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, χ. 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 determined χ 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.