Some homologues of alkyloxy cyanobipheny1 (6OCB, 8OCB, 9OCB, 10OCB) have been investigated by employing positron lifetime spectroscopy. In each of these compounds, temperature dependent positron lifetime measurements have been carried out, both in the heating as well as cooling cycles. Besides detecting many interesting features like solid-crystalline polymorphism, anti-parallel bimolecular association, formation of cybotactic groups in the nematic phase, positron annihilation parameters have been able to reveal anomalous structural changes taking place in these compounds.

The results of phase transformation studies carried out in four homologues of 4-n-alkyl-4'cyanobiphenyls, using positron annihilation spectroscopy, are presented. The homologues investigated are, hexyl-, heptyl-, octyl- and decyl-cyanobiphenyls (6CB, 7CB, 8CB and 10CB). In these compounds, the positron lifetime measurements were performed as functions of temperature. The positron annihilation parameters are found to exhibit strong dependence on temperature. It was found that the ortho-positronium pick-off lifetime shows changes which strongly support (i) a gradual disappearance, instead of an abrupt one, of some memory of the more ordered solid phases on passing to the liquid crystalline phases, (ii) the strong tendency for the molecules in the mesophases to undergo anti-parallel bimolecular association and (iii) the formation of cyabotactic groups of a smectic phase in a nematic medium. Changes were also observed in the ortho-positronium formation probability which apparently indicate a systematic transformation of the solid phase from 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 polymorphism has been observed in 8CB. A change in molecular packing in the solid phase of 10CB has been observed.

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