Temperature dependence of the smectic A layer spacing in two re-entrant nematogens

Abstract

Precise X-ray determinations have been made of the temperature variation of the layer spacing in the smectic A phase of two pure re-entrant nematogens, 4-cyanophenyl-3-methyl-4(4-n-dodecylbenzoyloxybenzoate (12CPMBB) and 4-cyano- phenyl-3-methyl-4(4-n-undecylbenzoyloxy)benzoate (11CPMBB).

The observed behaviour is similar in both cases: on cooling from the normal nematic-smectic A transition point, the spacing decreases at first over a part of the smectic range, and then increases with decrease of temperature. The results provide clear evidence of a structural rearrangement taking place in the smectic bilayer as a precursor to the formation of the re-entrant nematic phase.