Elastic and optical properties of some 4'-n-Alkyl-4-cyanobiphenyls

Abstract

The bend elastic constants (k₃₃) and order parameters (S) derived from optical measurements in the nematic phase of pentlyl, hexyl, heptyl and octyl cyanobiphenyls (5CB, 6CB, 7CB and 8CB respectively) are reported. The odd-even effect is clearly seen in the K₃₃ values of all four compounds and in the S values of the first three. 5CB, 6CB and 7CB show only the nematic mesophase but their elastic constants indicate the existence of smectic-like short range order. For example, even the lowest homologue, 5CB, does not obey the law k₃₃ ∞ S² given by the mean field throry. 7CB, has about the same order parameter as 5CB but a much higher elastic constant. 8CB, which shows a smectic A phase as well, exhibits a lower k₃₃ than 7CB near the nematic-isotropic transition point but a pronounced pretransitional increase near the A-N transition.