A STRUCTURAL STUDY OF THE SMECTIC MESOPHASES OF TWO BIPHENYL COMPOUNDS AND AN X-RAY INVESTIGATION OF THE MISCIBILITY CRITERION

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Abstract
The biphenyl compounds 4'-n-octyl-4-cyanobiphenyl and 4'-n-octyloxy-4-cyanobiphenyl form smectic phases of a novel type. The X-ray diffraction patterns are unlike any other previously reported ; they contain a sharp inner ring (corresponding to the layer thickness) and two diffuse outer rings. The layer thickness indicates some form of bilayer structure with molecules partially overlapping in the centre of the layer. A number of alternative models for the precise molecular arrangement are suggested. This smectic phase is miscible with the conventional SA phase formed by 2-(4'-n-octadecyloxybenzylideneamino) fluorene. X-ray investigations indicated that the layer thickness changes smoothly across the composition range. In spite of the difference in molecular length and the difference between the structures of the two pure smectic phases, it appears that in mixtures there is a complete and intimate mixing of the molecules

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