbones separated at a distance of 2-3 nm by the pendant groups of the copolymers. The ability to form an ordered morphology gradually decreased as the content of ionic groups in the copolymer was increased. The ionic conductivity reached values of 5 · 10^-9 and 8 · 10^-7 S cm^-1, at 20 and 80 °C respectively. Finally, we demonstrate that these values can be readily increased by blending with an amorphous polyether or by addition of a mixture of liquid carbonates, resulting in ionic conductivities reaching 6 · 10^-6 S cm^-1 at 80 °C.