Dynamics of the crystalline phase and the ordered smectic phases of TBBA

The dynamics of the crystalline phase and of the ordered smectic phases of terephthal bis-butyl aniline (TBBA) were studied using coherent neutron inelastic scattering. The experiments were carried out on a hydrogenated single-domain sample using the IN2 spectrometer at the Institut Laue-Langevin. The study proved the existence of both longitudinal and transverse modes propagated parallel to the molecular layers for the three phases studied. On the other hand, the shearing mode, which is propagated perpendicular to the molecular layers, was observed only for the crystalline phase: however, the spectral distribution of the scattered neutrons is modified - at least for the smectic Bsub(c) phase - and this modification will have to be studied further later on. Thus, the crystal-smectic B phase transition does not bring about an important change in the lattice dynamics (phonons of wave vectors perpendicular to the moleculear direction); in particular, the sound velocity values are changed only slightly. The mobilization of the molecules around their principal axis causes at most a slightly increased damping of the collective modes. The two ordered smectic modes studied thus behave like ordered solids; they can be regarded as plastic crystals. (author)