Solid polymer electrolyte membranes were prepared as semi-interpenetrating networks by photo-induced polymerization of mixtures of poly(ethylene glycol) (PEG) methacrylate macromonomers in the presence of poly(methyl methacrylate) (PMMA) and lithium bis(trifluoromethanesulfonyl)imide salt. The composition of the membranes was varied with respect to the PMMA content, the degree of cross-linking, and the salt concentration. Infrared analysis of the membranes indicated that the lithium ions were coordinated by the PEG side chains. Calorimetry results showed a single glass transition for the blend membranes. However, dynamic mechanical measurements, as well as a closer analysis of the calorimetry data, revealed that the blends were heterogeneous systems. The ionic conductivity of the membranes increased with the content of PEG-grafted polymethacrylate, and was found to exceed 10(-5) S cm(-1) at 30 degrees C for membranes containing more than 85 wt.% of this component in the polymer blend. (c) 2005 Elsevier B.V. All rights reserved.