Solid polymer electrolytes containing poly(ethylene oxide) (PEO) and 1-ethyl-3-methylimidazolium hydrogensulfate (EMIHSO4), an ionic liquid, were investigated. Thermal and structural properties of this polymer electrolyte were characterized using DSC, XRD, and FTIR. After addition of EMIHSO4, the melting point of PEO decreased by ca. 25 degrees C. Thermal and structural analyses indicated that the crystallinity of PEO substantially decreased with increasing EMIHSO4 content. The interaction between ionic liquid and polymer suggested further dissociation of the ionic liquid. Thus. EMIHSO4 acted as ionic conductor and plasticizer. (C) 2012 Elsevier B.V. All rights reserved.