Glassiness of thermotropic liquid crystals across the isotropic-nematic transition

The orientational dynamics of thermotropic liquid crystals across the isotropic-nematic phase transition have traditionally been investigated at long times or low frequencies using frequency domain measurements. The situation has now changed significantly with the recent report of a series of interesting transient optical Kerr effect (OKE) experiments that probed orientational relaxation of a number of calamitic liquid crystals (which consist of rod-like molecules) directly in the time domain, over a wide time window ranging from subpicoseconds to tens of microseconds. The most intriguing revelation is that the decay of the OKE signal at short to intermediate times (from a few tens of picoseconds to several hundred nanoseconds) follows multiple temporal power laws. Another remarkable feature that has emerged from these OKE measurements is the similarity in the orientational relaxation behavior between the isotropic phase of calamitic liquid crystals near the isotropic-nematic transition and supercooled molecular liquids, notwithstanding their largely different macroscopic states. In this article, we present an overview of the understanding that has emerged from recent computational and theoretical studies of calamitic liquid crystals across the isotropic-nematic transition. Topics discussed include (a) single-particle as well as collective orientational dynamics at a short-to-intermediate time window, (b) heterogeneous dynamics in orientational degrees of freedom diagnosed by a non-Gaussian parameter, (c) fragility, and (d) temperature-dependent exploration of underlying energy landscapes as calamitic liquid crystals settle into increasingly ordered mesophases upon cooling from the high-temperature isotropic phase. A comparison of our results with those of supercooled molecular liquids reveals an array of analogous features in these two important classes of soft matter systems. We further find that the onset of growth of the orientational order in the parent nematic phase induces translational order, resulting in smectic-like layers in the potential energy minima of calamitic systems if the parent nematic phase is sandwiched between the high-temperature isotropic phase and the low-temperature smectic phase. We discuss implications of this startling observation. We also discuss recent results on the orientational dynamics of discotic liquid crystals that are found to be rather similar to those of calamitic liquid crystals.     

Gravitational field-induced orientational transition of aligned nematic liquid crystals

We demonstrate induced orientational transition of liquid crystal (LC) E7 between two optical fibre tips in a gravitational field as a result of minimising the free energy. The LC orientational transition is from a homeotropic state, with respect to fibre tip, to a tilted state as the spacing between two fibre tips is increased. The orientational distortion introduces birefringence in the optical fibre that changes the polarised direction of the output light. At short spacing, the undistorted homeotropic orientation of LC is preferred. However, at longer spacing, a distorted orientation is preferred. Once the LC director profile is known, a Finite Difference Time Domain method is used to calculate the optical properties, which agree well with the experimental results.     

Evidence of nematic, hexagonal and rectangular columnar phases in thermotropic ionic liquid crystals

Dithiolium salts, with amphipathic character, are compounds of choice for investigations of the influence of an ionic feature upon mesomorphic properties. In this way, salts bearing a branched chain have been studied by SAXS. In spite of their rod-like shape, they exhibit only columnar mesophases, the supramolecular organization of which is close to that of cylindrical inverted micelles. Moreover, the nematic columnar phase, characterized by the loss of lateral positional correlations of the columns themselves, is one of the first examples of such behaviour in the case of thermotropic liquid crystals.     

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.     

High figure-of-merit nematic mixtures based on totally unsaturated isothiocyanate liquid crystals

High birefringence and low viscosity isothiocyanate liquid crystal single compounds, and eutectic mixtures based solely on unsaturated rigid core structures, are reported. Extraordinarily high values of figure-of-merit were observed at room temperature for the formulated nematic mixtures. Potential applications of such mixtures for laser beam steering at λ = 1.55 µm using optical phased arrays are emphasized.     

Stable photoalignment layer for nematic liquid crystals based on ladder-like polysilsesquioxanes

A new kind of aligning material for liquid crystal cells, ladder-like polysilsesquioxanes (LPS) grafted with cinnamoyl side groups, has been developed to improve the thermal stability of the photoalignment layer. The LC aligning ability of the LPS-based alignment layers, fabricated by linearly polarized UV-induced polymerization (LPP), was characterized by polarizing optical microscopy, conoscopic observations and electro-optic response measurements. In particular, a practical and severe annealing test was adopted to examine the thermal stability of the alignment layer; this showed that even when LC cells were annealed at 100 C (much higher than the clearing point of the LC) for several hours, good LC orientation could remain when the cell was cooled to a constant measurement temperature. The results confirmed that the photoalignment layers exhibited not only good LC aligning ability, but also excellent thermal stability, so heralding their potential application in LCDs.     

A new family of four-ring bent-core nematic liquid crystals with a highly polar end-group

A new family of four-ring achiral bent-core compounds derived from 2-methyl 3-amino benzoic acid with the methyl group in the bent direction incorporated into the central core have been designed and synthesised. These compounds possess an alkoxy chain attached at only one end of the bent core molecule, while the other arm consists of a biphenyl moiety possessing a highly polar cyano-group. The molecular structure has been confirmed by elemental analysis and spectroscopic data, and the thermal behaviour and phase characterisation has been investigated by differential scanning calorimetry and polarising microscopy. All the compounds exhibit a wide-range enantiotropic nematic phase. A comparison with non-mesomorphic unsubstituted and 4-methyl-substituted homologues is also presented.     

Synthesis and mesomorphic properties of chiral nematic liquid crystals based on cholesterol

A new series of mesogenic compounds having a cholesteryl moiety has been synthesised by condensing p-amino benzoate of cholesterol and methoxy substituted 4-n-alkoxy cinnamoyl chlorides and their liquid crystalline properties has been studied. All the members of the series are enantiotropic and exhibit chiral nematic (N*) mesophase. The plot of transition temperatures versus number of carbon atoms in the alkoxy chain exhibits odd–even effect and falling tendency for N*–isotropic transition temperatures. The compounds exhibit oily streak textures that on slight disturbance change to the plane textures and show iridescent colours. High anisotropy, linearity and intermolecular hydrogen bonding confer rich mesomorphic properties on the system. Intermolecular hydrogen bonding arising from amide linkage can lead to supramolecular motifs.     

Phenomenological theory of the nematic to lamellar phase transition in lyotropic liquid crystals

A phenomenological theory is presented to describe the nematic to lamellar phase transition in lyotropic liquid crystals. The problem of the first or second order transition is explored by means of the variation of the surfactant concentration. The possibility of the tricritical point at the nematic to lamellar phase transition is discussed in a phenomenological way. The influence of the electrolyte on this transition is also discussed by varying the coupling between the electrolyte concentration variables and the order parameters. The theoretical prediction is found to be in good qualitative agreement with experimental results.     

Internal molecular disorder in the nematic and smectic phases of thermotropic liquid crystals studied by NMR SPDE experiments

The recently developed NMR SPDE experiment is shown to provide a new and particularly convenient technique for probing the conformational dynamics of mesogens in thermotropic liquid crystals. Measurements have been made in the nematic and smectic phases of the 4,4′-di-n-alkoxyazoxybenzenes. It is shown for the first time that the internal disorder of the alkyl end chains is intimately related to the molecular organization of these mesophases.     

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.     

New calamitic thermotropic liquid crystals of 2-hydroxypyridine ester mesogenic core: mesophase behaviour and DFT calculations

ABSTRACT

New three groups of 2-hydroxypyridine ester-based liquid crystals named, 5-[2-(4-substitutedphenyl)diazenyl]pyridin-2-yl 4?-alkoxybenzoate were synthesised. Each group differs from each other by the terminal polar substituent X (CH₃O, Cl and H) which contains five compounds with different numbers (n) of carbons in the alkoxy chain. Mesophase behaviour was investigated for the prepared homologues by differential scanning calorimetry and polarised light microscopy. Elemental analyses, FTIR and ¹HNMR spectroscopy were used for structure confirmation of the prepared compounds. Density functional theory (DFT) theoretical calculations are estimated to prove the experimental data. Stability ranges and the type of the mesophases observed for the investigated compounds were found to be mainly dependent on the length of the alkoxy chain, the polarity, as well as dipole moment and charge distribution. Moreover, the high terminal charge distribution in the case of the methoxy derivative could effect the end-to-end interactions resulting in a nematic phase rather than the ordered Smectic A phase observed in the case of Cl derivatives and the unsubstituted homologues which are non-mesomorphic. Results of the DFT discussed are found to be consistent with the present experimental investigations.     

Synthesis and transition temperatures of liquid crystals incorporating a 1,4-tetrafluorophenylene unit

Two homologous series of mesogenic materials which incorporate a 1,4-tetrafluorophenylene moiety and a terminal cyano- or nitro-substituent have been synthesized by using palladiumcatalysed coupling procedures. The compounds exhibit nematic and/or smectic A phases.     

Thermal characterisation of thermotropic nematic liquid-crystalline elastomers

Nematic liquid-crystalline elastomers (LCEs) are weakly cross-linked polymeric networks that exhibit rubber elasticity and liquid-crystalline orientational order due to the presence of mesogenic groups. Three end-on side-chain nematic LCEs were investigated using real-time synchrotron wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and thermogravimetry (TG) to correlate the thermal behaviour with structural and chemical differences among them. The elastomers differed in cross-linking density and mesogen composition. Thermally reversible glass transition temperature, T_g, and nematic-to-isotropic transition temperature, T_ni, were observed upon heating and cooling. By varying the heating rate, T_g⁰ and T_ni⁰ were determined at zero heating rate. The temperature dependence of the orientational order parameter was determined from the anisotropic azimuthal angular distribution of equatorial reflections seen during real-time WAXS. Results show that the choice of cross-linking unit, its shape, density, and structure of co-monomers, all influence the temperature range over which the thermal transitions take place. Including multi-ring aromatic groups as cross-linkers increased the effective stiffness of the cross-linking, resulting in a higher glass transition temperature. The nematic-to-isotropic transition temperature increased in the presence of multi-ring aromatic structures, as either cross-linkers or mesogens, particularly when the multi-ring structures were larger than the low-molar-mass mesogen common to all three samples.