Ionic conductivity in physical networks of polyethylene-polyether-polyethylene triblock copolymers

A self-assembling ABA triblock copolymer complexed with lithium bis(trifluorometliylsulfonyl)imide (LiTFSI) salt showed ion conductivities (sigma) approaching 10(-5) S/cm at 20 degreesC. 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 degreesC and a PE melting point at 100 degreesC. When the melting region of the PE phase was reached, sigma 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 sigma 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 sigma increased by a factor of approximate to3 after annealing above 110 degreesC. 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.