Liquid Crystalline Films Research Articles

Locomotion and transport in a hexatic liquid crystal.

The swimming behavior of bacteria and other microorganisms is sensitive to the physical properties of the fluid in which they swim. Mucus, biofilms, and artificial liquid-crystalline solutions are all examples of fluids with some degree of anisotropy that are also commonly encountered by bacteria. In this article, we study how liquid-crystalline order affects the swimming behavior of a model swimmer. The swimmer is a one-dimensional version of G. I. Taylor's swimming sheet: an infinite line undulating with small-amplitude transverse or longitudinal traveling waves. The fluid is a two-dimensional hexatic liquid-crystalline film. We calculate the power dissipated, swimming speed, and flux of fluid entrained as a function of the swimmer's wave form as well as properties of the hexatic film, such as the rotational and shear viscosity, the Frank elastic constant, and the anchoring strength. The departure from isotropic behavior is greatest for large rotational viscosity and weak anchoring boundary conditions on the orientational order at the swimmer surface. We even find that if the rotational viscosity is large enough, the transverse-wave swimmer moves in the opposite direction relative to a swimmer in an isotropic fluid.

Fluctuation-induced interactions in nematics with disordered anchoring energy

We examine fluctuation-induced (pseudo-Casimir) interactions in nematic liquid-crystalline films confined between two surfaces, where one of the surfaces imposes a strong homeotropic anchoring (ensuring a uniform mean director profile), while the other one is assumed to be a chemically disordered substrate exhibiting an annealed distribution of anchoring energies. We employ a saddle-point approximation to evaluate the free energy of interaction mediated between the two surfaces and investigate how the interaction force is influenced by the presence of disordered surface anchoring energy. It is shown that the disorder results in a renormalization of the effective surface anchoring parameter in a way that it leads to quantitative and qualitative changes (including a change of sign at intermediate inter-surface separations) in the pseudo-Casimir interaction force when compared with the interaction force in the absence of disorder.

Molten Salt Assisted Self Assembly (MASA): Synthesis of Mesoporous Metal Titanate (CoTiO3, MnTiO3, and Li4Ti5O12) Thin Films and Monoliths

Mesoporous metal titanates are very important class of materials for clean energy applications, specifically transition metal titanates and lithium titanates. The molten salt assisted self-assembly (MASA) process offers a new synthetic route to produce mesoporous metal titanate thin films. The process is conducted as follows: first a clear solution that contains two solvents (namely the hydrated salt (Co(NO3)2·6H2O or Mn(NO3)2·6H2O, or LiNO3·xH2O, and ethanol), two surfactants (cethyltrimethylammonium bromide, CTAB, and 10-lauryl ether, C12EO10), an acid and titanium source (titanium tetrabutoxide, TTB) is prepared and then spin or spray coated over a substrate to form a thin or thick lyotropic liquid crystalline (LLC) film, respectively. Finally, the films are converted into transparent spongy mesoporous metal titanates by a fast calcination step. Three mesoporous metal titanates (namely, CoTiO3, MnTiO3, and Li4Ti5O12) have been successfully synthesized and structurally/thermally characterized using microscopy, spectroscopy, diffraction, and thermal techniques. The mesoporous cobalt and manganese titanates are stable up to 500 °C and collapse at around 550 °C into nanocrystalline Co3O4–TiO2 and Mn2O3–TiO2; however, lithium titanate is stable up to 550 °C and crystalline even at 350 °C. The crystallinity and pore size of these titanates can be adjusted by simply controlling the annealing and/or calcination temperatures.

Single step micro-patterned liquid crystal photoalignment by patterned quarter-wave plate

In this article, a method for fabricating the patterned liquid crystal photoalignment structures, in a single step, is proposed. A patterned quarter-wave plate formed by a photoaligned liquid crystalline polymer film is used as a photomask to generate photoaligned micro-patterns. Whereas other existing alternatives include complicated fabrication scheme, the proposed method provides an edge with simple one-step alignment. Moreover, the proposed method offers highly accurate, high resolution, multi micro-pattern alignment with great repeatability and therefore could find application in wide range of photonic and display devices demanding the micro-patterned, single of multi domain, alignment. In addition, the structure could be used for optical polarization information storage and patterned alignment structure storage, which could easily be reproduced.

Factors affecting photoisomerization of 4-styrylpyridine

Photoisomerization of 4-styrylpyridine at different irradiation wavelengths (λ 365 and 254 nm) and in different solvents was studied by UV and 1H NMR spectroscopy. A procedure for recording the UV spectra of 4-styrylpyridine dissolved in liquid crystalline films freely suspended within a frame. The results of quantum-chemical calculations of the geometry and UV spectra of 4-styrylpyridine isomers are presented.

Hierarchical thin film architectures for enhanced sensor performance: liquid crystal-mediated electrochemical synthesis of nanostructured imprinted polymer films for the selective recognition of bupivacaine.

Nanostructured bupivacaine-selective molecularly imprinted 3-aminophenylboronic acid-p-phenylenediamine co-polymer (MIP) films have been prepared on gold-coated quartz (Au/quartz) resonators by electrochemical synthesis under cyclic voltammetric conditions in a liquid crystalline (LC) medium (triton X-100/water). Films prepared in water and in the absence of template were used for control studies. Infrared spectroscopic studies demonstrated comparable chemical compositions for LC and control polymer films. SEM studies revealed that the topologies of the molecularly imprinted polymer films prepared in the LC medium (LC-MIP) exhibit discernible 40 nm thick nano-fiber structures, quite unlike the polymers prepared in the absence of the LC-phase. The sensitivity of the LC-MIP in a quartz crystal microbalance (QCM) sensor platform was 67.6 ± 4.9 Hz/mM under flow injection analysis (FIA) conditions, which was ≈250% higher than for the sensor prepared using the aqueous medium. Detection was possible at 100 nM (30 ng/mL), and discrimination of bupivacaine from closely related structural analogs was readily achieved as reflected in the corresponding stability constants of the MIP-analyte complexes. The facile fabrication and significant enhancement in sensor sensitivity together highlight the potential of this LC-based imprinting strategy for fabrication of polymeric materials with hierarchical architectures, in particular for use in surface-dependent application areas, e.g., biomaterials or sensing.

Molecular Orientation at the Near-Surface of Photoaligned Films Determined by NEXAFS

Because of the potential for use in display applications, photoalignment using photosensitive materials has received considerable attention. Herein, the influence of film thickness on thermally stimulated photoinduced molecular reorientation structures of photoreactive liquid crystalline polymer films at the near-surface and in bulk is investigated using near-edge X-ray absorption fine structure (NEXAFS) and polarization UV absorption spectroscopies, respectively. In films less than 90-nm thick, the uniaxial in-plane orientation order in bulk gradually decreases. In contrast, films thicker than 40 nm show a sufficient molecular orientation at the near-surface. The three-dimensional orientation structures are investigated using obliquely reoriented films fabricated by slantwise p-polarized UV light exposure. Films thicker than 40 nm display an effective slantwise orientation in bulk, but the slantwise angle at the near-surface is much smaller than that in bulk. Both the substrate–film interface and film th...

Free-surface molecular command systems for photoalignment of liquid crystalline materials

The orientation of liquid crystal molecules is very sensitive towards contacting surfaces, and this phenomenon is critical during the fabrication of liquid crystal display panels, as well as optical and memory devices. To date, research has focused on designing and modifying solid surfaces. Here we report an approach to control the orientation of liquid crystals from the free (air) surface side: a skin layer at the free surface was prepared using a non-photoresponsive liquid crystalline polymer film by surface segregation or inkjet printing an azobenzene-containing liquid crystalline block copolymer. Both planar-planar and homoeotropic-planar mode patterns were readily generated. This strategy is applicable to various substrate systems, including inorganic substrates and flexible polymer films. These versatile processes require no modification of the substrate surface and are therefore expected to provide new opportunities for the fabrication of optical and mechanical devices based on liquid crystal alignment.

Effects of 100 MeV O7+ ion beam irradiation on the optical, chemical and structural properties of NCO-terminated polybutadiene based liquid crystalline polyurethane

NCO-terminated polybutadiene based liquid crystalline polyurethane films were irradiated with O7+ ion beam at various fluences ranging 3 × 1010 to 1 × 1012 ions/cm2. Significant modifications in the structural, optical and chemical properties have been observed after irradiation. UV-visible spectra show a shift toward the higher wavelength regime after irradiation owing to the decrease in the band gap. FTIR spectra show the formation of NH2 group and CC bonds. The XRD spectra show the semi-crystalline nature of LCPU and with increasing fluence of irradiation a decrease in crystallinity is observed. However an increase in crystallinity at certain fluence followed by a decrease at the higher fluence of irradiation reveals the presence of secondary radiation induced crystallinity in LCPU.

Pseudo-Casimir interactions across nematic films with disordered anchoring axis

We study the effective pseudo-Casimir interaction forces mediated by a nematic liquid-crystalline film bounded by two planar surfaces, one of which imposes a random (disordered) distribution of the preferred anchoring axis in the so-called easy direction. We consider both the case of a quenched as well as an annealed disorder for the easy direction on the disordered surface and analyze the resultant fluctuation-induced interaction between the surfaces. In the case of quenched disorder, we show that the disorder effects appear additively in the total interaction and are dominant at intermediate inter-surface separations. Disorder effects are shown to be unimportant at both very small and very large separations. In the case of annealed disorder its effects are non-additive in the total inter-surface interaction and can be rationalized in terms of a renormalized extrapolation length.

Highly birefringent side-chain LC polymethacrylate with a dinaphthyl-acetylene mesogenic unit

We synthesised a highly birefringent liquid crystalline (LC) side-chain polymer based on polymethacrylate with a dinaphthyl-acetylene mesogenic unit in the side chain. Three polymer types with different carbon numbers in the terminal alkoxy chain (m = 1, 3 and 6) were prepared, and their optical properties as well as phase sequential behaviours were investigated. All polymers formed a nematic phase with a wide temperature range, and a smectic C phase was detected between the nematic and crystal phases when m was extended to 6. Notably, for homologous m = 1, we successfully obtained a nematic glassy-state LC film with high optical performance under ambient temperature conditions. The optical properties of this LC film were shown to be consistent with those of a highly birefringent, low molar mass LC material, with birefringence, extraordinary and ordinary refractive indices of 0.36, 1.82 and 1.46, respectively at 550 nm.

Depth-selective microscopic observation of a photomobile liquid crystal polymer under UV illumination.

By using the depth selective imaging method, we studied the UV induced change in a photomobile liquid crystalline polymer film. With 1 μm depth resolution, each slice inside the film was selectively observed. A network-like structure mixed with the ordered and disordered regions of molecules in the middle of the film, and a rubbed polymer layer at the bottom of the film were observed. In each slice of the film, the phase change induced by UV light was observed strongly dependent on the director direction, which indicates the ordering change of the liquid crystalline molecules in the director direction. It took several tens of seconds for the ordering change caused by the collaborative interaction between the molecules. Furthermore, it was suggested that the UV induced change travelled from the bottom layer to the middle layer on the micron order.

Labyrinthine instability in freely suspended films of a polarization-modulated smectic phase

We report on fingering and labyrinthine instabilities of the layer dislocation lines in freely suspended polar liquid-crystalline films. These polar fingerlike and labyrinth structures reversibly form upon a transition into a modulated phase. External electric fields of several kV/m applied in the film plane can reversibly influence the formation of the finger textures. We show that the labyrinthine pattern is intrinsically related to regular splay deformations of the polarization.

Formation and properties of liquid crystalline supramolecules with anisotropic fluorescence emission

Two kinds of liquid crystalline (LC) supramolecules with anisotropic fluorescence emission were prepared by ionic self-assembly from a copolymer, derived from acrylic acid and 6-(4′-cyanobiphenyl-4-yloxy)hexyl acrylate, and oppositely charged fluorescent molecules: stilbene-based rod-like dye (Stil) or tetraphenylethene-based propeller-like dye (TPE). The supramolecules exhibited a nematic LC nature and the different structure of the fluorescent units showed different effect on the LC behaviour of the supramolecules. After being spin-coated onto an oriented PVA film, the liquid crystalline supramolecule film was thermally annealed and the anisotropic fluorescence emission was investigated. A different thermal amplification effect was found depending on the different structure of the fluorescent unit. This conclusion provides a new perspective on the design of a high-performance fluorescence polarization film.

Accordion‐like Actuators of Multiple 3D Patterned Liquid Crystal Polymer Films

This work describes the fabrication, characterization, and modelling of liquid crystalline polymer network films with a multiple patterned 3D nematic director profile, a stimuli‐responsive material that exhibits complex mechanical actuation under change of temperature or pH. These films have a discrete alternating striped or checkerboard director profile in the plane, and a 90‐degree twist through the depth of the film. When actuated via heating, the striped films deform into accordion‐like folds, while the film patterned with a checkerboard microstructure buckles out‐of‐plane. Furthermore, striped films are fabricated so that they also deform into an accordion shaped fold, by a change of pH in an aqueous environment. Three‐dimensional finite element simulations and elasticity analysis provide insight into the dependence of shape evolution on director microstructure and the sample's aspect ratio.

Thermotropic, side-chain ordered polymeric coatings: gas permeability switching via a thermal stimulus

Rapid access to large areas of stimuli-responsive materials is attractive in the field of membrane science for purification systems and molecular valves. Thermotropic liquid crystalline (LC) systems show sharp property changes through the smectic–LC transition induced by temperature. Comparison of similar amorphous and liquid crystalline systems allows for the elucidation of the characteristics of the LC phase. Lightly crosslinked C6F13 and C8F17 perfluorinated side-chain acrylate networks were UV cured as thin films resulting in amorphous and thermotropic liquid crystalline thin films, respectively. Thermal and morphological characterization indicates a restructuring of the liquid crystalline phase through the isotropic transition giving rise to an increase in transport and free volume properties. The amorphous film showed no dramatic change in transport or free volume properties.

Influence of molecular weight on the photoinduced reorientation of photo-cross-linkable liquid crystalline polymeric films

Influence of the molecular weight on the thermally enhanced photoinduced molecular reorientation of photo-cross-linkable liquid crystalline polymer films was investigated, where axis-selectively photoreacted side groups controlled the reorientation. Sufficient molecular reorientation (S>0.7) was achieved when the number-average molecular weight (Mn) was >1.8 × 104, but reorientation ability was inferior when Mn was less than 1.0 × 104. The efficiency of the molecular reorientation depended on both degree of the photoreaction and the molecular weight of the material, which is related to the insolubility of the photoreacted film.

Molecular dynamics study of ionic liquid film based on [emim][Tf2N] and [emim][TfO] adsorbed on highly oriented pyrolytic graphite

Molecular dynamics simulation was used to study the ionic liquid(IL) crystalline film based on 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide([emim][Tf2N]) and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate([emim][TfO]) on the graphite surface. Our results show that the cations are parallelly distributed to the surface in the 1/2 monolayer(ML) crystalline film. The [Tf2N]− anions are parallel to the surface with the oxygen atoms at the bottom, whereas the [TfO]− anions are perpendicularly distributed to the surface also with the oxygen atoms at the bottom in the 1/2 ML crystalline film. It has been found that the IL-vapor interface strongly influences the arrangement of ions at the interface. The anions in the top layer with the oxygen atoms outmost turn over to make themselves with the F atoms outmost so as to form C—H…O hydrogen bonds with the cations. The calculated orientational ordering shows that in the outmost layer at the IL-vapor interface, the cation rings present either parallel or perpendicular to the surface at 350 K.

Influence of a Change in Helical Twisting Power of Photoresponsive Chiral Dopants on Rotational Manipulation of Micro‐Objects on the Surface of Chiral Nematic Liquid Crystalline Films

Herein we report a group of five planar chiral molecules as photon-mode chiral switches for the reversible control of the self-assembled superstructures of doped chiral nematic liquid crystals. The chiral switches are composed of an asymmetrically substituted aromatic moiety and a photoisomerizing azobenzene unit connected in a cyclic manner through methylene spacers of varying lengths. All the molecules show conformational restriction in the rotation of the asymmetrically substituted aromatic moiety in both the E and Z states of the azobenzene units resulting in planar chirality with separable enantiomers. Our newly synthesized compounds in pure enantiomeric form show high helical twisting power (HTP) in addition to an improved change in HTP between the E and Z states. The molecule with a diphenylnaphthalene unit shows the highest ever known initial helical twisting power among chiral dopants with planar chirality. In addition to the reversible tuning of reflection colors, we employed the enantiomers of these five compounds in combination with four nematic liquid crystalline hosts to study their properties as molecular machines; the change in HTP of the chiral dopant upon photoisomerization induces rotation of the texture of the liquid crystal surfaces. Importantly, this study has revealed a linear dependence of the ratio of the difference between HTPs before and after irradiation against the absolute value of the initial HTP, not the absolute value of the change in helical twisting power between two states, on the angle of rotation of micro-objects on chiral nematic liquid crystalline films. This study has also revealed that a change in irradiation intensity does not affect the maximum angle of rotation, but it does affect the speed of rotational reorganization of the cholesteric helix.

Electrical and NO2 sensing properties of liquid crystalline phthalocyanine thin films

Abstract In this study, the electrical and nitrogen dioxide (NO2) sensing properties of a series liquid-crystalline octakisalkylthiophthalocyanine (C6S)8PcM [M = 2H, Ni, Cu, Zn] thin film were investigated. The thin films were coated on to the Interdigital Transducer (IDT) with gold electrodes, using jet-spray technique in an inert ambient from a solution of phthalocyanines in chloroform. The electrical properties of liquid-crystalline phthalocyanines were studied within wide temperature range from 293 to 423 K under the high purity nitrogen (N2) flow and in the frequency range from 500 Hz to 1 MHz for ac electrical measurements. It was observed that the I–V curves of all phthalocyanines showed a significant hysteresis at low temperature and the maximum dc conductivity obtained for (C6S)8PcZn thin film. The ac conduction mechanism of all thin films could be explained with quantum-mechanical tunneling (QMT) model. The NO2 gas sensing measurements were done in the concentration range of 1–10 ppm at the temperatures of 100 °C and 150 °C. The maximum sensor response (%) was obtained as approximately 10,000 and the response time was observed to be a few seconds for (C6S)8PcH2 thin film at 150 °C.