Properties of polymer electrolytes composed of poly(ethylene oxide) and ionic liquids according to hard and soft acids and bases theory

We investigated the miscibility of 12 different ionic liquids (ILs) with poly(ethylene oxide) (PEO) under excess salt conditions, using hard and soft, acids and bases (HSAB) theory. The hardness of the component ions of a series of ILs was estimated from the energy calculation of their HOMO and LUMO. ILs composed of relatively hard acids, such as imidazolium cation, and soft bases such as bis(trifluoromethanesulfonyl)imide anion, displayed excellent miscibility with PEO regardless of mixing ratio. Good miscibility implies that the ions strongly interact with PEO chains, and accordingly the ionic conductivity decreased with increasing PEO content. On the other hand, the same ionic conductivity was found regardless of mixing ratio when ILs composed of soft acid and soft base such as N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide was mixed with PEO. These data suggest that high ionic conductivity should be supported by the micro-phase separation of polymers and ionic liquids. Copyright © 2011 John Wiley & Sons, Ltd.