Ionic liquid: 2,3-dimethyl-1-octylimidazolium tetrafluoroborate (DMOImBF(4)) has been synthesized and found to be liquid at room temperature (25 degrees C). This room temperature ionic liquid (DMOImBF4) was incorporated in poly (vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP) to obtain films of polymer electrolytes. The addition of ionic liquid, salt (ammonium tetrafluoroborate) and plasticizer (propylene carbonate, PC) has been found to. increase the conductivity of polymer electrolytes to 10(-4) S/cm at 25 degrees C. Line narrowing observed in variable temperature H-1 and F-19 NMR line width suggests that cations and anions are mobile in these electrolytes. These polymer electrolytes have been found to be thermally stable up to 200-250 degrees C and hence can be used in various applications at temperature above 100 degrees C. (C) 2007 Elsevier B.V. All rights reserved.