Positron annihilation study of a low-molecular-weight organic glass-forming liquid (BMMPC)

Positron lifetime spectra were obtained from 1,1-bis(paramethoxy-meta- methyl- phenyl)-cyclohexane (BMMPC) vs. temperature T between 23 and 348 K. At low temperatures, almost up to the glass transition at T g , a fast pick-off annihilation mechanism is proposed for explaining the variations in the annihilation parameters of positronium (Ps). Above the Vogel temperature T 0 the lifetime of o-Ps is coherently interpreted in terms of a thermodynamic model which assumes the molecules to occupy solid- and liquid-like states. The transition temperature of the model calculated from o-Ps lifetimes is significantly less by ∼ 20 − 30 K than that obtained from all other physical quantities available, including free positron lifetimes.