Positron lifetime study of the transition from glassy to normal liquid state for two phenyl ethers

Abstract

Positron lifetime measurements are reported as a function of temperature for bis[m-(m-phenoxyphenoxy)phenyl] ether and m-phenoxyphenyl-m-(m-phenoxyphenoxy)phenyl ether. The decay curves were analysed in terms of three lifetime components; two short lifetimes of typically 200 and 500 ps associated with the decay of para-positronium(p-Ps) and positrons and a longer component attributed to decay of the ortho-positronium (o-Ps). The o-Ps lifetime is nearly independent of temperature in the region from 213 to 233 K, above this it becomes strongly temperature dependent. The change in the temperature dependence at ≈ 233 K is associated with the glass transition for these liquids. A further change in the temperature dependence of the o-Ps lifetime was observed above 313 K where it becomes once more only weakly temperature dependent. Below the glass transition (T_g) Ps is probably trapped in pre-existing holes, which may be slightly enlarged due to the presence of Ps. Above 313 K Ps is apparently localized in a thermal equilibrated bubble. The size of the bubble is probably independent of the size of the pre-existing holes, as described by the “bubble model”. The transition between those two cases occurs between T_g and 313 K, reflecting the changes in the free volume and the thermal motion of the molecules as the temperature is increased. The results are correlated to viscosity data and a molecular interpretation is presented.