Electrical, Mechanical, Structural, and Thermal Behaviors of Polymeric Gel Electrolyte Membranes of Poly(vinylidene fluoride-co- hexafluoropropylene) with the Ionic Liquid 1-Butyl-3-Methylimidazolium Tetrafluoroborate Plus Lithium Tetrafluoroborate Shalu, Sujeet Kumar Chaurasia, Rajendra Kumar Singh, Suresh Chandra Department of Physics, Banaras Hindu University, Varanasi 221005, India Correspondence to: R. K. Singh (E - mail: rksingh_17@rediffmail.com) ABSTRACT: Polymeric gel electrolyte membranes based on the polymer poly(vinylidene fluoride-co-hexafluoropropylene) [P(VdF– HFP)] with different weight percentages of the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate plus 0.3M lithium tet- rafluoroborate (LiBF 4 ) salt were prepared and characterized by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, Fourier transform infrared (FTIR) spectroscopy, complex impedance spectroscopy, pulse echo techniques, and Vickers hardness (H) testing. After the incorporation of the IL plus the salt solution in the P(VdF–HFP) poly- mer, the melting temperature, glass-transition temperature (T g ), degree of crystallinity, thermal stability, elastic modulus (E), and hardness (H) gradually decreased with increasing content of the IL–salt solution as a result of complexation between the P(VdF– HFP) and IL. This was confirmed by FTIR spectroscopy. A part of the IL and LiBF 4 were found to remain uncomplexed as well. The ionic conductivity (r) of the polymeric gel membranes was found to increase with increasing concentration of the IL–salt solution. The temperature-dependent rs of these polymeric gel membranes followed an Arrhenius-type thermally activated behavior. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41456. KEYWORDS: crystallization; differential scanning calorimetry (DSC); glass transition; ionic liquids; mechanical properties Received 4 January 2014; accepted 26 August 2014 DOI: 10.1002/app.41456 INTRODUCTION Researchers around the globe are focusing on ion-conducting polymer gel electrolyte membranes (generally consisting of alkali–metal salt complexes) because of their potential applica- tions in electrochemical devices, such as batteries, fuel cells, supercapacitors, and solar cells, and high ionic conductivity (r) at room temperature, which is equivalent to that of liquid elec- trolytes. 1–4 Polymeric gel membranes have high r values, and they also offer high energy density, safe handling, ease in thin- film formation, packing flexibility, and light weight, and they provide good electrode–electrolyte contact; this makes them suitable candidates for technological applications. 5 Poly(vinyli- dene fluoride-co-hexafluoropropylene) [P(VdF–HFP)] has emerged as a promising host matrix for the preparation of poly- meric gel membranes having excellent mechanical strength and electrochemical stability. Out of poly(vinylidene fluoride) (PVdF) and P(VdF–HFP) (developed by Bellcore in 1996) 6 , the latter has received relatively more attention as a promising host polymer for polymer electrolytes because of its excellent mechanical strength, electrochemical stability, good hydropho- bicity, better amorphous domains, and high dielectric constant (8.4); this helps with a higher dissociation of charge carriers. The presence of the strong electron-withdrawing functional group (ACAF) in P(VDF–HFP) makes this polymer highly anodically stable. The addition of the HFP unit improves the uptake of liquid electrolytes by introducing the amorphous domain into the polymer and thereby increasing r. The crystal- line region provides enough mechanical stability to help in obtaining self-standing films. P(VdF–HFP)-based gels are opti- cally transparent. 7–10 Therefore, the P(VdF–HFP) copolymer is considered to be a promising alternative for preparing polymer gel electrolyte membranes compared to other existing polymers. Polymer gel electrolyte membranes based on polymers, such as poly(ethylene oxide) (PEO), poly(methyl methacrylate) (PMMA), PVdF, and polyacrylonitrile (PAN) mixed with suitable ionic salts added to low-molecular-weight organic solvents, such as propyl- ene carbonate (PC), ethylene carbonate (EC), poly(ethylene glycol) (PEG), and dimethylformamide (DMF), as plasticizers have been reported earlier. 11–16 These polymer gel electrolytes have high rs at ambient temperature but are not mechanically stable and are V C 2014 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2015, DOI: 10.1002/APP.41456 41456 (1 of 13)