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, 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 × 105 to 5 × 107 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, σPEO, and find that σ/[φσPEO] of our highest molecular weight sample is close to 0.67, the theoretical upper limit for transport through randomly oriented lamellar grains.

Within a polymer film, free-volume elements such as pores and channels typically have a wide range of sizes and topologies. This broad range of free-volume element sizes compromises a polymer's ability to perform molecular separations. We demonstrated free-volume structures in dense vitreous polymers that enable outstanding molecular and ionic transport and separation performance that surpasses the limits of conventional polymers. The unusual microstructure in these materials can be systematically tailored by thermally driven segment rearrangement. Free-volume topologies can be tailored by controlling the degree of rearrangement, flexibility of the original chain, and judicious inclusion of small templating molecules. This rational tailoring of free-volume element architecture provides a route for preparing high-performance polymers for molecular-scale separations.