Thermodynamics and Phase Behavior of Poly(ethylene oxide)/ Poly(methyl methacrylate)/Salt Blend Electrolytes Studied by Small- Angle Neutron Scattering

We studied the effect of added salt on the thermodynamics of a miscible polymer blend system: poly(ethylene oxide) (PEO) blended with poly(methyl methacrylate) (PMMA). In the absence of salt, PEO/PMMA blends are known to exhibit a negative Flory-Huggins parameter, x. Not surprisingly, the salt-free PEO/PMMA blends are miscible, regardless of composition. The addition of salt, which in our case was lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), induced phase separation in majority-PMMA blends, while majority-PEO blends remained miscible. The effect of added salt was studied at two salt concentrations, r = 0.05 and r = 0.10; r is defined as the molar ratio of lithium ions to ether oxygens (r = [Li]/[EO]). The immiscibility window, which was absent at r = 0, grew upon addition of a small amount of salt (r = 0.05). Further addition of salt to r = 0.10 results in shrinking of the immiscibility window. With small-angle neutron scattering (SANS) profiles from homogeneous blends, we determinedx in both the presence and absence of salt. We measure the composition dependence of this parameter and use it to predict the phase behavior of PEO/PMMA/LiTFSI blends. We find good agreement between theory and experiment.