Investigation of molecular modes in K18 employing positron lifetime spectroscopy

Molecular modes executed by the liquid crystal forming compound n-p-cyano-p-hexylbiphenyl (K18) have been studied using positron lifetime spectroscopy (PLS). Positron lifetime measurements have been carried out during the heating cycle of samples prepared by (i) slow cooling from the isotropic phase and (ii) quenching from either the liquid crystalline phase or the isotropic phase of the compound. The material in the quenched samples, unlike the slow-cooled sample, exhibits strong temperature dependence before it undergoes a glass transition. The temperature dependence of o-Ps pick-off lifetime in the quenched samples shows broad peaks at various characteristic temperatures. These peaks have been attributed to various intra- and inter-molecular motions associated with these compounds. The characteristic frequencies of the modes observed in the present work agree well with the literature reported values obtained from FIR studies. The two peaks corresponding to the collective modes observed in the sample quenched from the liquid crystalline phase have been found to be missing in the sample quenched from the isotropic phase. The present work demonstrates the usefulness of PLS in the study of molecular motions.