Effect of Molecular Weight on the Mechanical and Electrical Properties of Block Copolymer Electrolytes

The relationship between ionic conductivity, morphology, and rheological properties of polystyrene- block -poly(ethylene oxide) copolymers (SEO) doped with a lithium salt, Li[N(SO 2 CF 3 ) 2 ], is elucidated. We focus on lamellar samples with poly(ethylene oxide) (PEO) volume fractions, φ, ranging from 0.38 to 0.55, and PEO block molecular weights, M PEO, ranging from 16 to 98 kg/mol. The low-frequency storage modulus ( G ‘) at 90 °C increases with increasing M PEO from about 4 × 10 5 to 5 × 10 7 Pa. Surprisingly, the conductivity of the SEO/salt mixtures with the molar ratio of Li to ethylene oxide moieties of 0.02 σ, also increases with increasing M PEO, from 6.2 × 10 -5 to 3.6 × 10 -4 S/cm at 90 °C. We compare σ with the conductivity of pure PEO/salt mixtures, σ PEO, and find that σ/[φσ PEO ] of our highest molecular weight sample is close to 0.67, the theoretical upper limit for transport through randomly oriented lamellar grains.