Correlation of Positron Annihilation and Other Dynamic Properties in Small Molecule Glass-Forming Substances

Positron annihilation spectroscopy is used to characterize the ortho-positronium lifetime, τ₃, in a broad range of small molecule organic glasses over a wide temperature range. The magnitude and thermal variations of τ₃ reflect changes in the dielectric α-relaxation time τ_α and its non-Arrhenius or “fragile” characteristics. τ₃ also displays striking similarities with the fast relaxations reflected in the hydrogen-weighted mean square atomic displacements 〈u²〉. The parallel temperature variations of 〈u²(T/T_g)〉, τ₃(T/T_g), and τ_α(T/T_g), and their similar patterns of change between the different glasses, are discussed in terms of anharmonicity in the intermolecular caging potentials and the subnanometer density heterogeneities.