Effect of Added Salt on Disordered Poly(ethylene oxide)-Block-Poly(methyl methacrylate) Copolymer Electrolytes*

We studied the effect of salt addition on a diblock copolymer system with a negative Flory-Huggins interaction parameter, χ, indicative of attractive interactions between the two blocks. The system studied is PEO-b-PMMA with added LiTFSI salt. We studied two asymmetric PEO-PMMA block copolymers, PEO-PMMA(10-33) and PEO-PMMA(10-64), where the numbers refer to the molar masses of the blocks in kg mol^-1. The SAXS profiles for PEO-PMMA(10-33) were featureless at all salt concentrations and in the neat state. In contrast, PEO-PMMA(10-64) exhibited SAXS peaks when the salt concentration was between 0.22 ≤ m ≤ 0.44, where m is molality given in mol Li/kg polymer. The thermodynamics of PEO-PMMA/LiTFSI mixtures are quantified by an effective χ parameter, χ_eff, obtained from fits of the SAXS data. The appearance of SAXS peaks only in the more asymmetric block copolymer is inconsistent with the SCFT of Cochran and Frederickson. However, the predictions of ionic SCFT developed by de la Cruz and coworkers predicts that in systems with negative χ, the ordered phases are only found in a narrow chimney, where the volume fraction of the ionic block is about 10%. Our experiments provide considerable support for ionic SCFT.