Correlation of dielectric relaxation and positron annihilation in glass-forming liquids

Dielectric relaxation measurements are reported on $\mathrm{bis}[m-(m-\mathrm{phenoxy}\left)\mathrm{phenoxyphenyl}\right]$ ether from room temperature (298 K) down to its glass transition temperature (213 K). Combination of these data with orthopositronium annihilation measurements published earlier indicate that the higher-temperature elbow in the latter data ($T_r$) coincides with the temperature at which the orthopositronium lifetime ${\tau }_3$ is comparable with the dipolar relaxation time. A similar correlation is observed for ortho-terphenyl. With the use of ultrasonic velocity data, it is possible in the latter system to compute a mean cavity dimension and discuss the way in which the dynamic behavior of the supercooled liquid influences the positron annihilation behavior. The implications of these observations in terms of theories of viscous liquids are discussed.