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.

The ionic conductivity, σ, of mixtures of nearly symmetric polystyrene-block-poly(ethylene oxide) copolymers and Li[N(SO 2 CF 3 ) 2 ] (LiTFSI) salt was measured as a function of molecular weight, salt concentration, and temperature. The molecular weight of the poly(ethylene oxide) block, M PEO , was varied from 7 to 98 kg/mol. The molar ratio of lithium to ethylene oxide, r, was varied from 0.02 to 0.10. In general, σ increases with increasing M PEO for all values of r. The data can be summarized by plots of normalized conductivity, σ n , versus M PEO , where σ n = σ/( fφ PEO σ PEO ), φ PEO is the PEO volume fraction in the copolymer, σ PEO is the conductivity of PEO homopolymer, and f is a morphology-dependent factor set equal to 2/3 for our lamellar samples. The temperature-dependent conductivity data at a given salt concentration collapse onto a single curve when plotted in this format. At r = 0.085, σ n values reach a plateau in the vicinity of unity in the high M PEO limit. At other values of r, σ n continues to increase with M PEO within the experimental range and reaches a value of around 0.5 in the high M PEO limit.