Copolymers of poly(acrylonitrile-co-ethyl methacrylate), P(AN-EMA), with three different EMA content and parent homopolymers were synthesized by emulsion polymerization. The chemical composition of copolymers were identified by FTIR, H-1-NMR and C-13-NMR spectroscopy. The thermal properties of copolymers were modified by changing the EMA content in copolymer compositions. Various amounts of LiClO4 salt loaded (PAN-co-PEMA) copolymer films were prepared by solution casting. The dielectric properties of these films at different temperatures and frequencies were investigated. It was found that the dielectric constant and ac-conductivity of copolymer films were strongly influenced by the salt amounts and EMA content in copolymers. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012