Positron annihilation lifetime spectroscopy as a probe of free volume in plasticized solid polymer electrolytes

A recent report on the correlation between enhanced polymer mobility and ionic conductivity at room temperature in plasticized polyether-urethane solid polymer electrolytes (Forsyth et al.[1]), has prompted the present investigation. Positron annihilation lifetime spectroscopy (PALS) has been used to study the effect of plasticizer addition on the room temperature free volume characteristics of the crosslinked polyether-urethane. The addition of low molecular weight plasticizers to the polyether-urethane results in a constant or decreasing mean free volume cavity radius, as measured by the orthoPositronium lifetime τ₃, and a decreasing relative concentration of free volume cavities as measured by the ortho-Positronium intensity, I₃. It is postulated that the plasticizers interrupt polymer-polymer interactions by occupying the inter- and intra-chain free volume. The plasticizer structure influences the polymerplasticizer interactions which affect inter- and intra-chain separation and hence the free volume of the system. The decrease in polymer-polymer interaction and the increase in polymer-plasticizer interaction in turn influence the glass transition temperature behaviour.

Graphene networks and their influence on free-volume properties of graphene-epoxidized natural rubber composites with a segregated structure: rheological and positron annihilation studies

Epoxidized natural rubber-graphene (ENR-GE) composites with segregated GE networks were successfully fabricated using the latex mixing combined in situ reduced technology. The rheological behavior and electrical conductivity of ENR-GE composites were investigated. At low frequencies, the storage modulus (G′) became frequency-independent suggesting a solid-like rheological behavior and the formation of GE networks. According to the percolation theory, the rheological threshold of ENR-GE composites was calculated to be 0.17 vol%, which was lower than the electrical threshold of 0.23 vol%. Both percolation thresholds depended on the evolution of the GE networks in the composites. At low GE concentrations (<0.17 vol%), GE existed as individual units, while a "polymer-bridged GE network" was constructed in the composites when GE concentrations exceeded 0.17 vol%. Finally, a "three-dimensional GE network" with percolation conductive paths was formed with a GE concentration of 0.23 vol%, where a remarkable increase in the conductivity of ENR-GE composites was observed. The effect of GE on the atom scale free-volume properties of composites was further studied by positron annihilation lifetime spectroscopy and positron age momentum correlation measurements. The motion of ENR chains was retarded by the geometric confinement of "GE networks", producing a high-density interfacial region in the vicinity of GE nanoplatelets, which led to a lower ortho-positronium lifetime intensity and smaller free-volume hole size.