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

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