Liquid Crystalline Films Research Articles

Relaxation processes in dipolar liquid crystal film on metals surfaces

The influence of electron injection from the aluminium electrode into organic liquid crystalline film, comprising the polar molecules, such as 4- n-pentyl-4′-cyanobiphenyl, on the orientational relaxation process is investigated. The numerical study of the system of hydrodynamical equations that includes both director reorientation and fluid flow, excited by that reorientation, for number of dynamic regimes has been carried out. It is shown that the relaxation process of the director n ˆ to its equilibrium position n ˆ eq is sensitive to electron injection from the metal electrode into organic dipolar film, whereas, the effect of backflow on that process is vanishingly weak.

Circular Differential Scattering of Light in Films of Chiral Polyfluorene

Circular differential transmission in thick films (1.1 mum) of poly{9,9-bis[(3S)-3,7-dimethyloctyl]-2,7-fluorene} is investigated. The vitrified liquid crystalline film obtained after annealing shows high circular differential transmission of light in the wavelength range where the polyfluorene does not absorb (lambda > 450 nm). Using a specifically designed chiroptical setup, we show that circular selective reflection of light in which the circular polarization of the light is retained after reflection, a process which is characteristic for cholesteric liquid crystalline films, makes a negligible contribution to the differential transmission. Using an integrating sphere, we show that circular differential scattering can account for the observed circular differential transmission for lambda > 450 nm.

Photoinduced orientation of liquid crystalline copolymer films containing 4-oxybenzoic acid side groups and photo-cross-linkable mesogenic side groups

The cooperative molecular reorientation in methacrylate copolymer films with hexamethylene spacer groups terminated with 4-oxybenzoic acid (BA) and 4-(4-methoxycinnamoyloxy)biphenyl (MCB) in their side chains was investigated by irradiating with linearly polarized ultraviolet light (LPUV) and subsequent annealing. A high degree of cooperative in-plane reorientation of both the BA and MCB groups was obtained when the composition of the BA groups was greater than 50 mol% and hydrogen (H)-bonded LC mesogenic dimers of BA molecules existed. On the other hand, the molecular reorientation was restricted when the BA groups did not form H-bonds. It was clarified that the amount of axis-selectively photoreacted MCB groups and the H-bonds of the BA groups that exhibit a LC nature play important roles in the thermally enhanced molecular reorientation.

Photoorientation of Liquid Crystalline Azo-Dendrimer by Nanosecond Pulsed Laser for Liquid Crystal Alignment

Upon irradiation with nanosecond pulsed UV laser, surface periodic microstructure and photoorientation occurred simultaneously on the azobenzene functionalized liquid crystalline dendrimer films. The orientation of azo groups was perpendicular to the polarization of pulsed laser and dependent on the laser fluence. The in-plane orientation of azo groups presented parabola-shaped behavior with the increase of laser fluences. The out-of-plane reorientation of azo groups was closely related to thermal effect induced in the procedure of nanosecond pulsed irradiation, which led to the transformation of phase state of azo groups from smectic liquid crystal phase to isotropic phase. Annealing of irradiated film at its liquid crystalline temperature resulted in dramatic enhancement of in-plane orientation and out-of-plane redistribution of azo groups. Liquid crystal alignment on the irradiated film was controlled by a cooperative effect of surface topography and anisotropic molecular orientation simultaneously. Bo...

Synthesis and Structural Characterization of a Highly Ordered Mesoporous Pt−Ru Alloy via “Evaporation-Mediated Direct Templating”

Highly ordered 2D-hexagonal mesoporous Pt−Ru alloy particles were deposited by an electrochemical process after a lyotropic liquid crystalline (LLC) film including both Pt and Ru species was prepared on a conductive substrate from a diluted surfactant solution by casting. We have determined the alloy state by using high-resolution scanning electron microscopy (HR-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopic (EDS) mapping, and X-ray photoelectron spectroscopy (XPS). The HR-SEM images show that the porous nanostructures are formed over the entire area on the external surface of each particle. The TEM images prove that the pore wall is composed of connected nanoparticles of ca. 3 nm in size and that the lattice fringes assigned to a fcc structure are randomly oriented. The EDS mapping shows both elements of Pt and Ru are well distributed within the pore wall. From the XPS data, Ru atoms in the alloy are thought to basically be in the zerovalent metallic state, Ru(0). Th...

Synthesis of Novel Glass-Forming Liquid Crystals Containing Acrylic Acid Trimer Core Unit and Mesogenic Moiety, and Their Use in Cholesteric Reflection Films

A novel medium molecular mass liquid-crystalline (LC) material to form glassy nematic phase has been developed to provide uniformly aligned LC films. The LC material consists of an acrylic acid trimer as a core structure and a mesogenic moiety. The resulting LC trimer showed a nematic LC phase and vitrified to form monodomain films at room temperature. This glassy LC trimer is miscible with a chiral photochromic dopant, providing monodomain, aligned, defect-free cholesteric reflection films, the color of which is tunable by UV irradiation.

Control of emission outcoupling in liquid-crystalline fluorescent polymer films

We report the improvement of light outcoupling efficiency of thermally annealed thin liquid crystalline light-emitting polymer films. Unlike the generally observed phenomena from thermally annealed fluorescent polymer films, in the liquid-crystalline fluorescent polymer (LCFP) films, the transverse electric waveguide mode was suppressed due to the light scattering by liquid-crystalline domains in the film. In addition, the optical anisotropy of the annealed LCFP was increased. As a result, the light extraction of photoluminescence and electroluminescence emission in the annealed LCFPs was enhanced by ∼2 times because the emitted light loss by waveguide was minimized. This approach to improve light extraction by forming scattering domains inside the LCFP film gives an insight to improve the outcoupling efficiency of the EL devices in the viewpoint of the material design and film morphologies without employing external geometric device structures.

Preparation of novel liquid crystalline polymeric film material and its photoinduced alignment behavior

Two kinds of novel oxetane monomers, containing biphenyl, flexible spacer and cinnamoyl group, were synthesized. The liquid crystal phase behavior of the corresponding polymers was investigated and isotropic–nematic, nematic–smectic and smectic–crystal transitions were identified during cooling. Both polymers exhibited clear fan texture morphology under a polarized optical microscope. Irradiation of a polymer film with linearly polarized UV light led to cycloaddition of the cinnamoyl groups, causing simultaneous alignment of the liquid crystal unit.

Orientational order and mechanical properties of poly(amide- block-aramid) alternating block copolymer films and fibers

Mechanical properties and orientational order of a series of uniaxially oriented block copolymer films and fibers comprised of alternating rigid aramid blocks of poly( p-phenylene terephthalamide) (PPTA) and flexible blocks of polyamide 6,6 (PA 6,6) have been investigated. The prepared block copolymer films differ in aramid content and average block length. The films were prepared by shearing the polymer solutions (in sulphuric acid) followed by rapid coagulation of the solutions in water. From wide angle X-ray scattering (WAXS) and optical polarisation microscopy (OPM) it was found that films with a mole fraction of PPTA of at least 0.5 show a liquid crystalline (LC) phase. It was found that the mechanical properties of LC block copolymer films are similar to the properties of isotropic films, as determined with dynamical mechanical analyses (DMAs) and from tensile tests. This was attributed to the relative low 〈 P 2 〉 ¯ parameter of the LC films obtained by using WAXS. Copolymerisation of the PPTA blocks with the flexible polyamide blocks resulted in an increase of storage and Young's modulus, a decrease of the elongation at break while the tensile strength was unaffected compared to normal PA 6,6. Block copolymer fibers have been spun from liquid crystalline solutions by means of a dry-jet wet spinning process. The only variable parameter was the imposed draw-ratio in the air-gap of the spinning process. Increasing the draw-ratio resulted in an increased molecular orientation, Young's modulus and tensile strength of the fibers while its effect on the maximum elongation at break was small. Heat treatment at 300 °C of the fibers resulted in an increase of the Young's modulus, a minor increase of the strength and a decrease of the elongation at break. Scanning electron microscopic (SEM) photographs of the fractured surfaces of the block copolymer fibers do not show a fibrillar fracture surface, which is typically observed for pure PPTA fibers.

Development of Liquid Crystalline Polymer Film “Nisseki LC Film” for Viewing Angle Compensation of Various LCD Modes

To improve quality of images of LCDs such as their viewing angle performance and coloration, liquid crystalline retardation film technology has been developed by using rod-like liquid crystalline polymer. The resulting liquid crystalline retardation film has several advantages over conventional uni- or biaxial stretched retardation film. By using liquid crystalline polymer, the direction of the principal axis can be determined freely for roll-to-roll lamination. Optical well-controlled structures such as twisted nematic, hybrid nematic and homeotropic structures can be stabilized for ideal compensation of various LCD modes including STN, TN, ECB, VA and IPS LCDs.

Surface Relief Grating Couplers Using Azobenzene Liquid-Crystalline Polymer Films

Input and output grating couplers with a grating period of 1.3 µm were fabricated in an azobenzene liquid-crystalline polymer film by holography. When a He-Ne laser beam at 633 nm was incident to the input grating, the beam was coupled, propagated in the polymer film, and then was taken out from the output grating. The coupling efficiency of the input and output coupler was estimated as about 1.1% in TE mode. Upon irradiation of the film with UV light to cause trans-cis photoisomerization of the azobenzene moiety, the intensity of the output beam was repeatedly changed.

Rotational Reorganization of Doped Cholesteric Liquid Crystalline Films

In this paper an unprecedented rotational reorganization of cholesteric liquid crystalline films is described. This rotational reorganization results from the conversion of a chiral molecular motor dopant to an isomer with a different helical twisting power, leading to a change in the cholesteric pitch. The direction of this reorganization is correlated to the sign of the change in helical twisting power of the dopant. The rotational reorganization of the liquid crystalline film was used to rotate microscopic objects 4 orders of magnitude larger than the bistable dopants in the film, which shows that molecular motors and switches can perform work. The surface of the doped cholesteric liquid crystalline films was found to possess a regular surface relief, whose periodicity coincides with typical cholesteric polygonal line textures. These surface features originate from the cholesteric superstructure in the liquid crystalline film, which in turn is the result of the presence of the chiral dopant. As such, the presence of the dopant is expressed in these distinct surface structures. A possible mechanism at the origin of the rotational reorganization of liquid crystalline films and the cholesteric surface relief is discussed.

Orientational properties of 1,3,4-oxadiazoles in liquid-crystalline materials determined by electronic absorption and fluorescence measurements

The electronic absorption and fluorescence spectra of two 4-donor/4′-acceptor substituted 2,5-diphenyl-1,3,4-oxadiazole derivatives in isotropic solvents (methanol, toluene), in a commercial mixture of low molecular mass liquid crystals – ZLI 1695 (MERCK, Darmstadt), and in a liquid-crystalline polysiloxane with 4-methoxy-4′-pentoxy-phenylbenzoate side-groups are presented and discussed. The orientational order parameters of the 1,3,4-oxadiazoles, dissolved in the liquid-crystalline films with uniform planar orientation of the mesogenic molecules and in aligned films of the polysiloxane, are determined. The possibility of application in OLEDs of the 1,3,4-oxadiazoles dissolved in homogenously oriented liquid-crystalline polymers is suggested.

Reversible Full‐Range Color Control of a Cholesteric Liquid‐Crystalline Film by using a Molecular Motor

By using a chiral molecular motor as a dopant in a cholesteric liquid-crystalline film, fully reversible control of the reflection color of this film across the entire visible spectrum is possible. The large difference in helical twisting power between the two isomeric forms of the motor allows efficient light- and heat-induced switching of the helicity of the cholesteric liquid-crystal superstructure.

Spatial Solitons in Twisted Nematic Layer

In this work we investigate light beam propagation in twisted nematic liquid crystalline film. Due to the optical reorientational nonlinearity light beam is self-focusing and finally the spatial solitary wave is created. With increasing the nonlinear effect the direction of light beam propagation is also changing. The samples were filled with 6CHBT nematic liquid crystal and the propagation of light beam at the distance of few millimeters was measured. Nonlinear self-focusing was obtained for a light power of few tenths of milliwats.

Effects of Temperature on the Alignment and Electrooptical Responses of a Nematic Nanoscale Liquid Crystalline Film

The surface-induced alignment and electrooptical (EO) dynamics of a 50-nm-thick liquid crystalline (4-n-pentyl-4'-cyanobiphenyl; 5CB) film were studied at three temperatures: 25 and 33 degrees C (near the crystalline-nematic and nematic-isotropic transition temperatures, respectively) and 29 degrees C (a median temperature in the stability region of the nematic phase). The ZnSe surfaces that entrap the liquid crystal (LC) film have been polished unidirectionally to produce a grooved surface presenting nanometer-scale corrugations, a structure that induces a planar and homogeneous orientation in the nematic phase. The present work attempts to understand the influences of temperature on the surface-induced alignment and corresponding EO dynamics of the material. 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 film. The field-driven orientation rates vary sensitively with temperature across a range that spans the stability limits of the nematic phase; the relaxation rates, however, vary very little across this same temperature range. We propose that these differences in LC behavior arise as consequence of the interplay of the temperature dependence of the elastic constants, viscosity, and degree of orientational order of the LC medium. A simple theoretical model provides some understanding of these behaviors.

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.

Correlation of liquid crystal alignments on gratings with the thickness of liquid crystal films

With scanning electron microscopy, atomic force microscopy and polarizing optical microscopy, we have investigated the correlation of the alignment properties of a nematic liquid crystal (LC) 4-pentyl-4′-cyanobiphenyl with the thickness of LC films on holographic gratings. The spatial period and the height of the embossed ridges of the gratings are about 0.7 and 0.2 μm, respectively. Our results indicate that the LC alignment properties depend on the thickness, d, of the LC film on the gratings: (1) uniform LC alignments are achieved when 10<d<30 μm; (2) partially uniform LC alignments are recorded when 1<d<10 μm; and (3) non-uniform LC alignments are resulted when d<1 μm or d>30 μm. The liquid crystalline film thickness dependent alignment properties can be explained in terms of the evolution of order–parameter as a function of the thickness of LC films.

Patterned retarders prepared by photoisomerization and photopolymerization of liquid crystalline films

Isomerizable diacrylates derived from cinnamic acid are designed, synthesized and mixed with liquid crystalline diacrylates with the aim of making films with alternating birefringent and isotropic domains by applying the E–Z isomerization process at room temperature. The effects of the structure of the isomerizable‐mesogenic group on the isotropization efficacy, the efficiency of the E–Z isomerization reaction, and film formation are discussed. Compounds derived from cyclohexyl cinnamate are proved to be good candidates that meet a whole set of parameters related to processing and application. These compounds exhibit a low nematic‐to‐isotropic transition temperature. In addition, they show no yellowing upon irradiation, unlike similar compounds derived from phenyl cinnamate. To elucidate the origin of isotropization of the film by irradiation, the pure Z‐isomer is prepared by photolysis of the E‐isomer and subsequent chromatographic separation of both isomers. Analysis of reference samples containing the pure isomers reveals that the decrease in transition temperature can be attributed exclusively to the E–Z photoisomerization process. Finally, thin films with alternating birefringent and isotropic parts of 100×100 µm2 are obtained by using a combination of photoisomerization in air and photopolymerization in a nitrogen atmosphere, which is referred to as photo‐patterning.

First approach to the use of liquid crystal elastomers for chemical sensors

Liquid crystalline thin films elastomers that are able to bind pesticides have been developed. The synthesis involves grafting mesogen and crosslinkable groups on a polysiloxane chain in the presence of a template molecule. The molecular imprinted material is obtained after thin film deposition, UV crosslinking and washing. Experiments of readsorption of pesticide are presented. Development of a multisensor platform based on thermal and capacitive sensors is described and tests of deposition of the polymer film are presented.