Gel electrolytes on the basis of oligo(ethylene glycol), dimethacrylates-thermal, mechanical and electrochemical properties in relationship to the network structure

Gel electrolytes were prepared by photoinitiated polymerization of oligo(ethylene glycol), dimethacrylates ((EG),DMA) with n = 3, 9 and 23 in the presence of a plasticizer, e.g. various oligo(ethylene glycol), derivatives, and LiCF3SO3. The thermal, mechanical and electrochemical properties of the gel electrolytes were studied in dependence on their network structure and the type of the plasticizer. The molecular weights M-c of the chains between the network junction points were determined for the pure networks from their dynamic storage modulus at T approximate to T-g + 60 K above the glass transition temperature. Gel electrolytes on the basis of poly((EG),DMA) with n greater than or equal to 9 are single phase systems at room temperature. On the contrary, gels prepared by photopolymerization of ((EG)(3)DMA) in a solution of LiCF3SO3 in oligo(ethylene glycol)ll dimethylether ((EG)(11)DME) show two glass transition temperatures, one of the plasticizer salt solution and one of the polymer above 100 degrees C. These heterogeneous materials are able to embed up to 75 wt% of (EG)(11)DME, preserving high mechanical stability, showing ionic conductivities around 10(-4) S/cm at 25 degrees C and cationic transference numbers t(+) approximate to 0.55. The increase in thr polarity of the electrolytes by copolymerization with the polar comonomer cyanomethyl methacrylate (CyMA) or by the addition of polar plasticizers reduces the ionic association, but does not result in a remarkable increase in the ionic conductivity. Investigations of the gel electrolytes by cyclic voltammetry indicate that their electrochemical stability will be higher if the polar groups are fixed at the polymer and not at the plasticizer. (C) 2000 Elsevier Science Ltd. All rights reserved.