Silver nitrate-acetonitrile and iodine-benzene complexes as studied by NMR in thermotropic liquid crystals

Silver nitrate-acetonitrile and π iodine-benzene complexes in thermotropic liquid crystals have been studied by ¹H, ²H, and ¹³C NMR spectroscopy and by optical microscopy. Evidence for at least two silver complexes in each liquid crystal is presented.     

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.     

Structure of planar solitons in nematic and smectic liquid crystals

We have undertaken a theoretical study on the structure of planar solitons in nematic and smectic liquid crystals. In nematics we find a soliton solution which can be looked upon as an intertwine between two solitons. In a nematic obtained by unwinding a cholesteric and in a nematic with very high dielectric anisotropy, we have worked out energetics of solitons. A 2π soliton in a ferronematic or a smectic C becomes unstable due to the splay bend elastic anisotropy. The structures of π solitons in smectic A and smectic C in the neighbourhood of a S_A-S_C phase transition have also been studied.     

Surface tension measurements in freely suspended bubbles of thermotropic smectic liquid crystals

We present an experimental technique for the inflation of freely suspended films of thermotropic smectic liquid crystals to macroscopic bubbles. These bubbles have many properties in common with well known lyotropic soap bubbles, but on the other hand represent a completely different class of material with different physical properties and new phenomena. We describe the generation of such smectic bubbles and compare their physical properties with those of lyotropic soap bubbles. The measurement of inner pressure versus curvature is used for the determination of surface tensions of the smectic materials.     

Cybotactic nematic phases of photoisomerisable hockey-stick liquid crystals

New five-ring hockey-stick liquid crystalline materials with 4-bromoresorcinol as the central core unit and an azobenzene-based side arm were synthesised and their mesophase behaviour was investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and under a triangular wave electric field. Additional structural modification was done by introducing a lateral fluorine atom in the terminal ring of one of the side arms. It is found that regardless of the alkyl chain length or the lateral fluorine substitution, all of the prepared materials are liquid crystalline exhibiting nematic phases composed of cybotactic clusters of the SmC-type (N_CybC) in addition to a monotropic SmC phase for the longest homologue.     

Behavior of a thermotropic nematic liquid crystal confined to controlled pore glasses as studied by 129Xe NMR spectroscopy

The behavior of nematic liquid crystal (LC) Merck Phase 4 confined to controlled pore glass (CPG) materials was investigated using 129Xe nuclear magnetic resonance (NMR) spectroscopy of xenon gas dissolved in the LC. The average pore diameters of the materials varied from 81 to 2917 A, and the measurements were carried out within a wide temperature range (approximately 185-370 K). The spectra contain lots of information about the effect of confinement on the phase of the LC. The theoretical model of shielding of noble gases dissolved in liquid crystals on the basis of pairwise additivity approximation was applied to the analysis of the spectra. When pore diameter is small, smaller than approximately 150 A, xenon experiences on average an isotropic environment inside the pore, and no nematic-isotropic phase transition is observed. When the size is larger than approximately 150 A, nematic phase is observed, and the LC molecules are oriented along pore axis. The orientational order parameter of the LC, S, increases with increasing pore size. In the largest pores, the orientation of the molecules deviates from the pore axis direction to magnetic field direction, which implies that the size of the pores (approximately 3000 A) is close to magnetic coherence length. The decrease of magnetic coherence length with increasing temperature is clearly seen from the spectra. When the sample is cooled rapidly by immersing it in liquid nitrogen, xenon atoms do not squeeze out from the solid, as they do during gradual freezing, but they are occluded inside the solid lattice, and their chemical shift is very sensitive to crystal structure. This makes it possible to study the effect of confinement on the solid phases. According to the measured 129Xe NMR spectra, possibly three different solid phases are observed from bulk liquid crystal in the used temperature region. The same is also seen from the samples containing larger pores (pore size larger than approximately 500 A), and the solid-solid phase-transition temperatures are the same. However, no first-order solid-solid phase transitions are observed from the smaller pores. Melting point depression, that is, the depression of solid-nematic transition temperature observed from the pores as compared with that in bulk LC, is seen to be very sensitive to the pore size, and it can be used for the determination of pore size of an unknown material.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

First-order transition between nematic phases in lyotropic liquid crystals

Experimental evidence for the existence of a first order transition between N_c-N_d uniaxial lyonematic phases as a function of the variable M_d (number of decanol molecules per amphiphilic molecule) is reported. The relevance of this evidence to molecular models for micellar aggregates is discussed. The evidence is for a change in micellar symmetry at this transition.     

The relationship between liquid crystal alignment in the nematic and smectic C* phases

The pretilt angle in the nematic phase is related to two different surface dependent factors, while the quality of surface stabilized ferroelectric liquid crystal (SSFLC) devices is dependent on only one of them. We have synthesized two series of polyalkyleneimides and found that the pretilt angles for thick nematic cells are not related to the cone angles in SSFLC cells.     

Order parameters and densities in the smectic C, smectic A and nematic phases of a liquid crystal mixture

Measurements of the orientational and translational order parameters for the nematic, smectic A and smectic C phases of a commercial liquid crystal material are reported. The order parameters have been obtained by analysis of the angular distribution of the intensity of X-rays scattered by a sample aligned by a magnetic field. Results are presented as a function of temperature, and it is found that the apparent orientational order parameter in the smectic C phase decreases with decrease in temperature. This is explained using a model of random tilt. The experimental order parameters are compared with those calculated from a mean field model.     

End-chain ordering and even-odd effects in nematic thermotropic liquid crystalline mesophases studied by deuterium magnetic resonance

Deuterium quadrupole splittings are reported for the nematic mesophases of all isomers of 4,4′-di-n-octyl-d₄(d₆)-oxyazoxybenzene with CD₂(CD₃) groups at corresponding segments in the two end chains. These splittings do not decrease monotonically with increasing separation of the chain segment from the aromatic core as has previously been assumed. Comparison with similar measurements for 4,4′-di-n-heptyloxyazoxybenzene reveals an “even—odd” effect in the chain ordering, but only in so far as to reflect differences in the molecular order in the two mesophases. The results are used to test earlier theoretical calculations by Mar?elja.     

Internal orientations in externally deformed nematic polymer liquid crystals

The system of linear polymer liquid crystal (PLC) macromolecules, each modeled by semiflexible chain of alternate connected flexible and stiff rodlike sequences, is externally deformed. As a result, two orientation phases for hard rods are generated. One of them is nematic N+ with orientation parameter s>0; this has the place in monomer liquid crystal (MLC) systems and in PLCs. The nematic N- phase with s>0 appears in deformed PLCs only. This causes the fact that orientation of PLC chains, as a whole, is generated also by the system deformation. A discussion of that effect is the main goal of this article. The change of orientation is also discussed dependent on changes of the system temperature and parameters of the chain structure, such as the liquid crystal component concentration, contour lengths of stiff and flexible parts, and internal interactions parameters. Average shape of PLC chain and the shape anisotropy are calculated and discussed.     

Dielectric relaxation in coexisting nematic and smectic B phases

Mixtures of a polar solute 4-n-pentyl-4'-cyanobiphenyl in a non-polar nematic solvent exhibit two separated low frequency dielectric relaxations for concentrations of the solute between 2 mol% and 20 mol% over a limited temperature range. This behaviour is attributed to coexisting nematic and smectic B phases, in which the polar solute probe has different relaxation frequencies. The observed dielectric spectra can be accurately fitted to two Debye-like relaxations, and the strengths of the absorptions are proportional to the amounts of the coexisting phases. A microscopic determination of the phase diagram confirms the assignment of the coexisting phases, and it is concluded that there is a preference of the dipolar probe molecule for the smectic B phase, which is induced as a result of solute-solvent interactions.     

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (S_A1) or monolayer and partially bilayer (S_Ad) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A₁ phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC _m H_{2m + 1} terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A₁ phase slowly changes to be more like the smectic A_d phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (S_A1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A₁ phase is rapidly transformed into the smectic A_d phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.     

NMR spectra of chiral smectic liquid crystals differing in helical parameters

Chiral liquid crystals with three-ring rigid core were examined. The method based on the selectively reflected light was used to determine the helical pitch. The helical twist sense was worked out using the polarimetry method. ¹H and ¹³C NMR measurements were performed. The influence of three molecular structure parameters: the type of chiral centre, the length of non-chiral chain and substitution of benzene ring by fluorine atoms, on helical pitch, handedness of helical structure and values of chemical shift in proton and carbon spectra was determined. The change of the length of non-chiral terminal chain has the most significant influence on the temperature dependence of helical pitch. All tested parameters have the biggest influence on the values of chemical shift of atoms in the chiral centre.     

Rotational viscosity and molecular structure of nematic liquid crystals

An electro-optical technique was used to measure the rotational viscosity of nematic liquid crystals, whose structures were chosen to investigate the effects of linking groups and lateral groups on rotational viscosity. These results are compared with the theory developed by Osipov and Terentjev. It is shown that the theory is useful in predicting rotational viscosity for specific molecular structures. The agreement between theory and experiment is found to be good for all the liquid crystals studied.     

Orientational order of difluorinated liquid crystals: a comparative 13C-NMR, Optical, and dielectric study in nematic and smectic B phases

Structural and orientational order properties of 3Cy2CyBF2 and of 5CyCy2BF2 have been investigated by means of (13)C-NMR, optical, and dielectric spectroscopy methods. In the case of NMR, order parameters have been independently obtained from the analysis of either (13)C-(19)F dipolar couplings or (13)C chemical shift anisotropies, both measured from (13)C-{(1)H} NMR static spectra. The assignment of the (13)C resonances has been carried out thanks to the comparison with solution state spectra and DFT calculations, and the relevant geometrical parameters and (13)C chemical shift tensors needed to derive orientational order parameters have been calculated by DFT methods. In the analysis of (13)C-(19)F dipolar couplings, empirical corrections for vibrations and anisotropic scalar couplings have been included. Dielectric measurements have been performed over a broad frequency range for two orientations of the nematic director with respect to the measuring field. At low frequencies (static case) a positive dielectric anisotropy has been determined, which has enabled the calculation of the order parameters according to a well-tested procedure. At high frequencies the dielectric anisotropy changes its sign, a property which can be useful in designing a dual addressing display. The nematic order parameter determined from optical, dielectric, and NMR methods have been compared: their trends with temperature are very similar, apart from some slight shifts, and were analyzed by Haller and Chirtoc models. The differences among the results obtained by the four methods have been discussed in detail, also with reference to the assumptions and approximations used in each case, and to the results recently reported for similar fluorinated nematogens. The presence of a non-negligible order biaxiality has been related to the presence of a CH2CH2 bridging group, linking one cyclohexylic unit with either the other cyclohexyl or the phenyl ring.