Solid electrolyte membranes from semi-interpenetrating polymer networks of PEG-grafted polymethacrylates and poly(methyl methacrylate) Anette Munch Elme ´r, Patric Jannasch * Department of Polymer Science and Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden Received 30 September 2005; accepted 19 December 2005 Abstract 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 -C for membranes containing more than 85 wt.% of this component in the polymer blend. D 2005 Elsevier B.V. All rights reserved. Keywords: Solid polymer electrolytes; Semi-interpenetrating networks; PEG methacrylate macromonomers; Heterogeneous polymer blends 1. Introduction Electrolytes employed in electrochemical devices generally have two basic requirements, namely to effectively conduct ions and to separate the electrodes to avoid short-circuiting. In the ideal case, a polymer electrolyte should combine the conductivity of a liquid and the mechanical stability of a solid. Solid electrolytes containing lithium salt typically reach an ionic conductivity of approximately 10  5 S cm  1 at room temperature, while polymer gel electrolytes may reach above 10  3 S cm  1 [1], often at the expense of the mechanical stability. We are currently exploring the concept of using co- continuous blends as polymer electrolytes to obtain structural rigidity. In these blends, one component facilitates the ion conductivity, while the other provides the necessary mechanical stability. The aim is to blend the two polymers in order to combine their properties in a synergistic way. In our first study of this concept, we prepared electrolyte membranes from blends of amorphous cross-linked poly(ethylene glycol) (PEG)- grafted polymethacrylates with a semicrystalline poly(vinyli- denefluoride-co-hexafluoropropylene) (PVDF – HFP) doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt [2]. These blends had a morphology consisting of a micropo- rous PVDF–HFP structure embedded in a continuous phase of the cross-linked PEG-grafted polymethacrylate. The solid membranes showed promising properties for application as functional solid polymer electrolytes, combining high conduc- tivity with elasticity. We have also very recently studied the properties of gel electrolytes after allowing the solid mem- branes to take up controlled amounts of a liquid electrolyte to form gel electrolytes [3]. The presence of the liquid electrolyte led to softer membranes with a considerably higher ionic conductivity as compared to the corresponding solid electrolyte membranes. The objective of the present work was to investigate the possible advantages of using poly(methyl methacrylate) (PMMA), a fully amorphous polymer, to mechanically stabilize the soft PEG-grafted polymethacrylate networks. A further objective was to relate the membrane properties to the phase behavior of the blends. Consequently, the focus was set on 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.12.021 * Corresponding author. E-mail address: patric.jannasch@polymer.lth.se (P. Jannasch). Solid State Ionics 177 (2006) 573 – 579 www.elsevier.com/locate/ssi