Novel "polymer-in-salt" electrolytes were prepared by mixing random copolymers of acrylonitrile and butyl acrylate [poly(AN-co-BuA)] with lithium bis(trifluoromethanesulfone)imide (LiTFSI). The interaction of Li+ ions and polymeric groups was investigated by DSC and IR methods; electrical properties were studied by impedance spectroscopy. The properties of the new system were compared to those of the well-known poly(ethylene oxide) (PEO)-LiTFSI polymer electrolytes. The mixtures of poly(AN-co-BuA) and LiTFSI exhibit much lower glass transition temperature T-g than the parent copolymer (decrease of over 60 K for high salt content). By extrapolation of the dependence of the T-g on the salt content, the value of T-g for pure LiTFSI was estimated as 232 K. The decrease of T-g was correlated with the increase of the decoupling index, R-tau, which is proportional to the value of conductivity measured at T-g. The interactions of LiTFSI with poly(AN-co-BuA) lead to high flexibility of this system; a property that is opposite to the behavior of the PEO-LiTFSI system, where interactions between the salt and polyether lead to stiffening of the polymer with increasing salt content. (c) 2005 Elsevier B.V. All rights reserved.