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 poly(ethylene oxide)-block-poly(methyl methacrylate) (PEO-PMMA) with added lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. We studied two asymmetric 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 small-angle X-ray scattering (SAXS) profiles for PEO-PMMA(10–33) were featureless at all salt concentrations. In contrast, PEO-PMMA(10–64) exhibited SAXS peaks when the salt concentration was between 0.22 ≤ m (mol Li/kg polymer) ≤ 0.44. The appearance of SAXS peaks only in PEO-PMMA(10–64) is consistent with the predictions of ionic self-consistent field theory developed by de la Cruz and co-workers, which predicts that in systems with negative χ, ordered phases are only found when the volume fraction of the ionic block is about 10%.