Tunable polarised fluorescence of quantum dot doped nematic liquid crystals

The concentration, excitation photon wavelength, and polarisation dependent fluorescence of quantum dot (QD)–liquid crystal (LC) mixtures has been studied at room temperature using high-resolution, steady-state fluorescence spectroscopy. The fluorescence of QD–LC mixture increases with increasing QD’s concentration but the spectral red shift of ~10 nm relative to the stock QD–Toluene solution remains independent of concentration. In vertical switching (VS) cells, an external electric field changes the LC alignment direction and enhances the apparent fluorescence intensity. The apparent fluorescence anisotropy compared to that at zero applied electric field monotonically increases up to ~27% at an applied electric field of 0.6 V/µm. These results are consistent with the formation of disc-like assemblages of QDs oriented on planes perpendicular to the director of the nematic liquid crystal (NLC). These findings have important utility in polarisation sensitive photonic devices.     

Blue phase liquid crystals affected by graphene oxide modified with aminoazobenzol group

Liquid-crystalline blue phases (BPs) are stable only for very narrow temperature range between the isotropic and the chiral nematic phase that severely hinders their applicability. Herein, the aminoazobenzol group was chemically grafted onto epoxy group of graphene oxide (GO) via addition reaction. Successful grafting of aminoazobenzol group was confirmed using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), UV–vis absorption spectra and thermogravimetric analysis (TGA). The resultant aminoazobenzol group–modified GO sheets, which is reduced (RGO-Az), were easily redispersable in common organic solvents or liquid crystals (LCs). By doping different contents of RGO-Az, nanosheets could stabilise BP and increase the BP range. When doped with 0.5 wt% RGO-Az, the mixtures show the wider range with 5.9°C than the range with 3.6°C of BPLCs without RGO-Az. Meanwhile, the phase sequence and the range of the aforementioned phases are reproducible upon heating and cooling, which shows that the BPs doped with RGO-Az nanosheets are thermodynamically stable.     

Poly(ionic liquid)s as phase-transporter for graphene oxide liquid crystals from aqueous to non-polar organic phase via noncovalent functionalization

We synthesise a novel poly(ionic liquid) (PIL) and develop a procedure that allows the phase transfer of graphene oxides (GO) from water to organic solvent, retaining graphene oxide liquid crystal (GOLC) phase in various organic solvents, especially non-polar organic solvents. PIL ([PEP-MIM]DBS) is exploited in this procedure as a noncovalent functional material that is capable of transporting GO from aqueous to non-polar organic phase. PILs can decorate GO noncovalently and keep GO nano-sheets exfoliated in solid-state PILs/GO hybrids, which can well redisperse in organic solvents without any agglomeration. This expands the number of known solvents which can support GOLC phase to dimethyl formamide, dimethyl sulfoxide, acetonitrile, tetrahydrofuran, and a number of other non-polar organic solvents, many of which were not known to afford GOLC phase prior to this report, such as dichloromethane, tetrachloromethane, dichloroethane and tetrachloroethane.     

Investigations of dual-frequency nematic liquid crystals doped with dichroic dye

In this work, we studied and analysed a particular variety of liquid crystals, the so-called dual-frequency nematic liquid crystals (DFNLCs). The interest was to perform dielectric spectroscopy and optical characterisation with two pure and doped mixtures of DFNLCs. By means the dielectric spectroscopy we observed a low value of crossover frequency at room temperature for one dye doped mixture. Additionally, in the spectra studies, a scattering of light at cross-frequency, and a stable and unstable focal conical state was observed, respectively, for the investigated mixtures.     

High-pressure phase transitions in liquid crystals

Simultaneous measurements of the rate of heat evolution and changes of the mechanical variable of a transformation such as volume or pressure, performed in a p-V-T controlled scanning calorimeter have been applied to investigations of phase transitions in liquid crystals. In the instrument, the phase transitions can be induced by a controlled change of pressure, volume or temperature under isothermal, isobaric or isochoric conditions respectively. The present investigations have ben performed on 4-n-penthyl-penthylthiol-4-decycloxybenzoate which demonstrates in the liquid crystal state a nematic and three smectic phases

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Zusammenfassung In einem p-V-T-kontrollierten Scanning Kalorimeter wurden zur Untersuchung der Phasenumwandlungen in Flüssigkeitskristallen simultane Messungen der Geschwindigkeit der Wärmeentwicklung und der Veränderungen von mechanischen Größen von Umwandlungen, wie z.B. von Volumen oder Druck durchgeführt. In diesem Gerät kann die Phasenumwandlung durch eine kontrollierte Veränderung von Druck, Volumen oder Temperatur unter isothermen, isobaren oder isochoren Bedingungen ausgelöst werden. Vorliegende Untersuchungen wurden an 4-n-Pentyl-phenylthiol-4-dezyloxybenzoat durchgeführt, welches im Flüssigkristallzustand eine nematische und drei smektische Phasen aufweist.

     

Computer simulation of liquid crystals

Summary We review recent progress in the computer simulation of liquid crystals, with special emphasis on hard particle models. Surprisingly, the simplest molecular models, taking account only of molecular size and shape, are sufficient to generate a wide variety of liquid crystalline phases, closely analogous to those observed in real life. Thermodynamic stability of different phases is very sensitive to shape, and presumably will also be sensitive to further details of intermolecular interactions as they are incorporated into the model. Realistic atom-atom potential models of liquid crystals are available, but the associated simulations are quite expensive. Thus, while idealized models may be used to study quite general, fundamental properties of mesophases, the modelling of specific liquid crystal systems in a realistic way remains a great challenge. Progress continues to be made on both these fronts.     

Ionic impurities in nematic liquid crystal doped with quantum dots CdSe/ZnS

We investigated the dielectric losses and the ionic currents in the nematic liquid crystal (NLC) doped with semiconductor quantum dots (QDs) of CdSe/ZnS core – shell type and covered with trioctylphosphine oxide (TOPO) molecules. The dielectric loss tangent of the NLC composites increased with increasing the QDs concentration from 0.1 to 0.3 wt%. The density of mobile ions in the composites increased linearly and the average values of ions mobility in the composites decreased with increasing the QDs concentration. The fast ions with the mobility of about 10^–10 m²/V·s and the slow ions with the mobility of about 10^–11 m²/V·s were detected in the NLC composites. The growth of the content of slow ions took place with increasing the QDs concentrations. Increasing the dielectric loss tangent was observed with increasing the duration of sonication time of the NLC composites to prepare homogeneous suspensions. The fragmentation of the CdS/ZnS shell as a result of the sonication may lead to the appearance of the slow ions in the NLC composites.     

Photo-responsive liquid crystalline elastomer with reduced chemically modified graphene oxide

Photo-responsive liquid crystalline elastomer (LCE) with reduced chemically modified graphene oxide (RMGO) was fabricated and studied. Mesogenic groups modified graphene oxide (MGO) was prepared, reduced and characterised, then the obtained RMGO was mixed with an acrylate monomer containing a side-on mesogen, a crosslinker and a photoinitiator. After being oriented with magnetic field, well-defined LCE micropillar as photo-responsive actuators were fabricated from the mixture by the method combining soft lithography and photo-polymerisation/photo-crosslinking. The LCE micropillars showed reversible thermo-mechanical deformation during the nematic-to-isotropic transition temperature. Upon irradiating with red light (650 nm), photo-mechanical responses of the RMGO-containing LCE was demonstrated. This micron-sized LCE actuators can be used for remote photo-responsive devices.     

Curvature and defects in nematic liquid crystals

ABSTRACT

The use of nematic liquid crystals for directed assembly of particles and for the creation of multistable systems depends on the ability to control the topological defects and the distortions of the director field. These are not only driven by topological constraints and by anchoring energies but also by the curvature field created by the bounding surfaces. This review explores the interaction between defects, topology, inclusions and curvature in nematics. I will introduce the relationship between curvature and the Frank elastic energy in nematics, and then I will give an overview of specific examples that show how this coupling can create unexpected behaviours, such as lock-and-key interactions, anchoring transitions on curved surfaces and memory effects.     

Molecular relaxations in dual-frequency nematic liquid crystals

The preparation of nematic liquid crystals mixtures results in changing of molecular relaxations in comparison to pure substances. Typical example is the creation of dual-frequency nematic liquid crystals using a base mixture and functional admixtures. In this paper, we present how dielectric properties of starting compounds change at mixture preparation. Three dual-frequency nematic mixtures of different composition were prepared and examined by means of dielectric spectroscopy in a wide frequency (100 Hz to 10 MHz) and temperature range (170°C to ?60°C). Parameters of detected modes for pure compounds and final mixtures were calculated and their relationships with crossover frequency are discussed.     

Impact of ferroelectric nanoparticles on the dielectric constant of nematic liquid crystals

ABSTRACT

The influence of ferroelectric BaTiO₃ nanoparticles on the dielectric constant in the isotropic phase of the isotropic-nematic phase transition is studied within Landau phenomenological theory. The ferroelectric BaTiO₃ nanoparticles dependence of transition temperature and dielectric constant in the isotropic phase is calculated. We compare our theoretical results with experimental data available in the literature.     

Photoinduced liquid crystal blue phase by bent-shaped cis isomer of the azobenzene doped in chiral nematic liquid crystal

A photoresponsive azobenzene molecule DCAZO2 with two cholesteryl groups linked to both sides of the azobenzene group is doped in a mixture of nematic liquid crystal E7 and chiral dopant S811 (61.9 wt% E7, 36.1 wt% S811 and 2.0 wt% DCAZO2). Cooled from isotropic phase to 33.0°C, chiral nematic liquid crystal (N*LC) was formed in the sample and then the temperature was kept unchanged at 33.0°C. UV light irradiation induces the trans–cis photoisomerisation and thus an obvious phase transition. When the azobenzene groups isomerise to a cis-saturated state, the UV light was turned off and the white light was turned on at the same time. The bent-shaped cis isomer then turns back to the planar trans isomer gradually. A blue–green platelet texture representing cubic blue phase (BP) was observed and the size of the platelets was increased along with the cis–trans isomerisation. UV–vis absorption spectra indicate that the photoinduced BP exists when the isomerisation degree is between 79% and 18%, and further cis–trans isomerisation change BP back into N*LC. The large geometric structure of the cholesteryl groups and the large bent angle θ of the cis isomer are supposed to be responsible for the interesting result.     

Effect of carbon nanotubes on phase transitions of nematic liquid crystals

Phase diagrams of multi‐wall carbon nanotube (MWNT)/nematic liquid crystal (E7) and buckminsterfullerene (C₆₀‐I _h)/nematic liquid crystal (E7) binary systems have been investigated by means of polarizing optical microscopy and differential scanning calorimetry. It was found that the isotropic–nematic phase transition temperature (T _NI) of the liquid crystal component was enhanced by the incorporation of MWNT within a small composition gap. A chimney‐type phase diagram can be identified in the MWNT/E7 mixture over a narrow range of ～0.1–0.2% MWNT concentration. Upon substituting the nanotubes with isotropic fillers such as fullerene, the (C₆₀‐I _h)/E7 blend showed no discernible change of T _NI in the same concentration range of the chimney of the MWNT/E7 mixture, suggesting a significant contribution of anisotropy (or the aspect ratio) of the nanotubes to the entropy of the system containing liquid crystal molecules. This enhanced T _NI phenomenon may be attributed to anisotropic alignment of liquid crystal molecules along the carbon nanotube bundles.     

Orientational instability of nematic liquid crystal in a homeotropic cell with boundary conditions controlled by an electric field

We study the orientational instability of the director in a homeotropic nematic liquid crystal (NLC) cell in a DC electric field. The electric field is applied along or perpendicular to a cell surface depending on whether anisotropy of dielectric permittivity of NLC is positive or negative. The easy axis on one of the cell polymer substrates is allowed to deviate in perpendicular to the substrate plane due to the influence of the NLC and the electric field. It was established that the orientational instability of the director can have a threshold as well as be thresholdless which depends on the character of the coupling of the easy axis with the electric field. The temporal behaviour of a director and the easy axis during transition to the stationary state after turning on electric field and returning to the initial homogeneous state after turning off the field was investigated. In the case of the negative anisotropy of the static permittivity, the comparison of experimental and calculated time dependences of the easy axis reorientation angle let us to estimate values of the mobile easy axis viscosity coefficient and coupling parameter describing the coupling between the easy axis and the electric field.     

Mesophase study of pure and doped cyanobiphenyl liquid crystals with salen-type systems

Two salen Schiff base ligands derived from the condensation of ethylendiamine on salicylaldehyde and 5-chlorosalicylaldehyde, namely N,N′-bis(salicylidene)ethylene-diamine (L1) and N,N′-bis(5-chlorosalicylidene)ethylene-diamine (L2) as well as two of their iron(III) and nickel(II) complexes, were prepared and then used as doping agents of two thermotropic liquid crystals (LCs) of cyanobiphenyl type, namely 4-cyano-4′-n-pentyl-biphenyl (5CB) and 4-cyano-4′-n-octyl-biphenyl (8CB). The study of the mesophase of pure and doped LCs was carried out by UV–visible spectroscopy equipped with a heating compartment for precise temperature control, differential scanning calorimetry and polarised optical microscopy. The characteristic nematic/isotropic and smectic-A/nematic transition temperatures of 5CB- and 8CB-based systems were measured and then compared to those of the literature concerning pure 5CB and 8CB. Optical microscopy results revealed the existence of Schlieren and focal conic textures of the nematic and smectic states, respectively, both of pure and doped LCs. The homogeneity of the obtained guest–host systems was proven by the linear evolution of their transition temperatures as function of the solute concentration, with correlation factors close to unity.     

Miscibility and intermolecular specific interactions in thermosetting blends of bisphenol S epoxy resin with poly(ethylene oxide)

Thermosetting blends composed of phloroglucinol‐cured bisphenol S epoxy resin and poly(ethylene oxide) (PEO) were prepared via the in situ curing reaction of epoxy in the presence of PEO, which started from initially homogeneous mixtures of diglycidyl ether of bisphenol S, phloroglucinol, and PEO. The miscibility of the blends after and before the curing reaction was established on the basis of thermal analysis (differential scanning calorimetry). Single and composition‐dependent glass‐transition temperatures (T_g's) were observed for all the blend compositions after and before curing. The experimental T_g's could be explained well by the Gordon–Taylor equation. Fourier transform infrared spectroscopy indicated that there were competitive hydrogen‐bonding interactions in the binary thermosetting blends upon the addition of PEO to the system, which was involved with the intramolecular and intermolecular hydrogen‐bonding interactions, that is, OH···O?S, OH···OH, and OH, versus ether oxygen atoms of PEO between crosslinked epoxy and PEO. On the basis of infrared spectroscopy results, it was judged that from weak to strong the strength of the hydrogen‐bonding interactions was in the following order: OH···O?S, OH···OH, and OH versus ether oxygen atoms of PEO. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 359–367, 2005     

H-shaped liquid crystals inducing nematic order in the isotropic liquid

ABSTRACT

We prepared a homologous series of H-shaped liquid crystals I-n and investigated their phase transition properties using optical microscopy and differential scanning calorimetry. All the compounds exhibited a nematic phase at room temperature. The phase transition behaviour is explained in terms of molecular shape anisotropy. Furthermore, those compounds were found to exhibit electro-optical switching in the isotropic liquid in the vicinity of the nematic–isotropic liquid transition, indicating that the microscopic nematic order with a certain coherence length of the molecules exists in the optically isotropic temperature range.     

Microscopic characterisation of self-assembled colloidal particles in electrohydrodynamic convection of a low-birefringence nematic liquid crystal

Electrokinetics of small particles immersed in anisotropic fluids is attracting attention in recent years. Here we focus on microscopic appearance of single as well as self-assembled particles moving in the electrohydrodynamic convection (EHC) of a nematic liquid crystal with low birefringence. Characterisation of the birefringent properties is made by polarised light microscopy under different illumination conditions. Because of the small optical anisotropy, the director distortion around the particles clearly exhibits distinctive colours on both sides depending on the height in the cell. The observation can be explained as the change in the net phase retardation of the light. It is also found that a caterpillar-like motion is possible by tuning temperature, although the horizontal size of the EHC rolls is relatively narrow.     

Defect-induced melting in nematic liquid crystals

In this paper we employ a relatively simple theory to show how a nematic disclination line can act as a nucleation site for the growth of the isotropic phase. With this theory we are able to find analytical expressions for the critical temperature of nucleation and the behaviour of the core radius as a function of temperature. We are then able to compare these results with a previous numerical model of this effect.