Unexpected phase behavior in polymer blend electrolytes detected via small angle neutron scattering*

Polymer blends are usually immiscible as the entropy gain from mixing is negligible due to the connectivity of long chain polymers. Small angle neutron scattering (SANS) offers an unambiguous method for determining phase behavior by utilizing contrast in scattering lengths. Using SANS, we show that it is possible to obtain homogeneous mixtures of two chemically distinct polymers with a lithium salt for electrolytic applications. This approach is motivated by the success of using mixtures of organic liquids in modern lithium-ion batteries. We study polymer/polymer/salt blend electrolytes with the following polymers: poly(ethylene oxide) (PEO), poly(1,3,6-trioxocane) (P(2EO-MO)), and poly(1,3-dioxolane) (P(EO-MO)). PEO/P(2EO-MO), PEO/P(EO-MO), and P(EO-MO)/P(2EO-MO) blends are all miscible in the salt-free state, but minute salt addition (r = 0.005, where r is the molar ratio of Li atoms in the salt to O atoms in the polymers) leads to phase separation. PEO/P(2EO-MO) and PEO/P(EO-MO) blends become miscible again when salt concentration is increased beyond a critical value (r ≥ 0.075). However, P(EO-MO)/P(2EO-MO) blends remain immiscible for all subsequent salt concentrations, despite their chemical similarity. Our observations are inconsistent with the “like dissolves like” rule.