Macromolecular composites of extruded thermotropic polymer sheets

Using two types of thermotropic liquid-crystal polymers, experimental results reconfirmed previous reports that the relaxation time of extruded liquid-crystal polymers is much longer than that of conventional polymers. The initial modulus and elongation of sheets extruded with liquidcrystal polymers were found to be strongly dependent on the gauge length. In addition, it was observed, for the first time, that the mechanical properties of the extruded thermotropic copolyesters follow the general equations for fiber-reinforced composites such that the angular dependence of the tensile strength of the extruded liquid-crystal polymeric sheets obeys the Tsai-Hill theory, while the angular dependence of the initial modulus follows the Lees equation. These results suggest that the highly oriented liquid-crystal domains may be considered as reinforcing fibers in the extruded articles, and their mechanical properties can be predicted using existing composite theories.     

Comment on "Phase-shifting shearing interferometer"

The role of the liquid-crystal layer in the phase-shifting shearing interferometer that was proposed by Griffin [Opt. Lett. 26, 140 (2001)] is analyzed. An extra voltage-dependent phase difference is produced when the phase retarder is oriented at an angle from the optical axis of the laser beam such that the interferogram will not repeat itself after a 2pi phase change of the liquid-crystal retarder in either the common Mach-Zehnder interferometer or the phase-shifting shearing interferometer.     

Molecular dynamics and strengthening of liquid-crystal polymers

The role of large-scale molecular motion in the self-organization and strengthening of liquid-crystal polymer fibers is discussed. It is shown that, at high temperatures, these objects are oriented liquid-crystal melts in which macromolecules remain extended but execute high-frequency conformational motions without leaving the tube approximately 20 Å in diameter. This large-scale motion is referred to as quasi-segmental motion. During annealing, the chains involved in quasi-segmental motion can accomplish longitudinal displacements (reptate) over considerable distances. It is this reptation that favors spontaneous self-organization and, consequently, strengthening of liquid-crystal polymer fibers upon heat treatment. The role played by the quasi-segmental motion of rigid macromolecules in the strengthening of polymers of different types is compared with the role played by the segmental motion of flexible chains in this process.     

Reorientational optical nonlinearity of nematic liquid-crystal cells near the nematic-isotropic phase transition temperature

We address the reorientational optical nonlinearity of homogeneously aligned neamtic liquid-crystal (NLC) cells. The propagation of light in NLC cells depend strongly on temperature. At a temperature approaching the clearing point, an undulating beam and multifocal points are observed in the NLC cell by use of a polarizing optical microscope. Using a conoscopic technique, we observed novel consecutive concentric and parabolic patterns projected onto a screen. Optical energy is considered to compete with thermal energy to affect NLC's orientation and to generate singularities in the steady state. A model of the configuration of the liquid crystal's orientation is proposed.     

Switchable holographic gratings formed in polymer-dispersed liquid-crystal cells by use of a He-Ne laser

We report on the formation of switchable holographic transmission gratings in polymer-dispersed liquid-crystal cells by use of the 633-nm wavelength of a He-Ne laser. We present results for the structure, diffraction efficiency, and switching characteristics of the gratings.     

N.M.R. spectra of oriented molecules

By analysis of the high resolution N.M.R. spectrum of 1,2,2,3-tetrachloro-propane partially oriented in a liquid-crystal solvent, it has been shown that this molecule takes up predominantly the G′G′ type of conformation.     

Electrochromic blueshift in polymer-dispersed liquid-crystal cells

Electrochromic blueshift in the absorption band of polymer-dispersed liquid-crystal cells is reported as a function of applied electric field. The changes in the peak absorption wavelength, absorption broadening, and their possible relationships with the nonlinear optical properties of polymer-dispersed liquid-crystal cells are discussed.     

双掺杂LiNbO3晶体中的稳态空间电荷场

建立了包括扩散、漂移和光伏打三种输运机制下双掺杂LiNbO3晶体在用双色光进行光存储的带输运方程,并得出了小调制度下稳态空间电荷场的分析解。对双色光的光强比、体光栅的波矢、双掺杂的浓度和晶体的 氧化还原程度对空间电荷场的影响进行了数值计算和讨论。     

Dielectric properties of an ytterbium-based nematic liquid-crystal complex

The dielectric properties of an yttrium-based paramagnetic thermotropic nematic liquid-crystal complex have been studied. The conditions for the measurement of the magnitude and sign of the dielectric anisotropy of the mesophase have been determined. The relaxation of the components of the relative permittivity of a macroscopically homogeneously oriented sample has been observed. A high polarity of the metallomesogenic complex has been revealed.     

Rotational reorientation of the director in twisted nematic liquid-crystal cells

The orientational relaxation of the director to its equilibrium orientation under electric, elastic, and viscous torques arising in twisted nematic liquid-crystal cells is investigated. It is shown that the relaxation time of the director depends strongly on the external electric field strength and weakly on the energy of anchoring of liquid-crystal molecules to the surfaces of the cell. The relaxation time of the director anomalously increases in electric fields close to the Fréedericksz threshold. It is established that, at specific strengths of the external electric field, the relaxation can occur in the form of traveling waves propagating from one edge to the other edge of the twisted nematic liquid-crystal cell. The calculations of the relaxation processes in the vicinity of the nematic-smectic A phase transition temperature demonstrate that the distortion of the director field is uniform over the entire cross section of the liquid-crystal cell and does not depend on the strength of anchoring of the liquid-crystal molecules to the surfaces of the cell.     

Two-dimensional vectorial photonic crystals formed in azo-dye-doped liquid crystals

A 2D, square lattice, vectorial photonic crystal is formed by vectorial holography using an azo-dye-doped liquid-crystal material. Four coherent beams are interfered to generate a highly stable, switchable polarization holography structure. The formation of the vectorial lattice by the liquid-crystal molecular orientation is confirmed by diffraction pattern and polarization microscopy analysis. Simulations of the alignment and diffraction pattern are in good agreement with the experimental results. Polarization sensitive diffraction behavior is also consistent with the Kakichashvili model.     

Transitional regimes of penetrative convection in a plain layer

Penetrative convection in a plain layer of water is considered for the interval of temperatures containing the point of density maximum. When the unstable and stable layers are equal in the static conductive state, the development of convective instability is investigated from the formation of steady structures to chaotic motion. Scales of the periodicity cell for regimes with a strong nonlinear effect were chosen with particular attention. Specifics are shown for steady structures with small-scale vortices near upper boundaries and for periodic motions with synchronized oscillations of the lower parts of vertically elongated profiles of temperature that move symmetrically. With the increase of supercriticality, the motion loses reflection symmetry, becomes doubly periodic, and finally becomes quasi-periodic before transition to chaotic motion. Domains of hysteresis are investigated for which motions with different structure and heat fluxes coexist, and it is illustrated by the dependence of the Nusselt number on supercriticallity.     

Photoelectric properties of a self-ordered molecular disperse structure based on a polar-dye solution in a liquid-crystal matrix

An analysis of photoluminescence spectra and nonlinear optical properties revealed the effect of polar-dye aggregates in a liquid-crystal matrix on the formation of an ordered molecular disperse structure. As the dye is aggregated in the liquid-crystal matrix, there is a certain concentration of the dye below which only second-harmonic generation occurs upon laser excitation at 1064 nm, while above this concentration, this generation occurs in combination with intense two-photon-induced photoluminescence. An applied electric field or intense ultraviolet illumination is found to change both the molecule orientation and the conditions of molecule aggregation.     

Recording of thin phase holograms in polymer-dispersed liquid-crystal composites based on fullerene-containing π-conjugated organic systems

The recording of thin phase holograms in promising fullerene-containing, polymer-dispersed liquid-crystal composites based on 2-cyclooctylamino-5-nitropyridine and N-(4-nitrophenyl)-(L)-prolinol using picosecond laser pulses is studied for the first time. The anisotropy of the refractive index substantially increases after laser irradiation and exceeds by a factor of five this anisotropy for liquid-crystal cells containing dyes. The results of our studies suggest that the fullerene-containing, polymer-dispersed liquid-crystal systems can be used as passive media for reversible data recording.     

Model of organization of the epitropic liquid phase

We propose a molecular-statistical model of organization of the epitropic liquid-crystal phase, according to which the presence of fluctuation associates in the bulk of the liquid and active (adsorption) centers on the solid substrate is essential for the formation of this phase. Using this model, it is possible to describe the experimental temperature dependence of the thickness of the epitropic liquid-crystal layer of nitrobenzene on quartz. Quantitative agreement between the theory and experiment is reached in the close proximity (～10 K) of the crystallization temperature. Important thermodynamic parameters of the model can be corrected from comparison with experimental results.     

Laser Nanofluidics of Liquid Crystals

Physics of the Solid State - Several scenarios of formation of hydrodynamic flows in nanoscale planar-oriented liquid-crystal (POLC) channels are described by numerical methods within nonlinear...     

Molecular mobility and strengthening of oriented liquid-crystal polymers

Broad-line proton magnetic resonance has been used to study cooperative mesophase molecular motion in the example of a fully-aromatic oriented liquid-crystal polymer Vectra A950. The molecular mobility is found to decrease as a result of heat treatment, which is considered to be the reason for a higher tensile (rupture) strength owing to an increase in the activation energy for the fracture process. Fiz. Tverd. Tela (St. Petersburg) 41, 859–861 (May 1999)     

Hydrodynamic mechanism of temperature gradient formation in thin nematic films

Physics of the Solid State - The temperature gradient formation mechanism in an initially uniformly heated hybrid-oriented liquid-crystal (HOLC) channel of microscopic sizes upon exposure to a...