Preparation and characterization of highly proton-conductive composites composed of phosphoric acid-doped silica gel and styrene-ethylene-butylene-styrene elastomer

Highly proton-conductive elastic composites have been successfully prepared from H3PO4-doped silica gel and a styrene-ethylene-butylene-styrene (SEBS) block elastic copolymer. Ionic conductivities of the composites depended on the concentration of H3PO4 and the heat treatment temperature of the H3PO4-doped silica gel. It was found that H3PO4 added is present mainly as free orthophosphoric acid in the silica gel. The composite composed of H3PO4-doped silica gel with a molar ratio of H3PO4/SiO2 = 0.5 heat-treated at temperatures below 200 degrees C and SEBS elastomer in 5 mass% showed a high conductivity of 10(-5) S cm(-1) at 25 degrees C in an dry N-2 atmosphere. The water adsorption during a storage in 25% relative humidity at room temperature for 1 day enhanced the ionic conductivities of composites by about one order of magnitude. Lower conductivities obtained in the composite with the H3PO4-doped silica gel heat-treated at 250 degrees C for 1 h were due to the formation of crystalline Si-3(PO4)(4). The temperature dependence of conductivity of the composites was the Vogel-Tamman-Fulcher type, indicating that proton was transferred through a liquidlike phase formed in micropores of the H3PO4-doped silica gels. The temperature dependence of the modulus of the composite was similar to that of the SEBS elastomer. The thermoplastically deforming temperature of the composite was around 100 degrees C, which was higher by 30 degrees C than that of the SEBS elastomer.