Influence of rubbing conditions of polyimide alignment layer on optical anisotropy of immobilized liquid crystal film

The relationship between the molecular orientation of a rubbed polyimide film (alignment layer) and that of mesogens in a photopolymerized liquid crystal (LC) coated on the film has been investigated using optical measurements. LC monomers were deposited on the alignment layer and were aligned in one direction. The LC monomers were subsequently photocured. Alignment layers under various rubbing conditions were prepared. It was found that the inclination angle of the refractive index ellipsoid and the optical retardation of photopolymerized LC films are strongly related to the optical anisotropy of the rubbed polyimide film. The photopolymerized LC film exhibited high optical anisotropy when alignment layers with an inclination angle of the refractive index ellipsoid smaller than 6° were used.     

A new reverse mode light shutter from silica-dispersed liquid crystals

Nanoparticle dispersions in liquid crystalline materials at low concentrations allow both investigating the formation of defects in liquid crystal (LC) and enhancing the light-scattering properties of LC optical devices. Reverse mode LC dispersions are LC devices, which look like transparent in their OFF state, when no electric field is applied, and opaque in their ON state. In this paper, a new reverse mode device, formed by a dispersion of a LC mixture in a silica nanoparticle crosslinked network, is presented. The morphology and the electro-optical properties of these silica nanoparticle/LC composites were investigated for two different LC mixtures with a negative dielectric anisotropy. The observed transmittances and relaxation times were found to depend strongly on the silica amount and chemical–physical properties of LC used in the sample preparation.     

Induced birefringence of rubbed and stretched polyvinyl alcohol foils as alignment layers for nematic molecules

Some optical properties of poly (vinyl alcohol) (PVA) films are investigated prior and after the rubbing and stretching of the samples. Birefringence of the prepared foils is enhanced as their stretching degree and rubbing become larger. These processing factors are also affecting the values of the ordinary and extraordinary refractive indices. The induced anisotropy in PVA foils is evidenced in the morphological characteristics observed by optical microscopy. The spreading and adhesion behaviors of a nematic liquid crystal on PVA foils are analyzed by means of contact angle measurements. Preliminary testing of the nematic on the PVA films revealed significant variations in transmitted light intensity during sample rotation under crossed polarizers. The high contrast between dark and bright patterns is indicative of uniform and homogenous alignment of nematic on the rubbed and stretched PVA foils.     

Small-angle neutron scattering from a hexagonal phase under shear

Summary The shear orientation of a micellar hexagonal liquid crystalline phase was investigated by small-angle neutron scattering. The hexagonal phase in the quiescent state showed a symmetrical scattering pattern typical of a polydomain structure. Enhanced scattering along the flow direction was observed during shear and the anisotropy of scattering intensity became stronger with increasing shear rate. The anisotropic scattering pattern corresponds to an orientation perpendicular to the flow direction and can be interpreted as a log-rolling state. The oriented sample did not relax after cessation of shear. The results from small-angle neutron scattering confirm data obtained previously from rheo-small angle light scattering measurements and are discussed in comparison to shear alignment of lyotropic liquid crystalline polymer solutions.     

Orientational phenomena in comb-shaped liquid crystal polymers induced by laser beam irradiation

Action of the laser illumination on optical properties of liquid crystalline (LC) comb-shaped acrylic and methacrylic copolymers containing cyanobiphenyl mesogenic groups and azo dye fragments has been studied. Under an illumination the cis-trans configurational changes of dye fragments take place and reorientation of the side groups towards a direction perpendicular to the electric vector of the actinic light occurs. As a result the photoinduced optical birefringence (δn = 0.05-0.11) appears. The detailed study of dichrotic and optical properties of polymeric films, kinetics and relaxation of δn, as well as mechanism of orientational transformations during a laser illumination are discussed. The induced optical anisotropy of the films can be changed reversibly by rotating the electric vector of the laser beam in respect to the film that may be used in reversible optical data storage.     

Photo‐optical properties and photo‐orientation phenomena in an immiscible blend of cholesteric copolymer with azobenzene‐containing polymer

Novel side chain liquid crystalline polymer blends containing immiscible non‐chiral and cholesteric polymers were prepared. A non‐chiral polymer was used containing azobenzene side groups capable of E–Z photoisomerization. Phase behaviour and miscibility properties of the blends were studied. Thin films of the blends were prepared by spin‐coating; it was shown that just after preparation these films are optically isotropic and homogeneous. Photo‐ and chiro‐optical properties of the films as well as photo‐orientation phenomena induced by the action of polarized light were investigated. It was found that the photo‐optical behavior of these films is rather complicated and considerably different from the properties of both individual components. This difference is associated with the strong influence of blend morphology on the photo‐optical properties.     

Anchoring and electro-optical dynamics of thin liquid crystalline films in a polyimide cell: experiment and theory

The surface-dependent anchoring and electro-optical (EO) dynamics of thin liquid crystalline films have been examined using Fourier transform infrared spectroscopy. A simple nematic liquid crystal, 4-n-pentyl-4'-cyanobiphenyl (5CB), is confined as 40, 50, and 390 nm thick films in nanocavities defined by gold interdigitated electrode arrays (IDEAs) patterned on polyimide-coated zinc selenide (ZnSe) substrates [Noble et al., J. Am. Chem. Soc. 124, 15020 (2002)]. New strategies for controlling the anchoring interactions and EO dynamics are explored based on coating a ZnSe surface with an organic polyimide layer in order to both planarize the substrate and induce a planar alignment of the liquid crystalline film. The polyimide layer can be further treated so as to induce a strong alignment of the nematic director along a direction parallel to the electrode digits of the IDEA. Step-scan time-resolved spectroscopy measurements were made to determine the rate constants for the electric-field-induced orientation and thermal relaxation of the 5CB films. In an alternate set of experiments, uncoated ZnSe substrates were polished unidirectionally to produce a grooved surface presenting nanometer-scale corrugations. The dynamical rate constants measured for several nanoscale film thicknesses and equilibrium organizations of the director in these planar alignments show marked sensitivities. The orientation rates are found to vary strongly with both the magnitude of the applied potential and the initial anisotropy of the alignment of the director within the IDEA. The relaxation rates do not vary in this same way. The marked variations seen in EO dynamics can be accounted for by a simple coarse-grained dynamical model.     

Reverse-mode polymer dispersed liquid crystal films prepared by patterned polymer walls

This article proposes a methodology to prepare polymer dispersed liquid crystal (PDLC) films working in the reverse-mode operation, where the ion-doped nematic liquid crystals (NLCs) with negative dielectric anisotropy (Δε) were locked by polymer walls. On-state and off-state of films were controlled by an electric field. In the absence of an electric field, it appears to be transparent. In the field, the homogeneous alignment NLCs form dynamic scattering, giving rise to opaque. The effect of the cylindrical holes with different diameters of photo masks and liquid crystal Δε on the electro-optical properties and transmittance wavelength range of 400–3000 nm light of samples were investigated. It was found that it exhibited very good electro-optical characteristics, high contrast ratio and excellent infrared energy-efficient of films used as switchable windows.     

Liquid crystalline dispersions of complexes formed by chitosan with double-stranded nucleic acids

Double-stranded molecules of nucleic acids (NAs) were shown to interact with chitosans to form under certain conditions (chitosan molecular mass, content of amino groups, distance between amino groups, pH of solution, etc.) multiple types of liquid crystalline dispersions. The dispersions formed are different in their spatial structures, and hence in the sense and magnitude of the abnormal optical activity. The physicochemical properties of these dispersions were investigated. Time- and temperature-stabilization of dispersions that possess abnormal optical activity were achieved by chemical crosslinking of chitosan molecules in the liquid crystalline dispersions formed from NA–chitosan complexes. The accessibility of these ‘NA–liquid crystalline elastomers' with respect to enzyme and drug action was tested. The multiplicity of liquid crystalline forms of DNA–chitosan complexes was possibly explained by the influence of the character of the dipole distribution over the surface DNA molecules on the sense of the spatial twist of the cholesteric liquid crystalline dispersions resulting from these complexes.     

Dynamics of chiral liquid crystals under applied shear

We simulate the alignment dynamics of cholesteric (chiral) rod-like liquid crystals by using a Landau-de Gennes (LdG) expression for microstructure evolution in response to flow. This study is motivated by recent advances in novel cholesteric nanorod dispersions. Prior work on the modelling of cholesterics has suffered from the restriction of helicity to only a single direction, often with a pre-imposed pitch, due to numerical difficulties. This has severely limited cholesteric modelling in regard to both accuracy and experimental relevance. Our simulations avoid this limitation. Relevant forces on rods include solvent-rod drag, nematic alignment, microstructure elasticity and chiral twist. Phase diagrams are developed to demonstrate the response of these systems to variations in chiral and flow forces. Our results indicate that for low shear rates, chiral and elastic forces prevent the rods from moving in response to flow. At high shear rates, the rods tumble and form unique transient structures (combinations of tumbling and cholesteric phases) as flow forces and chiral forces compete. Even if slight alignment is induced at the boundaries, the phase diagram substantially changes, chiefly by constraining the possible chiral phases. This work has immediate relevance to applications which exploit the optical properties of films solidified from cholesteric dispersions.     

Liquid crystalline dispersions of complexes formed by chitosan with double-stranded nucleic acids

Double-stranded molecules of nucleic acids (NAs) were shown to interact with chitosans to form under certain conditions (chitosan molecular mass, content of amino groups, distance between amino groups, pH of solution, etc.) multiple types of liquid crystalline dispersions. The dispersions formed are different in their spatial structures, and hence in the sense and magnitude of the abnormal optical activity. The physicochemical properties of these dispersions were investigated. Time- and temperature-stabilization of dispersions that possess abnormal optical activity were achieved by chemical crosslinking of chitosan molecules in the liquid crystalline dispersions formed from NA-chitosan complexes. The accessibility of these 'NA-liquid crystalline elastomers' with respect to enzyme and drug action was tested. The multiplicity of liquid crystalline forms of DNA-chitosan complexes was possibly explained by the influence of the character of the dipole distribution over the surface DNA molecules on the sense of the spatial twist of the cholesteric liquid crystalline dispersions resulting from these complexes.     

Photoreactive main‐chain liquid‐crystalline polyesters: Synthesis,characterization, and photochemistry

Three series of semiflexible and rigid main‐chain polyesters containing photoreactive mesogenic units derived from p‐phenylenediacrylic acid (PDA) and cinnamic acid have been synthesized by high‐temperature polycondensation. The thermal and mesomorphic properties of the polymers have been determined. The photochemical behavior of polymer P‐[1]‐T, which contains a PDA unit, has been studied both in solution and in films. In solution, [2+2] photocycloaddition, E/Z photoisomerization, and photo‐Fries rearrangement can take place. In contrast, the dominant process in spin‐coated films is the [2+2] photocycloaddition reaction, which causes crosslinking of the polymer. In films, the photochemistry and induction of anisotropy are strongly influenced by the aggregation of the PDA phenylester unit. A dichroism of about 0.2 has been induced in films by irradiation with linearly polarized UV light, and thus the capability of these films to induce optical anisotropy and align liquid crystals has been demonstrated. Liquid‐crystalline cells have been made with polarized irradiated films of P‐[1]‐T as aligning layers. A commercial liquid‐crystalline mixture has been used for this study, and a similar liquid‐crystalline order determined by polarized Fourier transform infrared to a commercial cell with rubbed polyimide as an aligning layer has been detected. Because of crosslinking of the irradiated P‐[1]‐T photoaligning layer, the photoinduced anisotropy is stable at high temperatures, and the liquid‐crystalline molecules are insoluble in the irradiated polymer. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4907–4921, 2005     

Effect of Curing Process on the Dynamic Response of Polymer Dispersed Liquid Crystal Films

Summary: The time relaxation of the optical transmission of polymer-dispersed liquid crystal films was investigated after application of an electric rectangular pulse. These films, consisting of liquid crystalline microdomains dispersed in a polymer matrix, were obtained by polymerization induced phase separation using ultraviolet and electron beam curing techniques. The effects of the curing procedure and film thickness on the transmission properties were investigated. The electro-optical response was expressed via a hierarchy of order parameters. A good agreement between the experimental data and calculated transmission values was found.     

Nanoorganization and Phase Behavior of Mixtures of Nicotinic Acid with Dodecylbenzenesulfonic Acid

Abstract

Mesomorphically ordered structures and phase behavior of the mixtures of nicotinic acid (NICA) and dodecylbenzenesulfonic acid (DBSA) were investigated by X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and polarized optical microscopy (POM). The POM observations revealed that the NICA–DBSA mixtures spontaneously formed liquid crystalline phases, although both NICA and DBSA were not liquid crystalline molecules. The NICA–DBSA mixtures formed ordered lamellar structures in DBSA‐rich mixtures and hexagonal cylinder structure in NICA‐rich mixtures. The mesomorphically ordered structures and optical anisotropy were caused by hierarchical interactions in the NICA–DBSA mixtures. The phase diagram divided into five regions—optically isotropic disordered phase, optically isotropic lamellar phase, optically anisotropic lamellar phase, optically anisotropic cylinder phase, and crystalline solid phase—is drawn by summarizing the XRD and POM results.     

Mid-wavelength IR (MWIR) polarizers from glassy cholesteric liquid crystals

The investigation of glass-forming liquid crystalline materials for mid wave infrared polarization applications is driven by their low melt viscosity and ability to vitrify order, and thus functionality, into films with a wide range of thicknesses. Commercially available polarizers that function in the mid-wave infrared region suffer from poor polarization contrast, high cost, and limited size. In this work, we explore the feasibility of using cyclic siloxanebased liquid crystalline materials with chiral mesogens to form circular polarizers in the midwave infrared spectrum (3-5mum). Specifically, we have designed a cholesteric molecular blend to possess the proper helical twisting power to exhibit a selective reflection notch in the 4.0-4.5mum region. We have fabricated circular polarizers using shear under a number of processing conditions, and explored their performance as measured by polarization contrast. Processed films with reflection notches at the proper wavelength and near theoretical reflection performance have been prepared. A proper balance between film thickness and cooling rate has led to the demonstration of optically transparent and uniform films on 1 inch diameter substrates. The use of simple alignment layers was demonstrated to yield consistent formation of Grandjean monodomains, by reducing the tendency of the large pitch blends to spontaneously form a fingerprint molecular orientation observed in cells with untreated surfaces. The measured polarization contrast of 70:1 exceeds the values obtained from state-of-the art commercial polarizers in this wavelength region.     

A rheological,optical, and x-ray study of the relaxation of orientation of nematic PBZT

The ultimate properties of liquid crystalline polymers depend upon the achievement of high orientation, usually by means of flow fields. The properties are limited by disorientation which can occur before the product is solidified. Such cooperative orientation and disorientation phenomena also underlie the complex fluid rheology and product microstructure of these materials. The orientation and subsequent disorientation can be followed dynamically by optical and x-ray techniques. Normally, monitoring of orientation is possible only by “fast” techniqes, such as birefringence, these are not applicable to opaque and strongly scattering liquid-crystalline systems. To enable examination of the full dynamic response of concentrated nematic solutions of poly(1,4-phenylene-2,6 benzobisthiazole) (PBZT) the Daresbury Synchrotron Radiation Source was used. PBZT is among the most rigid macromolecules and serves as a good model for other materials of its class. The orientation process determined optically and from x-rays is correlated with fluid rheology and availabel theoretical approaches. During relaxation from near perfect mesogen alignment three principal stages of the disorientation process were identified corresponding to solvent disorientation (first stage of stress relaxation), banding (slow stress relaxation and mesogen disorientation), and finally a very slow banding to polydomain transition. © 1993 John Wiley & Sons, Inc.     

Dielectric and visco-elastic properties of laterally fluorinated liquid crystal single compounds and their mixture

The physical properties of three laterally fluorinated liquid crystalline compounds with negative dielectric anisotropy have been studied from static dielectric permittivity, optical birefringence, bend elastic constant, relaxation time and rotational viscosity measurements. Such negative dielectric anisotropy materials find use as components of mixtures for application in vertically aligned mode liquid crystal displays. Moreover, the physical properties of one phenyl cyclohexane compound with positive dielectric anisotropy have also been studied. A five-component mixture comprising these four mesogens and a non-mesogenic component has been formulated and its physical properties have been thoroughly investigated. An attempt has been made to strike a balance between the optical birefringence of the mixture to adjust the cell gap and the dielectric anisotropy and threshold voltage (V_th ) to ensure low driving voltages. The pretilt angle effect on the threshold voltage and the relaxation time has also been studied. At T?=?20°C, the response time decreases to 22% and 41% for the mixture for 2° and 5° pretilt as compared to zero pretilt. On the other hand, at the same temperature the V_th values are decreased by 5% and 9%, respectively.     

Thermal switching of the optical anisotropy of a macroscopically aligned film of a discotic liquid crystal

We have studied the temperature dependence of anisotropy in the optical absorption and charge transport properties of an aligned film of hexakis-dodecyl-hexa-peri-hexabenzocoronene (HBC-C12) formed by zone-casting on a quartz substrate. At room temperature the film displays a large anisotropy in (photo)conductivity, as determined using the flash photolysis time-resolved microwave conductivity technique, with charge transport in the casting direction favoured by a factor of at least 10. The anisotropy in the optical absorption is however negligible. At the temperature corresponding to the transition from the crystalline solid to the liquid crystalline mesophase (c. 110°C), the optical anisotropy increases abruptly, with absorption of light polarized in the direction perpendicular to the alignment direction favoured by a factor of c. 3. On cooling, the dichroism reverts to its initial very low value with a hysteresis of c. 30°C. The results are explained in terms of a reversible change in the orientation of the molecules with respect to the axis of the aligned columnar stacks from tilted (at c. 45°) in the crystalline phase to close to orthogonal in the liquid crystalline phase.     

非线性光学活性的侧链型聚丙二酸酯液晶的合成和性质

通过熔融缩聚法合成了含非线性光学活性硝基偶氮苯液晶基元的聚丙二酸酯侧链液晶聚合物 ,采用FTIR、NMR对其结构进行了表征 ,DSC确定其液晶转变温度 ,并用变温WAXD及偏光显微镜 (POM )研究其液晶性质 ,该聚合物为近晶型液晶聚合物 ,POM观察到典型焦锥织构 ,近晶相WAXD有对应层间距为2 74nm和 1 35nm的两个衍射峰 .聚合物旋涂膜经电晕极化 ,紫外光谱测试求得其取向序参数 =0 33 .     

Liquid crystalline thermosets based on anisotropic phases of cellulose nanocrystals

A new class of liquid crystalline thermosets (LCTs) was successfully produced containing lyotropic cellulose nanocrystals (CNCs) as the primary mesogenic component (up to 72 wt%) by the addition of non-mesogenic epoxy monomers. Cellulose-based LCTs were produced by totally aqueous processing methods and ultimately cured at elevated temperatures to produce ordered networks of ‘frozen’ liquid crystalline (LC) phases. Various degrees of birefringence were obtained via self-assembly of CNCs into oriented phases as observed by polarized optical microscopy and transmission electron microscopy. X-ray diffraction measurements highlighted the effects of texture of CNCs within LCT films compared to lyophilized CNCs. Cellulose-based LCT films uniquely exhibited thermo-mechanical properties of both traditional LCTs and LC elastomers, such as high elastic modulus (~1 GPa) under ambient conditions and low glass transition temperature (~?25 °C), respectively. The development of LCTs based on CNCs and aqueous processing methods provides a renewable pathway for designing high performance composites with ordered network structures and unique optical properties.