Synthesis and characterization of a new class of solid polymer electrolytes based on semi-interpenetrating polymer networks (semi-IPN) of poly(ethylene oxide)-polyurethane and poly(acrylonitrile) (PEO-PU/PAN) doped with lithium perchlorate is reported. With varying amount of PAN and LiClO4, room temperature conductivity increased over four orders of magnitude from 10(-10) to 10(-6) S cm(-1). The change in physical properties of the bulk is seen to govern the conductivity behavior. The temperature dependence of conductivity shows a transition from Arrhenius to VTF behavior above similar to 328 K, which becomes more prominent with increasing salt content. By correlating the trends in ESC and XRD with the temperature dependence of conductivity, the change over to VTF behavior is attributed to the transition from semicrystalline to amorphous phase. The glass transition temperature for all the compositions is well below room temperature, in the range of - 20 to - 50 degreesC and the semi-IPN structure improves the tensile strength and modulus while retaining the flexibility of the host polymer. These studies have shown the feasibility of the synthesized PEO-PU[PAN semi-IPNs to be used as an ideal host for solid polymer electrolytes. (C) 2004 Elsevier B.V. All rights reserved.