Determination of the second order susceptibility tensor in banana-shaped liquid crystals

Optical second harmonic generation (SHG) measurements have been carried out on the B2 phases of three liquid crystal materials composed of conventional banana-shaped molecules (P-10-O-PIMB, P-8-O-PIMB and P-8-PIMB). Quantitative values of the second order susceptibilities d_ij have been deduced from SHG measurements at oblique incidences on polydomain racemic samples under an electric field. The d_ij coefficients are in the range 1-10 pm V^-1.     

On the response speed of pi-cells

We discuss the role of the Miesowicz viscosities on director reorientation in pi-cells, showing that the symmetry of this system allows for a simplification of the dynamical equations. We consider practical aspects of material optimisation for increased pi-cell switching speed. We make an observation regarding the switching speeds of positive and negative dielectric anisotropy surface mode devices.     

Formation of lanthanide ion networks by orientation of the mesophases of their complexes in magnetic fields

A complex of a Tm(III) perfluoroalkylsulsphate with salicylaldimine Schiff's base was synthesized and its liquid crystallinity characterized by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. An additional phase transition was detected. 2D or 3D anisotropically-organized media were obtained by cooling of the lanthanide complex from different phases in an applied magnetic field. A huge positive magnetic anisotropy was observed for the resulting material.     

Shape of the nematic-isotropic interface in conditions of partial wetting

A nematic liquid crystal in contact with a solid substrate is studied in the partial wetting regime. Both a mesoscopic Landau-de Gennes theory and a macroscopic effective interface Hamiltonian approach are considered. A generalized Young equation for the balance of forces at the three-phase contact line is derived, which takes into account corrections due to distortions of the nematic director field. It is also shown that the asymptotic form of the separation of the nematic-isotropic interface from the substrate has a logarithmic correction to the usual linear behaviour. The characteristic length scale of this correction is given by the ratio K/(2σ_NI), where K and σ_NI are the average elastic constant and the nematic-isotropic surface tension, respectively, and is of the order of a few hundred angstroms. Then, a simple form of an effective interface Hamiltonian is proposed, and results consistent with the predictions of the Landau-de Gennes theory are obtained. It is shown, in the framework of this macroscopic approach, that the line tension associated with the contact line remains finite, when the thermodynamic limit is taken, if the anchoring at both the nematic-substrate and the nematic-isotropic interfaces is homeotropic. However, in the case of different anchoring directions, the line tension diverges logarithmically with the system size.     

New low viscosity liquid crystal compounds containing the 2,3,4-trifluorophenyl moiety for active matrix displays

The synthetic route and physical properties for new three-ring derivatives, trans -4- n -alkyl-4'-(2,3,4-trifluorophenyl)bicyclohexanes ( n CCHB3F) with alkyl chain length n from 2 to 5 are presented. They exhibit broad range nematic phases exceeding 40 K, and low viscosity with a low activation enthalpy; they also show low dielectric anisotropies and high birefringence. Their dielectric properties are compared with those of their three-ring homologues with a lateral -NCS group. The Maier-Meier theory is used to evaluate the temperature dependence of the value of 'dielectric anisotropy divided by order parameter ( Δ ε/ S )'. The temperature dependence of Δ ε/ S in several phenylbicyclohexane homologues is explained. Furthermore, the viscosity of nematic liquid crystal mixtures containing these materials is significantly reduced. These mixtures are suitable for the improvement of the switching time of liquid crystal displays using TFT Twisted Nematic (TN) modes.     

Helical superstructures in the mesophase of compounds derived from 2-cyanoresorcinol

The mesophase exhibited by four members of a new homologous series of compounds composed of banana-shaped molecules derived from 2-cyanoresorcinol is reported. The characteristic optical textures showing helical filamentary growth patterns, coupled with the results from X-ray diffraction studies of these seven-ring compounds containing a biphenyl moiety indicate that the mesophase is indeed B7.     

Formation of lanthanide ion networks by orientation of the mesophases of their complexes in magnetic fields

A complex of a Tm(III) perfluoroalkylsulsphate with salicylaldimine Schiff's base was synthesized and its liquid crystallinity characterized by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. An additional phase transition was detected. 2D or 3D anisotropically-organized media were obtained by cooling of the lanthanide complex from different phases in an applied magnetic field. A huge positive magnetic anisotropy was observed for the resulting material.     

Liquid crystalline properties of dissymmetric molecules IX. The effect of a terminal nitro group on the thermal properties of the smectic A phase in 4-nitrophenyl 4-(4-alkoxybenzoyloxyphenyl)benzoates

This paper describes the effect of alkoxy chain length on the physico-chemical properties of 4-nitrophenyl 4-(4-alkoxybenzoyloxyphenyl)benzoates. The homologous series shows a smectic A (SmA) phase commencing from the hexyloxy homologue. While the pentyloxy homologue shows only a nematic (N) phase under microscopic observation, X-ray diffraction shows that the N phase has a cybotactic domain with a layer structure of the SmA phase, where the layer spacings are longer than the calculated molecular lengths by c. Å. The layer spacings for the heptyloxy-nonyloxy homologues are longer than the calculated molecular lengths by 2.2-4.8 Å and are almost independent of temperature. The layer spacings are correlated with the calculated molecular lengths, and based on these results, a new molecular arrangement model for the SmA phase is proposed.     

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 ×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.     

Helical superstructures in the mesophase of compounds derived from 2-cyanoresorcinol

The mesophase exhibited by four members of a new homologous series of compounds composed of banana-shaped molecules derived from 2-cyanoresorcinol is reported. The characteristic optical textures showing helical filamentary growth patterns, coupled with the results from X-ray diffraction studies of these seven-ring compounds containing a biphenyl moiety indicate that the mesophase is indeed B 7 .     

Photochromic characteristics of monodisperse microcapsules containing azobenzene derivative-doped nematic liquid crystals

Highly mono-sized dye-doped liquid crystal (LC) microcapsules were prepared by the solute codiffusion method for application in photoswitchable devices. Azobenzene derivatives, which can be photoisomerized by irradiation with UV (366 nm) and visible (433 nm) light, were used as a photoresponsive dichroic dye. The microcapsules have a spherical shape and a single dye-doped LC domain. After UV light (366 nm) irradiation, PDLC films prepared using the microcapsules attained a photostationary state within 10 min and this state maintained stability. PDLC films made by employing a dichroic dye having electron donor and acceptor groups in the molecule showed especially good photoisomerization properties. The reversible trans-cis -photoisomerization occurred sharply by irradiating alternately with visible and UV light.     

Formation of a host nanostructure for ferroelectric liquid crystals using thiol-ene polymers

Liquid crystal displays are a subject of intense research interest because of their application to high definition display devices. Recently, polymer stabilized ferroelectric liquid crystals (PSFLCs) have been investigated due to the enhanced electro-optic properties of FLCs. We have utilized thiol-ene photopolymerizations to form a PSFLC system. Thiol-ene photopolymerizations are radical reactions, which proceed via a step growth reaction mechanism. During the polymerization, the polymer network structure is trapped into place due to the rapid transition from low molecular mass monomers and oligomers to high molecular mass polymer. This aspect is evidenced by phase transition data for the FLC, which indicates that the monomer and polymer are not phase separated from the FLC. Infrared dichroism shows that both monomer and polymer are ordered in parallel with the smectic layers of the FLC. Small angle X-ray scattering (SAXS) data show that both monomer and polymer are swelling the smectic layers. Thus, a polymer nanostructure is produced that serves as an ordered, stabilizing host for the FLC.     

Molecular alignment and field-induced reorientation in a twisted nematic liquid crystal pi-cell

A twisted nematic pi-cell has been studied by optical transmission measurement, polarized Fourier-transform infrared (pFTIR) absorption spectroscopy, and Raman spectroscopy. Our pFTIR results suggest that the LC molecules undergo a restricted rotation about the molecular long axis. The rise and decay times of the optical response were found to be 6 ms and 1.6 ms, respectively. The switching dynamics of the twisted pi-cell was also studied using time-resolved Raman spectroscopy. A normal mode associated with the C-H out-of-plane wag on the LC core was found to be enhanced after the electric field was switched off. Our data show that LC molecules in the twisted pi-cell do not rotate like a rigid molecule during the field-induced reorientation process. The methods employed in this study have yielded valuable information about LC alignment and field-induced reorientation with respect to functional group specificity.     

Comparative analysis of basic physical properties of a ferroelectric liquid crystal and a polymer dispersed ferroelectric liquid crystal

A polymer film of polyvinylbutyral with dispersed droplets of a ferroelectric liquid crystalline mixture (FLC309c) has been prepared and characterized. The collective processes have been studied by dielectric relaxation spectroscopy in the frequency range 10 Hz to 13 MHz. In comparison with the FLC309c mixture, the polymer dispersed ferroelectric liquid crystal (PDFLC309c) based on FLC309c exhibits a Goldstone-like mode relaxation with a much higher relaxation frequency, but a smaller dielectric strength than the Goldstone mode observed for the FLC309c mixture. The spontaneous polarization of PDFLC309c decreases by nearly one order of magnitude in comparison with FLC309c, while the tilt angle decreases by 20%. Considering these results, we believe that a non-switching region exists near the polymer boundaries and that significant deformations of the helical structure occur due to stronger anchoring.     

Thermotropic main chain liquid crystalline polyesters containing ferrocene and phosphate units: synthesis and characterization

A new series of liquid crystalline main chain copolyesters were prepared, having ferrocene in the mesogenic segment and a methyl phosphate group along with a methylene spacer. The even numbered methylene groups were varied from two to ten. Liquid crystalline behaviour was investigated on a hot stage optical polarized microscope. Thermal properties of the polymers were analysed by TGA and DSC, revealing that the polymers yield high char products, probably caused by the formation of phosphorus and iron oxides. The glass transition (T _g) temperatures of the polymers were found to be fairly low, the result of the incorporation of bulky phosphorus and ferrocene moieties in the chain. The phase behaviour was analysed and correlated with the structure of the polymers. The liquid crystalline textures of the polymers became more transparent with increasing spacer length. Energy minimized structures for the polymer repeating units reveal that both the ferrocene and phosphorus moieties produce more molecular entanglement, thus reducing the T _g and T _m of the polymers.     

The first lanthanide(III) monoporphyrin complex liquid crystal

Hydroxy [5,10,15,20-tetra[ p -decyloxy- m -methyloxy)phenyl]porphyrin Yb(III) exhibits a discotic hexagonal columnar phase, it is the first example of a monoporphyrin rare earth complex liquid crystal.     

A vector grouping algorithm for liquid crystal tensor field visualization

Tensor fields are at the heart of many science and engineering disciplines. Many tensor visualization methods separate the tensor into component eigenvectors and visualize those instead. Eigenvectors are normally ordered according to their eigenvalues: the eigenvectors corresponding to the smallest, median, or largest eigenvalues are in their corresponding groups. We show that this ordering strategy is undesirable for liquid crystal systems and propose a new approach, where the vectors are grouped to minimize some local energy. The grouping process is reminiscent of the epitaxial expansion in the anchoring of liquid crystals on a surface. The new algorithm is successfully applied to a liquid crystal system to study the biaxiality structure near a defect.     

Photochromic characteristics of monodisperse microcapsules containing azobenzene derivative-doped nematic liquid crystals

Highly mono-sized dye-doped liquid crystal (LC) microcapsules were prepared by the solute codiffusion method for application in photoswitchable devices. Azobenzene derivatives, which can be photoisomerized by irradiation with UV (366 nm) and visible (433 nm) light, were used as a photoresponsive dichroic dye. The microcapsules have a spherical shape and a single dye-doped LC domain. After UV light (366 nm) irradiation, PDLC films prepared using the microcapsules attained a photostationary state within 10 min and this state maintained stability. PDLC films made by employing a dichroic dye having electron donor and acceptor groups in the molecule showed especially good photoisomerization properties. The reversible trans-cis -photoisomerization occurred sharply by irradiating alternately with visible and UV light.     

Synthesis and properties of azo dye aligning layers for liquid crystal cells

The synthesis and properties of azo dyes that can be used for photoaligning liquid crystals (LCs) have been investigated. The structures and the synthetic procedure for the azo dyes are presented. The photoaligning of azo dyes takes place purely due to the reorientation of the molecular absorption oscillators perpendicular to the UV light polarization. The qualitative model for the phenomenon in terms of the rotational diffusion of the azo dye molecules in the field of the polarized light is discussed. The order parameters S = -0.4 (80% of the maximum absolute value S_m = -0.5) were measured from the polarized absorption spectra at the wavelength 372 nm. A temperature stable pretilt angle of 5.3° was obtained by a two-step exposure of the azo dye film using normally incident polarized light followed by oblique non-polarized light. The azimuthal anchoring energy of the photoaligned substrate was A_ϕ ≈10^-4 J m^-2, which is the same as the anchoring of the rubbed polyimide (PI) layer. The voltage holding ratio value of a photoaligned LC cell was found to be even higher than for a rubbed PI layer, which enables the applications of azo dyes as aligning layers in active matrix liquid crystal displays. The thermal stability of the photoaligned azo dye layers is sufficiently high, but UV stability has to be improved, e.g. by polymerization. A new LCD aligning technology based on polymerized azo dye layers is envisaged.     

The first columnar discotic liquid crystalline anthraquinone metal complexes

In this communication we present the synthesis and characterization of the first two members of a new class of metallomesogen. The Pd and Cu complexes of 1-hydroxy-2,3,5,6,7-pentakis(dodecyloxy)anthra-9,10-quinone were prepared by reacting the respective metal(II) acetate with the ligand. Preliminary mesophase characterization by DSC and polarizing optical microscopy indicates the formation of columnar mesophases in both the complexes.