Traian Beica

Dynamics of the surface layer in cyanobiphenyl–aerosil nanocomposites with a high silica density

Composites were prepared from an aerosil and 4-n-alkyl-4′cyanobiphenyls with five to eight carbon atoms in the alkyl chain. Their high silica density of ∼7 g aerosil in 1 cm3 of liquid crystal (LC) allows the observation of the behaviour of a thin cyanobiphenyl layer (having nearly a monolayer structure) on the silica particles. The systems are investigated by dielectric spectroscopy (10−2–109 Hz) in a large temperature range (220–370 K). All the composites show a (main) relaxation process at frequencies much lower than the processes observed for the bulk LC that was assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its characteristic frequencies obeys the Vogel–Fulcher–Tammann law, which is found to be typical for glass-forming liquids. The quasi two-dimensional character of the glass transition in the surface layer is discussed for the first time. At the nematic-to-isotropic transition temperature of the bulk, the composites show a continuous decrease of the characteristic frequencies as a function of the alkyl chain length, while the bulk LCs show the well known odd–even behaviour. The magnitude and temperature dependence of the slow relaxation process in the composites (molecules on an outer surface) agree with those of the same molecules confined to the nanopores of molecular sieves (internal surface).

Temperature dependence of the density of calamitic and discotic nematic lyotropic liquid crystals containing sodium lauryl sulphate/decanol/water

Experimental results for the temperature dependence of density at normal pressure for two compositions of sodium lauryl sulphate/decanol/water solutions, exhibiting either a calamitic or a discotic lyotropic nematic phase at room temperature, are presented. Within the limits of experimental precision (±1 2 10−5 g cm−3), the systems show no jump in density at the nematic to isotropic phase transition. Over the studied temperature range, the mean thermal expansion coefficients were also evaluated.

Nematic-isotropic interfacial tension anisotropy of a calamitic lyotropic liquid crystal

In a planar oriented sample of a calamitic nematic lyotropic system (mixture of sodium lauryl sulphate/water/decanol), isotropic pretransitional domains appear at the nematic to isotropic transition. The domains are oblong in shape with the long axis along the orientational direction. We show experimental evidence that this oblong shape is determined by the nematic–isotropic interfacial tension anisotropy. Two uniparametric models of simple angular dependences for the interfacial tension are tested. Using the differential system obtained from the Young–Laplace condition at the nematic–isotropic interface, the domain shape can be numerically calculated for each value of the interfacial tension anisotropy. By processing the values of the transmitted light through both an isolated isotropic domain and its surrounding nematic zone, we obtain the anisotropy of the interfacial tension as the main fitting parameter. An estimation of the ratio of the extreme values for the interfacial tension is given.

Orientational Structures in Chiral Nematic Liquid Crystals for Weak Anchoring

Abstract This work presents a unitary mode to describe orientational structures of extreme free energy for Unidimensional distortions of a chiral nematic layer with weak tilted anchoring at limiting surfaces, using structure parameters. The case of an applied electric field was also considered. A generalised expression for surface energy involving both polar and azimuthal deformations was introduced A classification of possible structures having monotonous or non-monotonous variations of the tilt angle is presented. The solving of the equation system that links the structure parameters in each case is given.

Dynamics of Cyanophenyl Alkylbenzoate Molecules in a Surface Layer Adsorbed onto Aerosil. I. Cyanophenyl Hexylbenzoate

A composite prepared from aerosil A380 and the liquid crystal (LC) 4-hexyl-4′-cyanophenyl benzoate (CP6B) was investigated by broadband dielectric spectroscopy in a large temperature range. The selected high silica density (ca. 7 g aerosil/1 g of CP6B) allows the observation of a thin layer (two-monolayer structure) adsorbed on the surface of the silica particles. For the composite one relaxation process is observed at frequencies much lower than that of the processes found for bulk CP6B. It is assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its relaxation rates obeys the Vogel–Fulcher–Tammann law, which is characteristic for glass-forming liquids. The quasi 2D character of the observed glassy dynamics in the surface layer is discussed. The temperature dependence of the CP6B relaxation in the composite is compared with that of related hexylcyanobiphenyl molecules in the surface layer of aerosil composite with a similar concentration.

Confining Effects in Composites Containing Molecular Sieves

Confinement of 4n-octyl-4′-cyanobiphenyl to nanoporous molecular sieves with hexagonal structure of cylindrical pores or with cellular structure is studied. It was found by differential scanning calorimetry and broadband dielectric spectroscopy that composites with the pores completely filled with liquid crystal show phase transitions characteristic for bulk, while the partially filled samples do not show these transitions. Thermogravimetric, differential thermal analysis and in situ infrared spectroscopic investigations have indicated first time that the dealkylation and oxidation of the confined liquid crystal is quite possible inside the nanopores of these molecular sieves, at temperatures much lower than for the bulk.

On the Crystal-to-Mesophase Phase Transition in Copper(II) Salts of Even-Chain Fatty Acids

The phase transition from the solid state to the liquid crystal state was investigated for the copper(II) salts of the fatty acids with 10, 12, 14, 16 or 18 carbon atoms in the chain, by measuring the temperature dependence of the conductance and capacitance of a cell containing the material under investigation. The temperatures deduced from these dependences agree reasonably with those found by DSC measurements and with those to be found in the literature. Differences in dielectric behaviour between the samples are qualitatively discussed.

Synthesis and Electro-Optical Studies on Composite Materials Polymer Particles/Nematic Liquid Crystal

We have synthesized polymer particles of the type alternated copolymers DVB- maleic anhydride. The dimensions of the obtained particles have been determined by electron microscopy and are in the range 1–2 μm. These particles have been mixed by stirring and ultrasound with nematic liquid crystals ZLI 1221 (Merck), in a concentration of 5% b.w.(polymer/LC). We applied cycles of temperature and electric field, corresponding to the TSDC method. We measured the depolarization currents and simultaneously the optical transmission as a function of the electric field and temperature. We obtained information regarding the conduction mechanism, the activation energy and the pre-exponential frequency factor in the Arrhenius diagrams. The dipolar current obtained is consequent to a superposition of a dipolar distribution with a wide spectrum of relaxation times.

Orientational Transitions in Nematic Liquid Crystals on Substrates with Controlled Topography

Abstract Orientational transitions induced by temperature variation, in cells with nematic liquid crystal having limiting substrates with controlled topography, are presented. The variable topography was obtained by SiOx deposition in vacuum at an incidence angle around 85°, or by depositions of polyvinyl alcohol (PVA) with different thicknesses, on glasses with the same SiOx deposition. The experimental results are explained by a phenomenological model based on the Landau-de Gennes free energy expansion, taking into account the anisotropy of the substrate.