Liquid Crystal 5CB Research Articles

Capillary Flows of Nematic Liquid Crystal

In this paper we report the new experimental results on the rise of a liquid crystal in flat capillaries with inner photosensitive surfaces. The capillaries with different surface orientations were prepared by the use of the photo-alignment technique. Such a surface treatment makes it possible to eliminate the noncontrollable influence of a nanorelief on the wetting process, which takes place in the rubbing treatment technique previously used in similar experiments. The dynamics of the capillary rise of a nematic liquid crystal 5CB (4-cyano-4′-pentylbiphenyl) in vertical plane capillaries with photo-aligned substrates were studied for the first time. It was found that the stationary value of a contact angle weakly depends on the direction of a planar surface orientation relative to the direction of a capillary rise. It has been shown that the application of strong electric fields resulted in a decreasing of the contact angle. The results, obtained for the nematic liquid crystal, are compared with the results of an investigation of the capillary flow in a shock-free ferroelectric smectic phase.

Intrinsically Visible Light-Responsive Liquid Crystalline Physical Gels Driven by a Halogen Bond.

Photoresponsive physical gels using liquid crystals (LCs) as solvents have attracted great interest owing to their potential applications. But, current investigations mainly focus on UV light, which is not environment-friendly enough. On the other hand, the halogen bond is a novel tool for constructing supramolecular gels because of good hydrophobicity, high directionality, tunable strength, and large size of halogen atoms. Herein, to construct an LC physical gel with both the advantages of a halogen bond and visible light response, azopyridine-containing Azopy-C10 is chosen as a halogen bond acceptor, while 1,2-bis(2,3,5,6-tetrafluoro-4-iodophenyl)diazene is selected both as the halogen bond donor and for the intrinsically visible light response. Such a binary gelator can self-assemble in the anisotropic solvent of nematic LC 5CB to form an LC physical gel. It experiences the gel-to-sol transition by green light irradiation. As the gelator concentration increases, the saturation voltage increases, but the switch-off time decreases. The combination of the halogen bond and controllable visible light-responsive LC physical gel provides the feasibilities of manipulating these smart soft materials.

Influence of planar anchoring on escaped-twisted configurations of liquid crystals in a cylindrical cavity

ABSTRACT Lyotropic chromonic liquid crystals (LCLCs) confined in cylindrical cavity can form an escaped-twisted (ET) configuration, resulting from the saddle-splay elastic constant is great much than twist elastic constant. The thermotropic liquid crystal 5CB does not meet the conditions for the formation of ET configuration, but it can be achieved under the appropriate planar anchoring and the action of chiral dopant. The equilibrium configuration of nematic liquid crystals confined in cylindrical cavities with azimuthal and axial surface anchoring conditions respectively is studied by the elastic continuum theory and the difference iterative method. We obtain the analytical solution of the distribution of the director without chiral dopant and the numerical solution of the chiral dopant. We find that the planar anchoring, the saddle-splay elastic constant and the chirality determine whether an ET configuration is formed. Furthermore, the cavity radius and the bend elastic constant can affect the value of twist angle. It provides a good theoretical basis for the determination of anchoring energy and saddle-splay elastic constant.

Effect of nature and degree of crosslinking agent of poly(hydroxy-butyl-methacrylate-co-2-ethyl-hexyl-acrylate) networks on the swelling properties in nematic liquid crystal 5CB

We experimentally measured the effect of nature and concentration of crosslinker on the photopolymerized time of the poly(hydroxy-butyl-methacrylate-co-2-ethyl-hexyl-acrylate)/5CB system. Initial mixtures are composed of monofunctional monomers hydroxy-butyl-methacrylate (HBMA) and 2-ethyl-hexyl-acrylate (2-EHA), and one of the three bifunctional monomers, poly-propylene-glycol-di-acrylate (PPGDA), tri-propylene-glycol-di-acrylate (TPGDA), or 1,6-hexane-diol-di-acrylate (HDDA), and 2-hydroxy-2-methylpropiophenone (Darocur 1173) as a photoinitiator. The copolymers were elaborated via UV irradiation of reactive formulation. The central composite face-centered design of experiments (DoE) has been used to determine the influence of temperature, crosslinking density and their interactions on swelling behavior of poly(HBMA-co-EHA/crosslinker) networks in liquid crystal 5CB. The experimental results and the predicted responses indicate a good correlation and therefore the validity of the used model.

Sustainable plant polyesters as substrates for optical gas sensors.

The fast and non-invasive detection of odors and volatile organic compounds (VOCs) by gas sensors and electronic noses is a growing field of interest, mostly due to a large scope of potential applications. Additional drivers for the expansion of the field include the development of alternative and sustainable sensing materials. The discovery that isolated cross-linked polymeric structures of suberin spontaneously self-assemble as a film inspired us to develop new sensing composite materials consisting of suberin and a liquid crystal (LC). Due to their stimuli-responsive and optically active nature, liquid crystals are interesting probes in gas sensing. Herein, we report the isolation and the chemical characterization of two suberin types (from cork and from potato peels) resorting to analyses of gas chromatography–mass spectrometry (GC-MS), solution nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS). The collected data highlighted their compositional and structural differences. Cork suberin showed a higher proportion of longer aliphatic constituents and is more esterified than potato suberin. Accordingly, when casted it formed films with larger surface irregularities and a higher C/O ratio. When either type of suberin was combined with the liquid crystal 5CB, the ensuing hybrid materials showed distinctive morphological and sensing properties towards a set of 12 VOCs (comprising heptane, hexane, chloroform, toluene, dichlormethane, diethylether, ethyl acetate, acetonitrile, acetone, ethanol, methanol, and acetic acid). The optical responses generated by the materials are reversible and reproducible, showing stability for 3 weeks. The individual VOC-sensing responses of the two hybrid materials are discussed taking as basis the chemistry of each suberin type. A support vector machines (SVM) algorithm based on the features of the optical responses was implemented to assess the VOC identification ability of the materials, revealing that the two distinct suberin-based sensors complement each other, since they selectively identify distinct VOCs or VOC groups. It is expected that such new environmentally-friendly gas sensing materials derived from natural diversity can be combined in arrays to enlarge selectivity and sensing capacity.

Phase transitions of liquid crystal confined in electrospun polymer nanofibres

Phase behaviour as well as phase transitions of $$4'$$ -pentyl-4-biphenylcarbonitrile (5CB) liquid crystal (LC) confined in an amorphous polymer matrix of electrospun nanofibres were investigated. The nanofibres were fabricated from simple monoaxial electrospinning using 5CB/polymer mixtures as well as coaxial electrospinning, where the polymer solution and neat 5CB constituted the shell- and core-forming fluids, respectively. The 5CB was found to be miscible with polystyrene (PS) and poly(4-vinyl pyridine) (P4VP). This was evident from the sharp drop in the glass transition temperature ( $$T_{\mathrm{g}}$$ ) of PS and P4VP in their mixtures with 5CB. Hence, the phase transitions of 5CB were completely suppressed in its mixture with PS and P4VP in electrospun nanofibres as ascertained from DSC and polarized optical microscopy measurements. However, the electrospun nanofibres composed of poly(vinyl pyrrolidone) (PVP) and 5CB showed phase-separated morphology. The phase-separated morphology was unambiguously characterized using SEM and TEM measurements. Furthermore, the phase separation resulted in 5CB exhibiting its liquid crystalline characteristics. However, the radial constraint of the nanofibres led to the formation of small-sized 5CB domains with limited spatial connectivity, which resulted in deviation from the known phase-transition characteristics of 5CB. It was also observed that the inherent orientation of the nanofibres favours the nematic to crystalline transition in the blend nanofibres. The present study gives new insight and understanding about phase behaviour of LC in electrospun polymer fibre and has technological relevance for the design of LC-based flexible fibrous components with tunable optical, thermal and dielectric properties.

Flexoelectric effects on escaped twisted configurations of nematic liquid crystal 5CB with chiral dopants in a cylindrical cavity

ABSTRACT In a cylindrical cavity with a degenerate planar anchoring, the lyotropic chromonic nematic liquid crystals without intrinsic chirality tend to exhibit an escaped twisted configuration for the large saddle-splay-to-twist elastic modulus ratio, which is not found in most thermotropic liquid crystals. In this work, we consider thermotropic nematic liquid crystal 5CB with a chiral dopant confined in a cylindrical cavity with a degenerate planar anchoring. Based on the elastic theory of liquid crystal and the variation method, Euler equation and boundary conditions are obtained. The numerical results show that the director of 5CB forms an escaped twisted configuration, and the twist angle is influenced by the chiral parameter and the saddle-splay elastic constant. Under an axial electric field, the dielectric effect and the flexoelectric effect affect the LC director distribution. The single flexoelectric coefficient e 1 has a large radial orientation effect on the director. The results provide a clearer understanding of the director configuration in the cylindrical cavity under the electric field and a theoretical basis for the measurement of the flexoelectric coefficients.

Nanosecond illumination source for speckle-free liquid crystalmicroscopy

ABSTRACT We present a setup for the speckle-free, low-light microscopy imaging at ~5 ns exposure times. The design is based on stroboscopic principle and uses fast and incoherent fluorescent emission from a dye, excited by a picosecond laser pulse. The basic spatial, spectral, and temporal beam properties are characterised, modelled and compared for two different designs of the source: (i) Rhodamine 6 G dye dissolved in ethanol and put in a glass cuvette, and (ii) random laser based on the dispersion of 300 nm TiO2 nanospheres in an ethanol solution of Rhodamine 6 G dye. The proposed solution of Rhodamine 6 G dye in ethanol put in a glass cuvette gives an excellent image quality with high contrast, excellent stability and tunable coherence. It delivers ~30 nJ of completely incoherent light per excitation laser pulse energy of ~40 μJ. It allows for the photographic measurements with 5 ns exposure time, demonstrated in imaging of thermally quenched 5CB liquid crystal. The set-up is used in studies of Kibble-Zurek mechanism of topological defect nucleation and growth at sub-microsecond time resolution and extremely fast cooling rates of ~40,000 K/s.

How does PEDOT grow like nanoropes? A theoretical investigation in pace with experimental insights

Poly 3–4 ethylene dioxythiophene (PEDOT) is a highly conducting polymer insoluble and intractable when synthesized under normal synthesis conditions. This polymer was found to assume spiral or ropelike nanostructures with some modification in physical properties when grown in presence of a liquid crystal 5CB. The present work focuses on finding out the underlying reason behind the unique morphology of PEDOT theoretically, using density functional theory (DFT) as implemented in the Gaussian 03 suite of programs. From the energy minimized structures of 5CB clusters as well as PEDOT-5CB combinations it was observed that in the clusters of 6 or 10 molecules, 5CB is self-assembled in spiral orientation which might have guided the similar growth of PEDOT. It was also established from molecular orbital studies that the neighboring 5CB environment undergoes some sort of nonbonding interaction with the growing PEDOT chains and has resulted in the modification of solubility and few other properties of PEDOT. Therefore the PEDOT nanostructure obtained under the present synthesis condition is supposed to be a hybrid material having optical and electrical properties dominated by the PEDOT counterpart.

Freedericksz Transitions in Twisted Ferronematics Subjected to Magnetic and Laser Field

The influence of the anchoring forces on the Freedericksz transition in twisted ferronematics simultaneously subjected to magnetic and laser fields is studied in this work. Using the elastic continuum theory and Gouchen model for molecular anchoring on the cell support plates, the critical field and the saturation field were calculated as a function of the laser intensity and anchoring strength for two types of ferronematics based on 5CB and CCN-37 liquid crystals.

Effect of Inhomogeneous Electric Field in a Cell with Side Electrodes: Nematic Liquid Crystal 5CB

The influence of the geometry of a cell with side electrodes and the parameters of applied pulsed-periodic voltage on the director orientation in nematic liquid crystal 5CB is analyzed. It is shown that the two-dimensional model of director motion taking into account the electric field inhomogeneity in the cell provides satisfactory agreement with experimental results.

Dielectric relaxation, electric conductivity, and electro-optic properties of SWCNT-doped liquid crystal 5CB

The effect of single-walled carbon nanotubes (SWCNTs) on the dielectric, conductivity, and electro-optic properties of nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) has been studied. It was shown that the additive of SWCNTs with concentration of 0.5% leads to an increase in the order parameter of 5CB. In this case, the clearing point is raised, the longitudinal component of the dielectric permittivity increases while the transverse component decreases. The incipient percolation effect promotes to the dominance of hopping electron conductivity over ionic conductivity, leading to an increase in specific conductivity. The elastic splay constant of 5CB is enhanced and, accordingly, the threshold voltage of the Freedericksz effect increases in this situation. An increase in distance between LC molecules leads to decreasing of viscosity. As a result, the flip-flop motion of molecules become easier and the switching times are also reduced.

Ultrahigh dielectric constant induced by the interfacial polarization between liquid crystal droplets and polymer matrix

Polymer composites mixed with high dielectric constant nanofillers are thought of as excellent candidates of high-performance dielectrics for electrostatic energy storage. In this contribution, we report a new kind of all-organic composites composed of liquid crystal (LC) 5CB and polymer PMMA. They present an ultrahigh dielectric constant of above 103 at high-temperature/low-frequency conditions, attributed to the effect of Maxwell–Wagner–Sillars polarizations proportional to the LC/polymer interface area. Moreover, adding a negligible amount (0.2 vol%) of BaTiO3 nanoparticles into composites for better 5CB-domain dispersion, we realize a further enhancement of dielectric constant at the same measurement conditions as the expectation.

Textural and electro-optical study of a room temperature nematic liquid crystal 4̍-pentyl-4-biphenylcarbonitrile doped with metal oxide nanowires in planar and in-plane switching cell configurations

ABSTRACT This work shows the doping effect of metal oxide nanowires (alumina nanowires) on the textural and electro-optical characteristics of nematic liquid crystal (5CB) in planar and in-plane switching (IPS) cell configurations. Results indicate the increase in nematic to isotropic phase transition temperature after doping in both planar and IPS cells. A decrease in threshold voltage in addition to increase in contrast ratio, birefringence, and band gap energy was observed after doping with alumina nanowires in both types of cells. Overall analysis shows that IPS cell configuration has more improved threshold voltage and contrast ratio in addition to decrease in birefringence and transmission intensity, as compared to planar cell configuration. No effect of cell configuration was observed on energy band gap and found same value for both planar and IPS cell configurations, as energy band gap is a material specific property.

Influence of cation substitution on dielectric properties and electric conductivity of 6CB liquid crystal with Me7GeS5I (me = Ag, Cu) superionic nanoparticles

The influence of Ag7GeS5I nanoparticles on dielectric properties of planar-oriented 6CB liquid crystal was investigated at 293 K within the frequency range from 6 to 106 Hz. The concentration of nanoparticles varied between 0 to 0.1 wt.%. The obtained results were compared with the influence of the same concentrations of Сu7GeS5I nanoparticles. It was shown that the Сu7GeS5I nanoparticles influence the conductivity of the liquid crystal significantly less than the Ag7GeS5I nanoparticles.

Comparative DFT Study of Parallel and Antiparallel Conformation of 5CB and 6CB Liquid Crystal Dimers

An investigation of the interaction of 5CB and 6CB liquid crystals along with their dimer configurations in different conformations. The total energy, thermal energy, HOMO-LUMO Gap, dipole moment, polarizability, constant volume heat capacity, entropy, zero-point energy (ZPE), and enthalpy of the different configurations of 5CB and 6CB liquid crystal dimers are affected during the molecular interaction. The different interaction properties of 5CB, 6CB, and 5CB-6CB dimers studied in parallel and antiparallel conformation with the help of density functional theory method lc-blyp by NWChem software package. The 5CB and 6CB liquid crystal dimers have the least dipole moment and negative entropy in the antiparallel conformation. The cross conformation of 5CB liquid crystal dimer (5CB-5CB) has the least isotropic polarizability; however, the parallel conformation of 5CB liquid crystal dimer has the highest isotropic polarizability. The isotropic polarizability is minimum in the antiparallel conformation of 6CB liquid crystal dimer (6CB-6CB), while isotropic polarizability is maximum in the parallel conformation of 6CB liquid crystal dimer. The 5CB liquid crystal dimer and 6CB liquid crystal dimer have minimum negative interaction energy in the antiparallel conformation, but 5CB-6CB liquid crystal dimer has positive interaction energy for all the possible conformations.

Enhancement of Birefringence in Reduced Graphene Oxide Doped Liquid Crystal.

We investigated the effect of reduced graphene oxide (rGO) doping on the birefringence of 5CB liquid crystal (LC). The characteristics of the synthesized rGO and LC-rGO composite with different rGO concentrations were analyzed by atomic force microscopy, X-ray photoelectron spectroscopy, white light polarized microscopy, voltage-dependent transmission measurement, and differential scanning calorimetry. We found that doping LC with an appropriate concentration of rGO enhances the birefringence of the LC. This is mainly due to the improved anisotropy of polarizability, which stems from the high shape anisotropy of rGO. However, the aggregation of rGO reduces the birefringence by decreasing the anisotropy of polarizability as well as the order parameter. Our study shows the promising potential of LC-rGO for developing various electro-optic devices that offer improved electro-optic effects.

The realisation of an electro-Optical Diffraction Grating using HeNe laser

This paper reports far-field of the electro-optical diffraction grating to the advantage of electro-optic response of 5CB liquid crystal. The diffraction grating was produced by intersacting of two laser beam of laser HeNe 632.8nm wavelength radiation transmitted through a 5CB liquid crystal dopped Sudan Black B (SBB) dye in a cell with a thickness of 5µm and applying a uniform electric filed across the cell. Different concentrations of SBB dye were prepared and optically characterised. Double side tape in a thickness of 5 µm was used to determine the cell thickness. Zero and two diffraction orders were observed. 16 µm grating period involve in a produced electro-optical diffraction grating. The effect of changing voltage was investigated. The possibility of realising the electro-optical diffraction grating made of 5CB liquid crystal doped biocompatible material chitosan doped SBB dye to enhance the absorption properties were also investigated and characterised using Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), UV-vis spectrophotometer and optical microscope. A fabricated Electro-Optic diffraction grating was experimentally characterized. Applications of liquid crystal and the electro-optic effect were discussed briefly.

Small-Angle Scattering and Radiation Polarization by a Stretched Polymer Film with Nematic Liquid Crystal Droplets Having a Single-Domain Structure

The coefficient of directed transmittance of a stretched polymer-dispersed liquid crystal film with a defect-free single-domain liquid crystal droplet structure formed by a stretched surfactant-doped film and the polarization degree of forward-transmitted light in the visible and near-infrared spectrum ranges are studied. Results are presented for the 5CB, E7, and E44 nematic liquid crystals. Dependences of the transmission coefficient and polarizing ability of the film on the photodetector field of view are studied. Relationships allowing one to determine film parameters at which the transmission coefficient and polarizing ability of the films simultaneously reach values close to limit ones (0.5 and ±1.0, respectively) are obtained in the Foldy–Twersky and anomalous diffraction approximations.

Controllable Selective Coupling of Dyakonov Surface Waves at a Liquid-Crystal-Based Interface

Highly directional and lossless surface wave has significant potential applications in the two-dimensional photonic circuits and devices. Here we experimentally demonstrate a selective Dyakonov surface wave coupling at the interface between a transparent polycarbonate material and nematic liquid crystal 5CB. By controlling the anisotropy of the nematic liquid crystal with an applied magnetic field, a single ray at a certain incident angle from a diverged incident beam can be selectively coupled into surface wave. The implementation of this property may lead to a new generation of on-chip integrated optics and two-dimensional photonic devices.