RESUMEN

The influence of magnetic field on the isotropic-to-nematic phase transition temperature is investigated in neat bent-core and calamitic liquid crystals, in their mixture, and in samples doped with spherical magnetic nanoparticles for two different orientations of the magnetic field. A magnetic-field-induced negative or positive shift of the transition temperature was detected depending on the magnetic field orientation with respect to the initial orientation of the nematic phase, and on the type of liquid crystal matrix.

RESUMEN

Liquid crystal layers sandwiched between a reference plate and a photosensitive substrate were investigated. We focused on the reverse geometry, where the cell was illuminated by a laser beam from the reference side. In planar cells both static and dynamic instabilities occurred, depending on the angle between the laser polarization and the director orientation on the reference plate. In cells where the molecules were aligned along the normal of the reference plate, a dynamic pattern was observed at all angles of polarization. A simple model based on a photoinduced surface torque accounts for the findings. Light scattering studies revealed some basic properties of the instabilities.

Asunto(s)

RESUMEN

Mixtures of a bent-core mesogen (ClPbis10BB) and a calamitic mesogen (6OO8), showing a nematic phase over the entire compositional range and one or two smectic phases (namely, SmA, SmC, or SmC(A)) below the nematic one over a wide concentration range, were investigated by means of (2)H NMR spectroscopy, exploiting selectively deuterated isotopomers of both mesogens. The analysis of (2)H NMR spectra recorded in the liquid crystalline phases on several representative mixtures gave information on the orientational order properties and the molecular organization within the phases as well as on the alignment properties upon application of a magnetic field. On the other hand, the analysis of (2)H longitudinal relaxation times (T(1Z) and T(1Q)) in 6OO8-d(2)/ClPbis10BB mixtures and pure 6OO8-d(2) allowed the influence of the bent-core mesogen on the dynamics of the calamitic one to be highlighted.

RESUMEN

The onset of electroconvection in binary mixtures of a bent-core and a rodlike nematic has been characterized by measuring the threshold voltage U(c) and the critical wave number of the pattern in a wide range of frequencies f. In the mixtures rich in bent-core molecules, a "conductive-prewavy2-patternless-prewavy1" morphological sequence has been detected with an unusual negative slope of U(c)(f) at high frequencies. This latter scenario seems to be related to the bent-core component, as it disappears with increasing the concentration of rodlike molecules. In addition, one of the parameters most relevant for electroconvection, the electrical conductivity, has also been varied by ionic salt doping. It has been found that the above effect of the banana-shaped molecules on the electroconvection scenarios can be suppressed by the conductivity.

RESUMEN

In this work the 4-(trans- 4'-n -hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) liquid crystal was doped with differently shaped magnetite nanoparticles. The structural changes were observed by capacitance measurements and showed significant influence of the shape and size of the magnetic particles on the magnetic Fréedericksz transition. For the volume concentration phi= 2 x 10(-4) of the magnetic particles, the critical magnetic field was established for the pure liquid crystal, and for liquid crystals doped with spherical, chainlike, and rodlike magnetic particles. The influence of the magnetic field depends on the type of anchoring, which is characterized by the density of anchoring energy and by the initial orientation between the liquid crystal molecules and the magnetic moment of the magnetic particles. The experimental results indicated soft anchoring in the case of spherical magnetic particles and rigid anchoring in the case of rodlike and chainlike magnetic particles, with parallel initial orientation between the magnetic moments of the magnetic particles and director.

RESUMEN

The orientational properties of the banana-shaped liquid crystal 4-chloro-1,3-phenylenebis{4-[4'-(10-undecenyloxy)]benzoyloxy} benzoate (ClPbis11BB) are reported in the nematic phase under the effect of an external magnetic field. A new hypothesis, which states that the central ring of the aromatic core is oriented perpendicularly to the external magnetic field, is proposed. In support of this hypothesis, a series of studies based on (2)H NMR spectroscopy, both in the bulk and in solution, are discussed. (2)H NMR measurements on three selectively deuterium-labelled isotopomers are presented, together with DFT results from B3LYP/cc-pvDz calculations performed on the aromatic core. The rather flat shape of the investigated intramolecular energy surface allows for several different conformations to be populated, the computed magnetic susceptibilities of which are consistent with the proposed hypothesis of peculiar orientation of banana-shaped molecules. Moreover, the orientation of the magnetic susceptibility tensor is shown to be strongly dependent on the internal conformation of the banana-shaped molecules.

RESUMEN

Novel, symmetrical, bent-core mesogens, namely substituted 1,3-phenylenebis{4'-[(ethoxycarbonyl)oxy]-1,1'-biphenyl}-4-carboxylates, are prepared with carbonate groups at both ends of the molecules. The mesophase properties are investigated for different electron-donating and -withdrawing substituents connected to the central ring at different positions. These compounds exhibit nematic and/or smectic A intercalated (SmA(int)) mesophases. Electro-optical studies show a very significant electric-field-induced biaxiality that is fast (<1 ms) and large (0.03) associated with a high birefringence (0.33 at 500 nm, 10 V mum(-1) field) for the unsubstituted analogue. For one substance this effect is observable from above 100 degrees C down to room temperature. Such electric-field-induced biaxiality may find practical applications in fast electro-optic modulation devices.