Effect of molecular weight on the mechanical and electrical properties of block copolymer electrolytes

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The relationship between ionic conductivity, morphology, and rheological properties of polystyrene-block-poly(ethylene oxide) copolymers (SEO) doped with a lithium salt, Li[N(SO2CF3)(2)], is elucidated. We focus on lamellar samples with poly(ethylene oxide) (PEO) volume fractions, phi, 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 degrees C increases with increasing M-PEO from about 4 x 10(5) to 5 x 10(7) Pa. Surprisingly, the conductivity of the SEO/salt mixtures with the molar ratio of Li to ethylene oxide moieties of 0.02 sigma, also increases with increasing M-PEO, from 6.2 x 10(-5) to 3.6 x 10(-4) S/cm at 90 degrees C. We compare sigma with the conductivity of pure PEO/salt mixtures, sigma(PEO), and find that sigma/[phi sigma(PEO)] of our highest molecular weight sample is close to 0.67, the theoretical upper limit for transport through randomly oriented lamellar grains.

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