Salt Doping in PEO-Containing Block Copolymers: Counterion and Concentration Effects

The phase behavior of lithium salt-doped poly(styrene-b-ethylene oxide) (PS−PEO) was studied as a function of salt concentration, lithium (Li) counterion (X), and annealing temperature. Three LiX salts were employed in this work: LiClO4, LiCF3SO3, and LiAsF6. Increasing the salt doping ratio ([EO]:[Li]) from 48:1 to 3:1 led to morphological changes from hexagonally packed cylinders to lamellar morphologies; however, comparison between the small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) results shows that the location of these morphological changes are altered by the different counterions. We interpret our SAXS, TEM, and differential scanning calorimetry data using strong segregation theory to estimate an effective interaction parameter (χeff) for our salt-doped copolymers. This χeff varies linearly with salt concentration for a single PS-PEO:LiX material; however, the slope for each χeff,PS−PEO:LiX is influenced by the nature of the counterion. We rationalize these effects by examining the Lewis acidity of each anion.