J. Phys. II France
Volume 5, Numéro 7, July 1995
Page(s) 979 - 1001
DOI: 10.1051/jp2:1995224
J. Phys. II France 5 (1995) 979-1001

Evidence of Tilted Dimeric Mesophase for Terminally Polar Polyphilic Mesogens

B.I. Ostrovskii1, F.G. Tournilhac2, L.M. Blinov1 and W. Haase3

1  Institute of Crystallography, Academy of Sciences of Russia, Leninsky pr. 59, Moscow 117333, russia
2  Ecole Supérieure de Physique et Chimie Industrielles, 10 rue Vauquelin, 75231 Paris Cedex 05, France
3  Institut für Physikalische Chemie, Technische Hochschule Darmstadt, Petersenstraße 20, 64287 Darmstadt, Germany

(Received 4 November 1994, revised 10 March 1995, accepted 31 March 1995)

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 A $_{\rm d}$ phase composed of antiparallel dimers, its tilted analogue-smectic C $_{\rm d}$ phase has been detected. The temperature variations of the interlayer spacing in the smectic C $_{\rm d}$ 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 C $_{\rm d}$ phase and the features of smectic A $_{\rm d}$-smectic C $_{\rm d}$ phase transition are discussed in terms of sterical coupling of molecules with differently tilted fragments.

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