SUPERCOOLING STUDIES IN N-P-CYANO-P-HEPTYL BIPHENYL USING POSITRON, LIFETIME SPECTROSCOPY

Supercooling studies have been performed in n-p-cyano-p-heptyl bipheny l using positron lifetime spectroscopy. Positron lifetime measurements have been performed during the heating cycle of samples prepared by e ither quenching or slow cooling from the nematic phase. The behaviour of these two samples is found to be different. In case of the quenched sample, the material seems to have transformed into a glassy solid. T he o-Ps pick-off lifetime in the quenched sample exhibits a strong tem perature dependence. Its value exhibits six broad peaks at 176, 195, 2 14, 233, 248 and 257K. These temperatures have been ascribed to variou s molecular motions. The sample exhibits a glass transition at about 2 68K.