On the study of liquid crystalline materials using positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS) has emerged as a powerful technique for investigating structural changes, phase transformations and micro environmental transformations in a variety of systems. Positron lifetime technique has been employed to investigate various liquid crystalline materials, both thermotropic and lyotropic. Investigations in thermotropic liquid crystalline materials, besides detection of various phase transformations, have revealed solid crystalline polymorphism in some systems. In some thermotropic liquid crystalline materials, the ortho-positronium (o-Ps) pick-off lifetime shows changes which strongly support (i) retention of some memory of more ordered solid phases on passing to the liquid crystalline phase, (ii) strong tendency for the molecules in the mesophases to undergo anti-parallel bimolecular association, and (iii) the formation of cybotactic groups in a nematic medium. The present paper illustrates some of these features. A work on supercooled states has demonstrated the possibility of using PAS for studying various intra- and inter-molecular motions associated with the molecules of liquid crystalline materials. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim