We report an X-ray diffraction and infra-red dichroism study of the structure and phase transitions for a new homologous series of cyanoterminated polyphilic liquid crystals. Analysis of the X-ray scattering profiles in the direction parallel and perpendicular to the smectic layers provides detailed data on lamellar ordering in smectic phases of polyphilics. The infra-red dichroism measurements allow molecular conformations and the tilt of various molecular moieties in smectic phases to be determined. In addition to the partly bilayer smectic Ad phase composed of antiparallel dimers, its tilted analogue-smectic Cd phase has been detected. The temperature variations of the interlayer spacing in the smectic Cd phase are very small as compared to the classical smectic A–smectic C transitions. This unusual behaviour results from the fact that both the tilt angle for different molecular moieties and the relative displacement of molecules in dimers vary with temperature. The enhanced stability of dimeric smectic Cd phase and the features of smectic Ad–smectic Cd phase transition are discussed in terms of sterical coupling of molecules with differently tilted fragments.
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Островский et al. (1995) studied this question.