Study of Ion Diffusional Motion in Ionic Liquid-Based Polymer Electrolytes by Simultaneous Solid State NMR and DTA Dushyant Singh Rajput, † Koji Yamada, ‡ and S. S. Sekhon* ,†,§ † Department of Physics, Guru Nanak Dev University, Amritsar, 143005, India ‡ Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, Narashino, Chiba, 275-8575, Japan § Department of Physics, The University of the West Indies, St. Augustine, Trinidad and Tobago, West Indies * S Supporting Information ABSTRACT: Polymer electrolytes containing ionic liquid (IL), 2-methyl-1,3- dipropylimidazolium dihydrogenphosphate (MDPImH 2 PO 4 ) have been studied by 1 H solid state NMR and differential thermal analysis (DTA) simultaneously by using a specially designed probe. To the best of our knowledge, this is the first report of its kind for IL based polymer electrolytes. The variation of NMR line width with temperature for the IL and polymer electrolytes shows line narrowing at the glass transition and melting temperature. The onset of long-range ion diffusional motion also takes place at these temperatures and is accompanied by a sudden increase in ionic conductivity value by 2-3 orders of magnitude. The presence of amorphous and crystalline phases in IL-based polymer electrolytes has been observed from X-ray diffraction (XRD) studies, and the amorphous phase is the high conducting phase in these polymer electrolytes. The IL-based polymer electrolytes have been observed to be thermally stable up to 200 °C. The results obtained from ion transport studies have also been supported by Fourier transform infrared (FTIR), XRD, and cyclic voltammetry (CV) studies. ■ INTRODUCTION Ionic liquids (ILs) are primarily composed of bulky and asymmetric cations and large anions. Their physicochemical properties can be tuned by using different combinations of cations and anions. The thermal stability and other properties of hydrophobic and hydrophilic ILs based on imidazolium cations have been reported to depend upon the alkyl chain length of the imidazolium cation and the nature of the anion. 1-10 A change in viscosity, density, ion size, and degree of dissociation also affects the ionic conductivity of ILs, but at this stage it is very difficult to estimate the contribution of each parameter separately. ILs generally have higher viscosity due to strong Coulombic forces between the ionic species present in them. A number of ILs containing various fluoroanions have already been widely studied. 1-4,11-13 Also, ILs containing acidic counteranions are very important due to the possibility of proton conduction by hopping mechanism, and their potential applications as proton-conducting membranes in polymer electrolyte membrane fuel cells and other devices. 14-16 As ILs consist of loosely packed cations and anions, generally both cations and anions are reported to be mobile in ILs as well as in IL-based polymer electrolytes. The nature of mobile species and other ion transport properties of polymer electrolytes are generally studied by solid state NMR. 15-19 The glass transition temperatures were observed just below the NMR line narrowing temperatures, i.e., the motional correlation frequency at the glass transition were estimated to be ≤40-60 kHz (full width at half-maximum, fwhm). The ionic conductivity also shows a sudden increase at the temperature at which line narrowing takes place. The phase change in the materials can also be studied by differential thermal analysis (DTA), and the temperature at which line narrowing takes place can be related to different thermal temperatures (glass transition, melting and crystallization temperature) determined from DTA. 19 Gen- erally, both (NMR and DTA) measurements are performed independent of each other. However, a simultaneous recording of solid state NMR and DTA for the same sample under identical conditions can be very helpful in finding a correlation between the NMR and DTA results. In the present study, we have recorded 1 H solid state NMR and DTA simultaneously by using a specially designed probe on different samples based on an IL (MDPImH 2 PO 4 ). This has been done to find a correlation between the motional narrowing, thermal temper- atures, and ionic conductivity of different polymer electrolytes. The results obtained from ion transport studies have also been supported by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and cyclic voltammetry (CV). ■ EXPERIMENTAL SECTION Polyethylene oxide (PEO) with average molecular weight 5 × 10 6 (Aldrich), 2-methylimidazole (>99% Merck), sodium Received: November 27, 2012 Revised: January 25, 2013 Published: January 25, 2013 Article pubs.acs.org/JPCB © 2013 American Chemical Society 2475 dx.doi.org/10.1021/jp3116512 | J. Phys. Chem. B 2013, 117, 2475-2481