Phase transformations in some homologues of 4-n-alkyl-4 '-cyanobiphenyls investigated by positron annihilation spectroscopy

The results of phase transformation studies carried out in four homologues of 4-n-alkyl-4'cyanobiphenyls, using positron annihilation spectroscopy, ar e presented. The homologues investigated are, hexyl-, heptyl-, octyl- and d ecylcyanobiphenyls (6CB, 7CB, 8CB and 10CB). In these compounds, the positr on lifetime measurements were performed as functions of temperature. The po sitron annihilation parameters are found to exhibit strong dependence on te mperature. It was found that the ortho-positronium pick-off lifetime shows changes which strongly support (i) a gradual disappearance, instead of an a brupt one, of some memory of the more ordered solid phases on passing to th e liquid crystalline phases, (ii) the strong tendency for the molecules in the mesophases to undergo anti-parallel bimolecular association and (iii) t he formation of cyabotactic groups of a smectic phase in a nematic medium. Changes were also observed in the ortho-positronium formation probability w hich apparently indicate a systematic transformation of the solid phase fro m a close-pack structure to an open-pack structure as one goes from a lower to a higher homologue of the compounds investigated. Solid crystalline pol ymorphism has been observed in 8CB. A change in molecular packing in the so lid phase of 10CB has been observed.