STUDY OF MOLECULAR MOTIONS IN ALKYL CYANOBIPHENYLS USING POSITRON LIFETIME SPECTROSCOPY

Positron lifetime measurements have been carried out in two liquid-cry stal-forming homologues of alkyl cyanobiphenyls, 7CB and 10CB. In each of these two compounds measurements were performed during the heating cycle of samples prepared either by quenching or by slow cooling from the respective liquid crystalline phase. In both compounds, the behav ior of quenched and slow-cooled samples is found to be different. Unli ke the slow-cooled sample, the material in the quenched sample seems t o have transformed into a glassy solid. The o-Ps pick-off lifetime in the quenched sample exhibits a strong temperature dependence. In each case, its value exhibits six broad peaks at various characteristic tem peratures which have been ascribed to various motions associated with the molecules of these compounds. The quenched samples of 7CB and 10CB exhibit glass transitions at similar to 268 and similar to 266 K, res pectively. The present work demonstrates an interesting application of positron lifetime spectroscopy (PLS).