End-functionalized block copolymer electrolytes: Effect of segregation strength on ion transport efficiency

We report the synthesis, morphology and ion transport properties of poly(styrene-b-ethylene oxide) (PS-b-PEO) block copolymers with various end-functional groups. The PS-b-PEO having an -OH terminal group showed poorly defined morphology, while the inclusion of a terminal acid unit (-SO₃ H or -COOH) in PS-b-PEO caused the development of an ordered lamellar structure. The terminal acid moieties in PEO were found to increase the segregation strength of PS-b-PEO and reduce the degree of crystallinity of PEO. This observation is intriguing given that the end group concentration of PEO in PS-b-PEO employed in this study is as small as 0.4 mol%. Decreased conductivity was observed for PS-b-PEO tethered with acid groups when doped with lithium salts, which we ascribe to the slow segmental motion of acid group-terminated PEO chains. However, organization of the PEO domains of PS-b-PEO with terminal acid units into self-assembled nanostructures having sharp interfaces proved beneficial for increasing ion transport efficiency by creating less tortuous ion conduction pathways. At high levels of salt loading, the thermodynamic effects of the terminal group on ion transport properties of PS-b-PEO were attenuated. Our results suggest a unique methodology for controlling the morphology and ion transport properties of block copolymer electrolytes via attachment of a single end functional unit.