An Investigation of the Phase Behaviour of Poly(ethylene oxide)/Poly(methyl methacrylate) Blend Electrolytes*

In this study, we investigated the effect of added salt on a miscible polymer blend system. The system studied is PEO (8.5 kg/mol) blended with PMMA (47.3 kg/mol) with added LiTFSI salt. We studied the phase behavior of 5 PEO/PMMA blend systems: two majority PEO blends, one symmetric blend and two majority PMMA blends. Utilizing SANS, we found that the majority PEO blends were miscible both in the neat state and at all salt concentrations. We found that the majority PMMA blends were only miscible in the neat state. The symmetric PEO/PMMA blend system was miscible both in the neat state and at all salt concentrations except at r = 0.05, where r is the ratio of moles of Lithium to moles of ethylene oxide. The data points to a complex relationship between salt concentration, blend composition and phase behavior. We quantified the thermodynamics of PEO/PMMA/LiTFSI blends utilizing the framework of de Gennes RPA to calculate an effective χ parameter, χeff, from fits of SANS data. We found that χeff is constant as a function of salt concentration for all blends in this study.