Sol-gel hybrid materials based on hydroxylated poly[bis(methoxy-ethoxy-ethoxy)phosphazene] and silica: a ceramic ionic conductor

The sol-gel synthesis of hybrid materials based on poly[bis(methoxy-ethyoxy-ethoxy) phosphazene] (MEEP) and silica and their characterization are described. MEEP is a polymer with a low glass transition temperature of -71°C, that bears ethyleneoxide oligomers as phosphorus side groups, able to impart to the macromolecule metal co-ordination ability. This property makes MEEP an ionic conductor, but unfortunately its flow behaviour at room temperature limits its industrial development. The linkage of MEEP to a rigid inorganic network, which would overcome this problem, was pursued using the solÐgel method. The functionalization of MEEP by the introduction of free hydroxyl groups allows the formation of chemical bonds with hydrolysed tetraethoxysilane. Phosphazene-based hybrid materials with no phase separation could be obtained by this procedure. The ionic conductivity of these materials doped with lithium triflate has been investigated using a.c. impedance techniques. A conductivity value of about 3x10-5 Ohm-1 cm-1 was measured which is comparable to typical values of poly(ethylene oxide) solid electrolytes. A thermal treatment of the Li-doped samples decreases theconductivity but it still remains above 5x10-6 ohm-1 cm-1.