Alignment Of Liquid Crystalline Molecules Research Articles

Photoalignment and Photofixation of Chromonic Mesophase in Ionic Linear Polysiloxanes Using a Dual Irradiation System

Photoalignment technology enables macroscopic alignment of liquid crystalline molecules and their aggregates in a non-contact process by irradiating photo-responsive liquid crystalline compounds with linearly polarized light. Because photoalignment techniques prevent dust generation and uneven stretching, and accomplish fine and complex patterning, they are involved in the practical process of fabricating display panels, and continue to be applied in the research and creation of various anisotropic materials. Brilliant yellow (BY), a chromonic liquid crystal, has attracted considerable attention as the photoalignment sublayer in recent years, because of its ability to induce a high dichroic nature among many photo-responsive liquid crystalline materials. However, its dichroism is not maintained after prolonged exposure to a humid environment because of its intrinsic strong hygroscopicity of ionic BY molecules. In this study, to overcome this drawback, the photoalignment and successive photo-fixation of the BY columnar phase is proposed using UV-curable ionic polysiloxane as a matrix. Visible light was used for the photoalignment of the BY columnar phase, and UV light for photo-fixation. Consequently, the columnar chromonic phase is found to retain its orientation even after 4 h of exposure to a highly humid environment.

Fast photo- and electro-optical switching of the polymer- stabilised cholesteric liquid crystal composite prepared by the template method

ABSTRACTMultifunctional cholesteric liquid crystalline (LC) polymer-stabilised composite with photo- and electro-controllable photonic band gap was prepared and its fast photo- and electro-optical switching was demonstrated. The composite was prepared by a template method implying the polymerisation of cholesteric mixture consisting of several components, such as nematic liquid crystal compounds, chiral dopant, polymerisable mono- and diacrylates as well as photoinitiator. UV-induced polymerisation and subsequent removing of the low-molar-mass components enables to obtain porous cholesteric polymer matrix, which was used for filling with photochromic nematic mixture. Application of electric field or UV-irradiation shifts the photonic band gap to the short-wavelength spectral range. In the case of an electric field, this shift is associated with alignment of LC molecules along the electric field and deformation of the cholesteric helical structure. UV light action induces E-Z isomerisation of the azobenzene-containing molecules of photochromic mixture and decrease in their molecular anisometry leading to decrease in birefringence. Both electro- and photo-induced changes are completely reversible and switching the electric field off and/or irradiation with visible light completely recovers initial shape of selective reflection band. The switching is very fast and takes several seconds for the light action and less than one second for the electric field.

Effect of ZnO nanoparticles dispersed in liquid crystalline p-n-propoxy/propyl benzoic acids and mixtures – optical studies

In the present work, characterization studies are carried out on liquid crystalline (LC) p-n-propoxy benzoic acid (3oba), p-n-propyl benzoic acid (3ba) and the mixture of these two benzoic of acids dispersed with low weight concentrations of ZnO nanoparticles (nps). The existence of ZnO nps in above LC compounds are studied by scanning electron microscope and X-ray diffraction techniques. The LC phase transition temperatures of the compounds with the dispersion of ZnO nps are measured using polarizing microscope (POM) and differential scanning calorimeter (DSC). The nematic phase transition temperatures of 3ba, 3oba and mixture of 3ba and 3oba obtained from POM and DSC found to decrease with increase in the dispersion of various weight concentrations of ZnO nps. The birefringence studies are carried out for these compounds at different wavelengths (400, 500, 570, and 635 nm) with the dispersion of ZnO nps with increasing in weight concentrations (1, 1.5, 2 and 2.5 wt%) using modified spectrometer and the orientational order parameter ‘S’ of corresponding LC nanocomposites is measured from different optical methods viz., Kuczynski internal field model, Vuks isotropic model and effective geometry model . Among the three models the order parameter found to increase due to the strong alignment of LC molecules with the ZnO nps. The refractive indices measured at different wavelengths are fitted with two and three coefficient Cauchy’s model. For high birefringence LCs three coefficient Cauchy’s model is favorable than two coefficient model and vice versa. The results obtained are compared and discussed.

Morphology formation in binary mixtures upon gradual destabilisation.

Spontaneous liquid-liquid phase separation is commonly understood in terms of phenomenological mean-field theories. These theories correctly predict the structural features of the fluid at sufficiently long time scales and wavelengths. However, these conditions are not met in various examples in biology and materials science where the mixture is slowly destabilised, and phase separation is strongly affected by critical thermal fluctuations. We propose a mechanism of pretransitional structuring of a mixture that approaches the miscibility gap and predict scaling relations that describe how the characteristic feature size of the emerging morphology decreases with an increasing quench rate. These predictions quantitatively agree with our kinetic Monte Carlo and molecular dynamics simulations of a phase-separating binary mixture, as well as with previously reported experimental observations. We discuss how these predictions are affected by non-conserved order parameters (e.g., due to chemical reactions or alignment of liquid-crystalline molecules), hydrodynamics and active transport.

Quantum dot–polymer composites based on nanoporous polypropylene films with different draw ratios

Hybrid composites of polymers with fluorescent nanoparticles are unique materials combining good mechanical properties of the host polymer matrices and luminescent properties of the embedded nanoparticles. Here, we report on the optical characterization of novel inorganic–organic hybrid composites based on nanoporous polypropylene (PP) as a polymer matrix and quantum dots (QDs) as a fluorescent inorganic component. The first type of the composite films is prepared by absorption of CdSe/ZnS QDs onto the porous structure of the PP films, followed by annealing at 170°C, i.e., above the melting point of the PP. The second type of the composite films is obtained by filling of porous QD–PP composites with a nematic liquid crystalline (LC) mixture. Both types of composites are characterized by low light scattering, which makes it possible to study their optical properties by absorbance and fluorescence spectroscopies. TEM and confocal fluorescence microscopy have been used to analyze the aggregation of QDs in composites of different types. It has been shown that introduction of QDs into PP with a low draw ratio decreases the degree of alignment of LC molecules embedded into the pores of QD–PP composite films.

Points, skyrmions and torons in chiral nematic droplets.

Chiral nematic droplets with perpendicular surface alignment of liquid crystalline molecules frustrate the helical structure into convoluted 3D textures with complex topology. We observe the droplets with fluorescent confocal polarising microscopy (FCPM), and reconstruct and analyse for the first time the topology of the 3D director field using a novel method of director reconstruction from raw data. We always find an odd number of topological defects, which preserve the total topological charge of the droplet of +1 regardless of chirality. At higher chirality, we observe up to 5 point hedgehog defects, which are elastically stabilized with convoluted twisted structures, reminiscent of 2D skyrmions and toron-like structure, nested into a sphere.

LCD alignment layers. Controlling nematic domain properties

From simple pocket calculators, to mobile telephones and LCD-TV, over the past few decades devices based on liquid crystal display technology have proliferated into just about all conceivable aspects of everyday life. Although used in cutting-edge technology, it is surprising that a vital part in the construction of such displays relies essentially on a process invented almost 100 years ago. This essential part, the alignment layer, dictates the macroscopic uniform alignment of liquid crystalline molecules (mesogens) near its surface. The current method for manufacturing such layers is the mechanical rubbing of spin-coated polymers with a piece of velvet cloth. This very successful method is still at the basis of the production process of even the largest displays currently manufactured in industry. Unfortunately, the construction of ever larger displays with this technique is becoming a technological nightmare for engineers. Therefore, over the past decades, many alternatives to rubbing have been explored. This review will focus on advances towards achieving one of the most important goals in LCD technology: attaining rational control over the properties of nematic liquid crystal domains.

The phase transition temperatures of a liquid crystal determined from FT-IR spectra explored by principal component analysis

The FT-IR spectra of a thin layer of pure 4-chloro-2′-hydroxy-4′-pentyloxyazobenzene (CHPAB) were studied as a function of temperature. A detailed analysis of the intensity variations was performed by a method based on principal component analysis (PCA). It was shown that the phase transition temperatures obtained by means of PCA and those determined by differential scanning calorimetry (DSC), the most widely used technique in the field, were nearly identical. The PCA results revealed that the transition from solid to a liquid crystalline (LC) phase (smectic A) is more drastic phase transition in terms of infrared absorption changes. The nematic to isotropic phase transition is much less infrared sensitive. Very much smaller absorption changes are associated with the transition between the smectic and nematic mesophases. The pattern of the intensity changes strictly is correlated with the orientation of the CHPAB molecules towards the surface windows due to the surface-induced homeotropic alignment of LC molecules. The important role of hydrogen bonding interaction on the observed transition is disclosed.

Homeotropic-planar anchoring transition induced by trans-cis isomerization in ultrathin polyimide Langmuir–Blodgett films

The influence of the charge separation during the trans-cis conformational change between the surface azobenzene 6Az5PVA monolayer on polyimide Langmuir–Blodgett film on the surface charge density is investigated, using the Maxwell displacement current technique. It is theoretically shown, in the framework of the molecular model, which takes into account interactions between the surface polarization (in the quadrupole approximation) and surface electric field, that the experimentally observed changes of the surface charge density in ∼3×10−4 C/m2, caused by the charge separation in ∼2 pC, taking place during the conformational change, may lead to changing of the surface alignment of liquid crystalline molecules, such as 4-n-pentyl-4′-cyanobiphenyl (5CB), having contact with that solid substrate.

Surface polarization and effective anchoring energy in liquid crystals.

The influence of the surface polarization (SP) on the effective anchoring energy for both homeotropic and planar alignments of polar liquid crystals at a solid substrate is discussed from the energy point of view. It is shown that in a certain range of surface charge density the destabilizing SP mechanism may lead to destruction of the homeotropic or planar alignment of liquid crystalline molecules, such as 4-n-pentyl-4(')-cyanobiphenyl.

Photocontrol of Alignment of Liquid-Crystalline Molecules in Photoresponsive Polymer Networks

A novel composite system consisting of a low-mass nematic liquid crystal and a crosslinked polymer with azobenzene side chains exhibited large birefringence upon irradiation with linearly polarized light. The extent of photogenerated birefringence depended on p-substituents and alkylene spacer lengths of the azobenzenes. Three-dimensional orientation of liquid-crystalline molecules was also induced by slantwise irradiation with non-polarized light in this composite system. Rewritable photopatterning exhibiting good storage stability was carried out by slantwise irradiation with non-polarized visible light.

Steep electro-optical response of nematic liquid crystalline molecules induced by addition of polymeric chains

Abstract The steepness increases with a decrease in the ratio of elastic constants of the bend to the splay mode, K 3/K 1. The elastic constants of nematic directors are affected by the geometrical shape of a liquid crystalline molecule and a short range ordering for the alignment of liquid crystalline molecules. The magnitude of K 3/K 1 and the reorientational steepness of the nematic director were evaluated by an electric capacitance measurement of the (side chain-type liquid crystalline polymer (PS6EC)/low molecular weight liquid crystal (E7)) composite system cell. It became apparent from X-ray diffraction studies that the smectic-like short range ordering increases with increasing the fraction of PS6EC even in the nematic state of the composite system. Furthermore, a remarkably steep electro-optical response was successfully achieved.

Flow-induced high pretilt alignment of liquid crystalline molecules on polymer surface evaluated by optical second-harmonic generation

Abstract The alignment of liquid crystalline molecules at the interface between liquid crystal and polyimide surface was investigated on the basis of an optical second-harmonic generation method. It was confirmed for the first time that a uniform alignment of liquid crystalline molecules with a pretilt of about 10° was achieved by simply flowing the liquid crystal on the polyimide-coated substrate surface which produced no pretilt by a standard rubbing treatment.