Orientational ordering in nematic liquid crystals 1CB-d11 dissolved in 5CB-d6

The ²H-N.M.R. spectra of mixtures of the non-mesogenic compound 4-cyano-4'-methylbiphenyl (1CB) and the nematic liquid crystal 4-cyano-4'-n-pentylbiphenyl (5CB) are measured as a function of concentration and temperature. Concentrations of up to 25 mol% 1CB have no effect on the N.M.R. spectrum and therefore on the orientational order of 5CB at a given reduced temperature. The order matrix of the 1CB is calculated from the measured quadrupole couplings. The results are analysed in terms of a model for orientational order that includes two anisotropic terms: (a) interaction between the molecular quadrupole moment and the mean electric field gradient of the medium, and (b) short range repulsive interactions. An estimate of the molecular quadrupole moment tensor of 1CB is obtained from the analysis.     

Orientational ordering in nematic liquid crystals 1CB-d11 dissolved in 5CB-d6

The ²H-N.M.R. spectra of mixtures of the non-mesogenic compound 4-cyano-4′-methylbiphenyl (1CB) and the nematic liquid crystal 4-cyano-4′-n-pentylbiphenyl (5CB) are measured as a function of concentration and temperature. Concentrations of up to 25 mol% 1CB have no effect on the N.M.R. spectrum and therefore on the orientational order of 5CB at a given reduced temperature. The order matrix of the 1CB is calculated from the measured quadrupole couplings. The results are analysed in terms of a model for orientational order that includes two anisotropic terms: (a) interaction between the molecular quadrupole moment and the mean electric field gradient of the medium, and (b) short range repulsive interactions. An estimate of the molecular quadrupole moment tensor of 1CB is obtained from the analysis.     

Mechanisms of solute orientational order in nematic liquid crystals

The order parameters of a number of different solutes dissolved in a number of different nematic liquid-crystal solvents are measured and compared. It is shown that the order parameters can all be rationalized with the ansatz that there exist two independent ordering mechanisms operating in the liquid crystals employed. With this ansatz it is possible to fit the experimental order parameters to better than 5%. This opens the possibility of the accurate prediction of order parameters in ordered liquids.     

Nanoparticles in nematic liquid crystals: interactions with nanochannels

A mesoscale theory for the tensor order parameter Q is used to investigate the structures that arise when spherical nanoparticles are suspended in confined nematic liquid crystals (NLCs). The NLC is "sandwiched" between a wall and a small channel. The potential of mean force is determined between particles and the bottom of the channels or between several particles. Our results suggest that strong NLC-mediated interactions between the particles and the sidewalls of the channels, on the order of hundreds of k(B)T, arise when the colloids are inside the channels. The magnitude of the channel-particle interactions is dictated by a combination of two factors, namely, the type of defect structures that develop when a nanoparticle is inside a channel, and the degree of ordering of the nematic in the region between the colloid and the nanochannel. The channel-particle interactions become stronger as the nanoparticle diameter becomes commensurate with the nanochannel width. Nanochannel geometry also affects the channel-particle interactions. Among the different geometries considered, a cylindrical channel seems to provide the strongest interactions. Our calculations suggest that small variations in geometry, such as removing the sharp edges of the channels, can lead to important reductions in channel-particle interactions. Our calculations for systems of several nanoparticles indicate that linear arrays of colloids with Saturn ring defects, which for some physical conditions are not stable in a bulk system, can be stabilized inside the nanochannels. These results suggest that nanochannels and NLCs could be used to direct the assembly of nanoparticles into ordered arrays with unusual morphologies.     

A neutron reflection study of surface enrichment in nematic liquid crystals

The interfacial adsorption properties of several different dopants in cyanobiphenyl liquid crystals have been measured using specular neutron reflection. It was found that a partly fluorinated analogue of 11OCB, called F17, adsorbed strongly at the interface between 5CB and air but it was not adsorbed at the interface between 5CB and a solid substrate treated with cetyl trimethyl ammonium bromide (CTAB). The concentration dependence of the adsorption at the air interface was well described by the Brunauer, Emmett and Teller (BET) model, adapted for solutions rather than the gas phase. The isotherms are determined by two equilibrium constants: K(S) for adsorption of the dopant directly at the interface and K(L) for adsorption onto previously adsorbed dopant. The temperature dependence of K(S) indicated that the adsorption enthalpy is not influenced by the phase of the 5CB and its value of -29 kJmol(-1) is consistent with physical adsorption. The value of K(L) is zero in the isotropic phase but increases rapidly on cooling in the nematic phase suggesting that the F17 is less compatible with nematic than isotropic 5CB. The smallest layer thicknesses (~18 ?) suggest that the F17 molecules are approximately perpendicular to the surface. The other dopants studied were components of the E7 mixture: 8OCB and 5CT. No adsorption was found for 8OCB but 5CT showed adsorption at a CTAB treated solid interface when present in 5CB at the 10% level. In this case, the value of K(S) was much smaller than for F17 but the value of K(L) was such that an exponential concentration profile (predicted by the BET model) was observed with characteristic thickness of ~200 ?. The results demonstrate the potential for very precise control of surface properties in liquid crystal devices by using appropriate dopants.     

Colloidal particles as elastic triads in nematic liquid crystals

ABSTRACT

In this short review we summarise already published results to manifest very important role of high order elastic terms in the formation of colloidal structures in nematic liquid crystals (NLC). We reveal that every colloidal particle in nematics can be effectively represented as a triad of nonzero elastic moments. Usually colloidal particles in NLCs are treated with their elastic dipole and/or quadrupole moments only. But we demonstrate that octupole, hexadecapole and even 64-pole moments play an important role as well and determine parameters of different 1D, 2D and even 3D colloidal crystals in NLCs. In general the triad of the first three nonzero elastic moments can describe almost all colloidal structures observed so far. Dipole particles should be considered as hard spheroids with a triad of the dipole, quadrupole and octupole moments. Quadrupole particles should be treated as hard spheres with a triad of quadrupole, hexadecapole and 64-pole elastic moments

PACS numbers: 61.30.Dk, 82.70.Dd, 64.70.M?     

Monosubstituted ferrocene liquid crystals containing click triazole with a wide nematic phase temperature range

New monosubstituted ferrocenomesogens of the 1,4-substituted-1,2,3-triazole series 3a-3d and the 5-halogen-1,4-substituted-1,2,3-triazole series 4a-4d were successfully synthesised through a facile and versatile synthetic route using click reactions with moderate yields, in different atmospheres, and catalysed by different amounts of CuX (X = Br, I). Two series of compounds exhibited a typical nematic liquid crystal texture, and the 5-halogen-1,4-substituted-1,2,3-triazole series 4 had a wider mesomorphic temperature range than the corresponding series 3 due to the effect of the halogen atom substituent on the triazole. These compounds showed similar absorption spectra, but the fluorescence emission spectra of 4 were obviously redshift relative to 3. Cyclic voltammetry investigations revealed that the redox-active ferrocenyl groups of 3 and 4 follow one-electron transfer processes.     

Atomistic modelling of nematic liquid crystals: a study of structure-property relationships in steroidal mesogen based liquid crystals

In this study we report molecular mechanics calculations designed to predict and interpret structure property relationships in nematic liquid crystals. A family of liquid crystals with steroidal mesogens were studied and the results were compared with available X-ray data. Low energy conformations of dimers were analysed to provide quantitative information about the local intermolecular interactions and their anisotropic nature. Important contributions to the molecular packing could be identified and the geometry of the dimers and the extent of their positional correlation was successfully related to their observed packing behaviour. By monitoring the relative orientation of the two molecules, a qualitative study of liquid crystalline phase stability was accomplished. Simulations were also carried out with a modified energy function which includes a nematic contribution representing the cumulative intermolecular interactions owing to long range orientational order present in liquid crystals. Along with providing a systematic study of the relative importance of the various competing forces (steric repulsion, attractive forces, long-range electrostatic interactions) in the formation of liquid crystalline phases, this method can also be expected to be useful in predicting mesophase behaviour.     

Multiple short time power laws in the orientational relaxation of nematic liquid crystals

Relaxation in the nematic liquid crystalline phase is known to be sensitive to its proximity to both isotropic and smectic phases. Recent transient optical Kerr effect (OKE) studies have revealed, rather surprisingly, two temporal power laws at short to intermediate times and also an apparent absence of the expected exponential decay at longer times. In order to understand this unusual dynamics, we have carried out extensive molecular dynamics simulations of transient OKE and related orientational time correlation functions in a system of prolate ellipsoids (with aspect ratio equal to 3). The simulations find two distinct power laws, with a crossover region, in the decay of the orientational time correlation function at short to intermediate times (in the range of a few picoseconds to a few nanoseconds). In addition, the simulation results fail to recover any long time exponential decay component. The system size dependence of the exponents suggests that the first power law may originate from the local orientational density fluctuations (like in a glassy liquid). The origin of the second power law is less clear and may be related to the long range fluctuations (such as smecticlike density fluctuations)--these fluctuations are expected to involve small free energy barriers. In support of the latter, the evidence of pronounced coupling between orientational and spatial densities at intermediate wave numbers is presented. This coupling is usually small in normal isotropic liquids, but it is large in the present case. In addition to slow collective orientational relaxation, the single particle orientational relaxation is also found to exhibit slow dynamics in the nematic phase in the long time.     

A mesoscale modelling study of nematic liquid crystals confined to ellipsoidal domains

Director configurations of nematic liquid crystalline molecules packed in ellipsoidal domains have been investigated using mesoscale modelling techniques. Interactions between the directors were described by the Lebwohl-Lasher potential. Four different ellipsoidal shapes (sphere, oblate spheroid, prolate spheroid, and ellipsoid) were studied under homogeneous and homeotropic surface anchoring conditions. The model has been characterized by computing thermodynamic and structural properties as a function of ellipsoidal shape (prolate and oblate) and size. The predicted director configuration in ellipsoids resulting from homeotropic surface anchoring is found to be very different from that in spherical domains. The bipolar configuration involving homogeneous surface anchoring is nearly identical in the four cases. The effect of an external electric field, applied at different orientations with respect to the major axis of the ellipsoid, has been probed as a function of the magnitude of the field and the ellipsoidal size and shape. The orientation of directors is most easily accomplished parallel and perpendicular to the major axis for the oblate and prolate spheroids, respectively, for homeotropic anchoring, and along the bipolar symmetry axis for homogeneous anchoring. In domains with homeotropic surface anchoring, the oblate spheroid and elongated ellipsoid are predicted to be the most efficient geometries for PDLC applications; for homogeneous anchoring conditions, the prolate spheroid and elongated ellipsoid are predicted to be the most efficient.     

A mesoscale modelling study of nematic liquid crystals confined to ellipsoidal domains

Director configurations of nematic liquid crystalline molecules packed in ellipsoidal domains have been investigated using mesoscale modelling techniques. Interactions between the directors were described by the Lebwohl-Lasher potential. Four different ellipsoidal shapes (sphere, oblate spheroid, prolate spheroid, and ellipsoid) were studied under homogeneous and homeotropic surface anchoring conditions. The model has been characterized by computing thermodynamic and structural properties as a function of ellipsoidal shape (prolate and oblate) and size. The predicted director configuration in ellipsoids resulting from homeotropic surface anchoring is found to be very different from that in spherical domains. The bipolar configuration involving homogeneous surface anchoring is nearly identical in the four cases. The effect of an external electric field, applied at different orientations with respect to the major axis of the ellipsoid, has been probed as a function of the magnitude of the field and the ellipsoidal size and shape. The orientation of directors is most easily accomplished parallel and perpendicular to the major axis for the oblate and prolate spheroids, respectively, for homeotropic anchoring, and along the bipolar symmetry axis for homogeneous anchoring. In domains with homeotropic surface anchoring, the oblate spheroid and elongated ellipsoid are predicted to be the most efficient geometries for PDLC applications; for homogeneous anchoring conditions, the prolate spheroid and elongated ellipsoid are predicted to be the most efficient.     

Non-covalent intermolecular interactions of colloidal nematic liquid crystals doped with graphene oxide

Graphene oxide (GO) was doped to eutectic uniaxial nematic liquid crystals (NLCs) (E_5CN7) with different percentages to improve the physiochemical properties of NLCs effectively. GO shifts the N-I phase transition temperature to higher values. It has been observed that increasing GO concentration up to 0.75% increases the N-I phase transition temperature substantially while further increase of GO concentration results in an inverse trend. The GO/E_5CN7 non-covalent interactions change the N-I phase transition. The contribution of several terms such as anchoring and polarisation effects on N-I phase transition was quantified as well. The results suggest that the size is an important contributor to GO and liquid crystal interaction. The results show that E_5CN7@GO composites may act as promising candidates to enhance the efficiency of room temperature devices.     

A statistical model for transitions in nematic liquid crystals

The two-lattice model of Lennard-Jones and Devonshire has been modified and applied to the study of the transition from the nematic to the isotropic phase. We have used the concept of external degrees of freedom for molecules introduced by Prigogine and applied to the case of chain liquids.     

A direct assessment of refractive indices of nematic liquid crystals at broad VIS - MWIR range

We developed an interference method for determination refractive indices of nematic liquid crystals in a broad electromagnetic spectrum. Values of ordinary and extraordinary refractive indices were measured in VIS (from 0.4 to 0.8 μm), NIR (from 0.8 to 1.4 μm), SWIR (from 1.4 to 3.0 μm) and at an edge of MWIR (from 3.0 to 4.2 μm) regions. The main task of this work was to develop the efficient and accurate interference method. The absolute error of these measurements is not higher than 0.02. The method was tested on two newly designed mixtures. The first one was a high birefringence liquid crystal mixture for the laser rangefinder, while the second one is intentionally prepared for breathalyser application. The mixture does not possess high absorption bands at 3.4 μm. To measure a dispersion of the refractive indices cells with various thicknesses and dielectric mirrors were prepared. The paper presents a theoretical discussion and experimental results.     

Ordering transitions in nematic liquid crystals induced by vesicles captured through ligand-receptor interactions

We report that phospholipid vesicles incorporating ligands, when captured from solution onto surfaces presenting receptors for these ligands, can trigger surface-induced orientational ordering transitions in nematic phases of 4'-pentyl-4-cyanobiphenyl (5CB). Specifically, whereas avidin-functionalized surfaces incubated against vesicles composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) were observed to cause the liquid crystal (LC) to adopt a parallel orientation at the surface, the same surfaces incubated against biotinylated vesicles (DOPC and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(biotinyl) (biotin-DOPE)) caused the homeotropic (perpendicular) ordering of the LC. The use of a combination of atomic force microscopy (AFM), ellipsometry and quantitative fluorimetry, performed as a function of vesicle composition and vesicle concentration in solution, revealed the capture of intact vesicles containing 1% biotin-DOPE from buffer at the avidin-functionalized surfaces. Subsequent exposure to water prior to contact with the LC, however, resulted in the rupture of the majority of vesicles into interfacial multilayer assemblies with a maximum phospholipid loading set by random close packing of the intact vesicles initially captured on the surface (5.1 ± 0.2 phospholipid molecules/nm(2)). At high concentrations of biotinylated lipid (>10% biotin-DOPE) in the vesicles, the limiting lipid loading was measured to be 4.0 ± 0.3 phospholipid molecules/nm(2), consistent with the maximum phospholipid loading set by the spontaneous formation of a bilayer during incubation with the biotinylated vesicles. We measured the homeotropic ordering of the LC on the surfaces independently of the initial morphology of the phospholipid assembly captured on the surface (intact vesicle, planar multilayer). We interpret this result to infer the reorganization of the phospholipid bilayers either prior to or upon contact with the LCs such that interactions of the acyl chains of the phospholipid and the LC dominate the ordering of the LC, a conclusion that is further supported by quantitative measurements of the orientation of the LC as a function of the phospholipid surface density (>1.8 molecules/nm(2) is required to cause the homeotropic ordering of the LC). These results and others presented herein provide fundamental insights into the interactions of phospholipid-decorated interfaces with LCs and thereby provide guidance for the design of surfaces on which phospholipid assemblies captured through ligand-receptor recognition can be reported via ordering transitions in LCs.     

On the temperature dependence of the order parameter of liquid crystals over a wide nematic range

Based on the ideas of Landau-de Gennes theory applied to nematic liquid crystals, several forms for the variation of the order parameter as a function of temperature are investigated over a wide nematic range. These functional forms are used to fit the experimental order parameters, determined through the use of C-13 NMR, for 4-methoxybenzylidene-4'-butylaniline (MBBA) and 4-n-pentyl-4'-cyanobiphenyl (5CB), and the physical significance of the parameters is discussed. A comparison of the results shows that the functional form which fits the experimental data best is similar to the Haller equation, a useful relation which is usually regarded as empirical. In this case, the coefficients resulting from a semi-empirical approach based on the Landau-de Gennes treatment may be thought of as quantifying the importance of the structure and rigidity of the liquid crystal in determining the temperature dependence of the order parameter for that liquid crystal. In the process, we have also examined the pretransitional behaviour in the C-13 NMR chemical shifts of liquid crystals observed within a few tenths of a degree above the nematic to isotropic transition temperature.     

Derivatives of menthothiophene as chiral dopants for nematic liquid crystals

New chiral derivatives of thiophene fused with menthane, camphor or the 3R-3-methylcyclohexane ring were prepared and studied as chiral dopants in cholesteric liquid crystalline mixtures. The helical twisting power of the most effective compounds of this series, menthothiophenes, was strong enough to obtain selective reflection of visible light at 16 wt% concentration of the dopant in a non-chiral nematic host.     

On the temperature dependence of the order parameter of liquid crystals over a wide nematic range

Abstract

Based on the ideas of Landau-de Gennes theory applied to nematic liquid crystals, several forms for the variation of the order parameter as a function of temperature are investigated over a wide nematic range. These functional forms are used to fit the experimental order parameters, determined through the use of C-13 NMR, for 4-methoxybenzylidene-4′-butylaniline (MBBA) and 4-n-pentyl-4′-cyanobiphenyl (5CB), and the physical significance of the parameters is discussed. A comparison of the results shows that the functional form which fits the experimental data best is similar to the Haller equation, a useful relation which is usually regarded as empirical. In this case, the coefficients resulting from a semi-empirical approach based on the Landau-de Gennes treatment may be thought of as quantifying the importance of the structure and rigidity of the liquid crystal in determining the temperature dependence of the order parameter for that liquid crystal. In the process, we have also examined the pretransitional behaviour in the C-13 NMR chemical shifts of liquid crystals observed within a few tenths of a degree above the nematic to isotropic transition temperature.