We studied the effect of salt addition on a diblock copolymer system with a negative Flory-Huggins interaction parameter, χ, indicative of attractive interactions between the two blocks. The system studied is poly(ethylene oxide)-block-poly(methyl methacrylate) (PEO-PMMA) with added lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. We studied two asymmetric block copolymers, PEO-PMMA(10-33) and PEO-PMMA(10-64), where the numbers refer to the molar masses of the blocks in kg mol -1 . The small angle X-ray scattering (SAXS) profiles for PEO-PMMA(10-33) were featureless at all salt concentrations. In contrast, PEO-PMMA(10-64) exhibited SAXS peaks when the salt concentration was between 0.22 ≤ m (mol Li/kg polymer) ≤ 0.44. The appearance of SAXS peaks only in PEO-PMMA(10-64) is consistent with the predictions of ionic self-consistent field theory developed by de la Cruz and coworkers, which predicts that in systems with negative χ, ordered phases are only found when the volume fraction of the ionic block is about 10%.
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.
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