Thermodynamic Properties of Block Copolymer Electrolytes Containing Imidazolium and Lithium Salts

We report on the thermal properties, phase behavior, and thermodynamics of a series of polystyrene- block -poly(ethylene oxide) copolymers (SEO) mixed with the ionic species Li[N(SO 2 CF 3 ) 2 ] (LiTFSI), imidazolium TFSI (ImTFSI), and an equimolar mixture of LiTFSI and ImTFSI (Mix). Differential scanning calorimetric scans reveal similar thermal behavior of SEO/LiTFSI and SEO/ImTFSI at the same salt concentrations. Phase behavior and thermodynamics were determined using a combination of small-angle X-ray scattering and birefringence. The thermodynamics of our mixtures can be mapped on to the theory of neat block copolymer phase behavior provided the Flory−Huggins interaction parameter, χ, between the blocks is replaced by an effective χ (χ eff ) that increases linearly with salt concentration. The phase behavior and the value of m, the slope of the χ eff versus salt concentration data, were similar for SEO/LiTFSI, SEO/ImTFSI, and SEO/Mix blends. The theory developed by Wang [ J. Phys. Chem. B. 2008, 41, 16205] provides a basis for understanding the fundamental underpinnings of the measured value of m . We compare our experimental results with the predictions of this theory with no adjustable parameters.