(129e) A Study of the Thermodynamics of PEO/PMMA/Litfsi Blend Electrolytes

In this study, we examined the thermodynamics of added salt on a miscible polymer blend system. Our system of interest was Poly(ethylene oxide)/Poly(methyl methacrylate)/ Lithium bis(trifluoromethanesulfonyl)imide or PEO/PMMA/LiTFSI. We added LiTFSI to PEO (8.5 kg/mol) and PMMA (47.3 kg/mol) to create a series of blends. We studied 5 blends: two PEO-rich blends, two PEO-lean blends and one symmetric blend. We utilized small angle neutron scattering (SANS) to determine the phase behavior. We found that the two PEO-rich blends were miscible in both the neat state and at all salt concentrations. We found that the PEO-lean blends were only miscible in the neat state and were immiscible when salt was added. Most unusually, we found that the symmetric blends were completely miscible except at r = 0.05, where r is the moles of lithium per moles of ethylene oxide. This initial phase behavior pointed to a highly complex relationship between blend composition, salt concentration and miscibility. We utilized the framework of de Gennes random phase approximation to calculate an effective χ parameter (χ_eff) from the scattering profiles. We found that χ for neat blends has a linear dependence on composition and were able to analyze these parameters using the theoretical framework of Sanchez. We found that χ_eff is a non-linear function of composition for the salt containing samples, in contrast with the neat samples. The results from this study indicate that the thermodynamics in a polymer blend electrolyte system are a strong, non-linear function of blend composition and salt concentration.