Study of molecular motions in liquid crystal forming compounds using positron lifetime spectroscopy

Positron lifetime spectroscopy (PLS) has been used to study molecular motions associated with n ‐p ‐cyano‐p ‐heptylbiphenyl (K21), n ‐p ‐cyano‐p ‐decylbiphenyl (K30), n ‐p ‐cyano‐p ‐hexyloxybiphenyl (M18), and n ‐p ‐ethoxybenzylidene‐p ‐butylaniline (EBBA). In each of these liquid crystal forming compounds, temperature dependent positron lifetime measurements have been performed during the heating cycle of samples prepared by either quenching or slow cooling from the respective liquid crystalline phase of the compounds. Unlike the slow‐cooled sample, the quenched sample shows various characteristic peaks in the ortho‐positronium (o‐Ps) pick‐off lifetime measured over a wide range of temperature, before the system undergoes a glass transition. These peaks have been attributed to various intra‐ and inter‐molecular motions associated with these compounds. A comparative study of the four compounds is presented in this paper. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)