Solid electrolyte of fully conjugated, water–soluble rigid-rod polymer with articulated backbone for isotropic ionic conductivity

An intractable and fully conjugated, aromatic, heterocyclic rigid-rod polymer poly[1,7-dihydrobenzo[1,2-d:4,5-d′]diimidazo-2,6-diyl-(2-sulfo)-p-phenylene] (sPBI) was derivatized with isophthalic acid for an articulated rigid-rod polymer asPBI. This molecular backbone alternation significantly changed the intrinsic viscosity [η] of sPBI from 9.4 dL/g to 1.1 dL/g for asPBI containing minute (1/50 molar ratio) articulation moiety indicating more a coil-like polymer for asPBI. Both sPBI and asPBI were reacted with 1,3-propanesultone in dimethylsulfoxide containing lithium hydroxide for water–soluble polyelectrolytes sPBI-Li+ and asPBI-Li+, respectively. The polyelectrolytes were dissolved in aqueous solution with up to 5 wt.% of LiI dopant and cast into films. Direct-current conductivity (σ) was measured at room-temperature parallel to the film surface yielding σ∥=3.2 × 10−3 S/cm and 2.8 × 10−3 S/cm for sPBI-Li+ and asPBI-Li+ (1/15), respectively. X-ray scattering and electron microscopy suggested that the cast films of sPBI-Li+ was in-plane isotropic but out-of-the plane anisotropic, and of asPBI-Li+ was three-dimensionally isotropic. It suggested that cast films of asPBI-Li+ having articulated backbone acquired an isotropic microstructure as well as an isotropic room-temperature σ superior to those of other solid polyelectrolytes.