INFLUENCE OF PERMANENT MOLECULAR DIPOLES ON SURFACE ANCHORING OF NEMATIC LIQUID-CRYSTALS

We consider the ordering of molecular dipoles at a nematic surface and the influence of this ordering on the equilibrium director orientatio n and the anchoring energy. Both phenomenological and molecular approa ches are used. We show that a thin, spontaneously polarized layer will appear even at a structureless nematic surface without any charged im purities or gradients of the order parameter. The ordering of molecula r dipoles in this layer is determined by strong dipole-quadrupole inte ractions, modified by the presence of the surface. Surface polarizatio n gives a significant contribution to the anchoring energy of nematic liquid crystals composed of strongly polar molecules. The estimates in dicate that in such systems the dipolar contribution is very large and could even be predominant, for example, at a free surface of same nem atic liquid crystals. We show also that in the case of large longitudi nal dipoles the equilibrium director orientation at a free surface is homeotropic. in the case of large transverse dipoles the alignment is planar. The principal results of this study are consistent with experi mental observations on ferroelectric layers at nematic interfaces and with data on the orientation of typical polar nematics at a free surfa ce.