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The thermodynamics of block copolymer/salt mixtures were quantified through the application of Leibler's random phase approximation to disordered small-Angle X-ray scattering profiles. The experimental system is comprised of polystyrene-block-poly(ethylene oxide) (SEO) mixed with lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI), SEO/LiTFSI. The Flory-Huggins interaction parameter determined from scattering experiments, χSC, was found to be a function of block copolymer composition, chain length, and temperature for both salt-free and salty systems. In the absence of salt, χ0,SC is a linear function of (NfEO)-1 in the presence of salt, a linear approximation is used to describe the effect of salt on χeff,SC for a given copolymer composition and chain length. The theory of Sanchez was used to determine χeff from χeff,SC to predict the boundary between order and disorder as a function of chain length, block copolymer composition, salt concentration, and temperature. At fixed temperature (100 °C), Ncrit, the chain length of SEO at the order-disorder transition in SEO/LiTFSI mixtures, was predicted as a function of the volume fraction of the salt-containing poly(ethylene oxide)-rich microphase, fEO,salt, and salt concentration. At fEO,salt > 0.27, the addition of salt stabilizes the ordered phase; at fEO,salt < 0.27, the addition of salt stabilizes the disordered phase. We propose a simple theoretical model to predict the block copolymer composition at which phase behavior is independent of salt concentration (fEO,salt = 0.27). We refer to this composition as the "isotaksis point". © |
