POSITRON LIFETIMES IN SMECTIC LIQUID-CRYSTALS

The mean lifetimes of positrons in 4,4'-n-octyloxycyanobiphenyl and p-decyloxybenzylidene p'-amino 2-methyl butyl cinnamate were measured. The measurements of magnetic susceptibility and differential scanning calorimetry were also carried out to study the influence of molecular order and phase transition on the mean lifetime of positrons. The poistron lifetime spectra consist of two components. The mean lifetimes of the shortlived and the longlived components in these liquid crystals increase abruptly at the solid-smectic transition. It seems that the change in the shortlived component at the mesomorphic phase transitions depends on the magnitude of transition enthalpy. The longlived component shows a maximum in the smectic A phase. The maximum mean lifetime of the longlived component in the smectic A may be related to the size of defects in the well-defined fluid regions.