Ionic Conductivity in Physical Networks of Polyethylene−Polyether−Polyethylene Triblock Copolymers

A self-assembling ABA triblock copolymer complexed with lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt showed ion conductivities (σ) approaching 10-5 S/cm at 20 °C. The block copolymer had endblocks of polyethylene (PE) and midblocks of poly(ethylene oxide-co-propylene oxide) (PEOPO) and formed physical networks with PEOPO chains interconnected by crystalline PE domains. Thermal analysis showed a PEOPO glass transition at −68 °C and a PE melting point at 100 °C. When the melting region of the PE phase was reached, σ of the electrolytes was found to increase sharply to attain the same level as the corresponding electrolyte based on the neat PEOPO precursor block. Furthermore, the level of σ of the block copolymer electrolytes was found to be strongly dependent on the thermal history of the samples, and they showed a hysteresis behavior where σ increased by a factor of ≈3 after annealing above 110 °C. This may be explained by a reduction of the chain constraints of the PEOPO blocks in the physical network after annealing. The electrolytes behaved essentially like thermoplastics and were soft, tacky, self-supporting materials at room temperature.