Improved ionic conductivity of nitrile rubber/Li(CF 3 SO 2 ) 2 N composites by adding imidazolium-type zwitterion Edy Marwanta, Tomonobu Mizumo, Hiroyuki Ohno ⁎ Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184–8588, Japan Received 16 September 2006; received in revised form 9 November 2006; accepted 21 December 2006 Abstract Polymer electrolytes based on the composite of nitrile rubber (NBR; poly(acrylonitrile-co-1,4-butadiene)) and Li(CF 3 SO 2 ) 2 N (Li(Tf) 2 N) were prepared and their ionic conductivity was evaluated. NBR with nitrile content of 33% was mixed with 25 wt% Li(Tf) 2 N to show the highest ionic conductivity, while still in the rubbery state. Furthermore, addition of 9.2 wt% imidazolium-type zwitterion enhanced the ionic conductivity of about 8 times (5.6 × 10 - 6 S cm - 1 at 50 °C) with lithium ion transference number of 0.32. FT-IR investigations suggested that the interaction between NBR and Li + ion decreased by the addition of zwitterion. DSC measurements on the composites consisting of NBR, Li(Tf) 2 N, and zwitterion strongly suggested micro-phase separation between NBR-rich phase and zwitterion-rich phase. © 2007 Elsevier B.V. All rights reserved. Keywords: Nitrile rubber; Zwitterions; Ionic liquid; Ionic conductivity 1. Introduction For the applications of polymer electrolytes as components of energy conversion devices, both good flexibility and thermal stability are required as well as high ionic conductivity. For successful practical applications, a polymer must be able to relax elastically under stress arising from volume changes in adjacent phases [1]. Some studies of polymer electrolytes have been attempted to employ rubbers with or without organic solvents [2–5]. Synthetic rubbers such as nitrile rubber (NBR) and urethane rubber were employed with organic solvents for gel electrolytes [2–4]. Although the addition of organic solvents such as propylene carbonate and ethylene carbonate is effective to improve polarity and flexibility, obtained systems are still problematic because of their volatility. Therefore, we have mixed natural rubber with polyethylene oxide (PEO) and its derivatives to obtain flexible and non-volatile polymer electro- lytes with long shelf-life [5]. We also improved the polarity of natural rubber by epoxidation, and accordingly the natural rubber was converted into an ion conductive rubber [6,7]. NBR is expected to be suitable for polymer electrolytes because of its polar chemical structure and excellent mechanical properties such as high tensile strength and good elasticity. Incorporation of butadiene units provided amorphous elastomer with high chain flexibility and segmental motion, while nitrile units provided polarity which is useful for salt dissociation [8]. In our previous study, we employed NBR and ionic liquids for preparation of elastic polymer electrolytes [9]. Addition of lithium salts into NBR/ionic liquid composites induced micro- phase separation and enhanced the ionic conductivity of about 10 times. Ionic liquids are known as a new candidate of ion conductive matrices because of their high ionic conductivity, high polarity, non-volatility, etc. [10,11]. However, simple ionic liquids are not suitable as lithium ion conductive matrices. Since the component ions of the ionic liquid migrate along with the potential gradient, the transport number of the target ion such as lithium ion is pretty low. Migration and concentration of non- reactive ions of the ionic liquid also induce lowering of current, which should be avoided for the practical uses. To overcome these, we evaluated zwitterionic derivatives where cation and anion were chemically tethered as ion conductive matrices Solid State Ionics 178 (2007) 227 – 232 www.elsevier.com/locate/ssi ⁎ Corresponding author. Tel./fax: +81 42 388 7024. E-mail address: ohnoh@cc.tuat.ac.jp (H. Ohno). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.12.022