We present cloud point measurements on low molecular weight binary polymer blends doped with salts that exhibit unusual phase behavior. These blends include poly(ethylene-alt-propylene)/poly(ethylene oxide) (PEP/PEO) doped with lithium bis(trifluoromethane)sulfonimide (LiTFSI), NaTFSI, KTFSI, LiClO4, and sodium iodide NaI. The addition of salt dramatically decreases the miscibility of the binary blends and results in an asymmetric cloud point profile. The phase behavior is found to be governed by the concentration of the salt, the size of the anion, and the composition of the polymer mixture. The experimental results are compared with a recent theory, which predicts the effect of ions on the polymer phase diagram by taking into account both ion-induced cross-linking and self-energy effects. Furthermore, the coexistence curve of salt-doped PEP/PEO blends is determined quantitatively by 1H NMR spectroscopy when the volume fraction of PEO is maintained at 0.6. The coexistence curve does not coincide with the cloud point profile, which can be attributed to the effect of the redistribution of ions between the two coexisting phases. In the interest of generality, the cloud point profile of polystyrene/poly(ethylene oxide) (PS/PEO) doped with LiTFSI is also mapped out, in which similar phenomena are observed.
Bibliographical noteFunding Information:
This work was supported by the Office of Basic Energy Sciences (BES) of the U.S. Department of Energy (DoE), under Contract DE-FOA-0001664. We thank Zhen-Gang Wang (Caltech), Frank S. Bates, and Qile P. Chen for helpful discussions on the model and critically reading the manuscript.
© 2017 American Chemical Society.