Effect of Molecular Weight and Salt Concentration on Conductivity of Block Copolymer Electrolytes

The ionic conductivity, sigma, of mixtures of nearly symmetric polystyrene-block-poly(ethylene oxide) copolymers and Li[N(SO2CF3)(2)] (LiTFSI) salt was measured as it 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, sigma increases with increasing M-PEO for all values of r. The data can be summarized by plots of normalized conductivity, sigma(n), versus M-PEO. where sigma(n) = sigma/(f phi(PEO)sigma(PEO)), phi(PEO) is the PEO volume Fraction in the copolymer, sigma(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 formal. At r = 0.085 sigma(n) values reach a plateau in the vicinity of it unity in the high M-PEO limit. At other values of r, sigma(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.