J. Phys. II France
Volume 6, Numéro 9, September 1996
Page(s) 1259 - 1272
DOI: 10.1051/jp2:1996129
J. Phys. II France 6 (1996) 1259-1272

Polar Surface Bifurcation in Dimerized Nematic Phase of 4-n Alkyloxybenzoic Acids

M. Petrov and G. Durand

Laboratoire de Physique des Solides, Bâtiment 510, Université Paris-Sud, 91405 Orsay Cedex, France

(Received 14 March 1996, received in final form 23 May 1996, accepted 30 May 1996)

We have studied the mechanism of the spontaneous twist of surface orientation observed when cooling down 4-n heptyloxybenzoic and 4-n octyloxybenzoic acids (HOBA and OOBA) in their dimerized nematic phase, above a SiO evaporated dielectric surface. We apply an electric field parallel to the dielectric substrate. When the field is parallel to the high temperature surface projection of the director, we up or down shift the temperature of the twist surface bifurcation, linearly in field. For perpendicular field, of defined sign, we can induce surface transitions from one twisted state to the other one. These observations can be explained by a simple polar model: the order electric polarization close to SiO is almost parallel to the substrate, since the molecular tilt is around the "magic" angle. Surface undulations create an anisotropic surface depolarizing field, which forces the polarization to rotate. We estimate a strong flexoelectric coefficient for 4-n octyloxybenzoic acid $e \sim +10^{-3}$ cgs. This could be associated with the large electric quadrupole momentum of the hydrogen bonds in the nematic dimers.

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