Composite microporous polymer electrolyte membranes comprising poly(vinylidene fluoride), PVDF, poly(ethylene glycol), PEG, and 1-ethyl-3-methylimidazolium tetrafluoroborate functionalized montmorillonite (EMIm-MMT) were prepared by a phase inversion method. The EMIm-MMT was subjected to FTIR, WXRD, TGA and element analysis in order to better understand the intercalation of imidazolium cations in sodium montmorillonite (Na-MMT) through exchange with intermellar sodium ions and improved thermal stability compared to organic-montmorillonite (O-MMT). The studies of porosity, weight uptake, and ionic conductivity of composite microporous membrane were performed as a function of EMIm-MMT content. The addition of EMIm-MMT can greatly enhance the thermal stability of the polymer electrolyte membrane. The maximum conductivity at 25 degrees C was found to be 7.77 mS cm(-1) for PVDF-PEG/6wt% EMIm-MMT. (c) 2008 Elsevier Ltd. All rights reserved.