Mixtures Of Nematogens Research Articles

Mean-field model for a mixture of biaxial nematogens and dipolar nanoparticles.

We analyze a mean-field model for mixtures involving biaxial nematogens and dipolar nanoparticles, taking into account not only orientational and isotropic pair interactions between nematogens but also orientational nematogen-nanoparticle interactions. We determine bulk equilibrium phase diagrams for a wide range of interaction strengths, identifying in each case the effect of the nanoparticles on the stability of nematic phases and on the appearance of multicritical points. Special attention is given to the limit of a low concentration of nanoparticles, in which their effect on the temperatures of both the first-order uniaxial-isotropic and the continuous biaxial-uniaxial transitions is investigated in detail.

Pretransitional effects in a mesogenic mixture under an electric field.

Nematogenic liquids are materials that are in high demand for many application purposes. Field-induced nematic order in isotropic liquids can be observed by the enhancement of electric permittivity during dielectric spectroscopy measurements under the bias electric field. For this reason, dielectric measurements were done on a nematogenic mixture of highly polar compounds. This paper presents a discussion on the influence of the bias electric field on the dielectric response. We observed a decrease of electric permittivity in the isotropic phase under the bias electric field. This effect is most likely caused by the field-induced creation of some complex molecular objects. In the paper, relaxation parameters of the molecular modes were analyzed. The field-induced orientational order was determined using a dichroic ratio in a guest-host system. The measurements show the significant order parameter in the isotropic phase under an electric field.

Induced smectic phase in binary mixtures of twist-bend nematogens.

The investigation of liquid crystal (LC) mixtures is of great interest in tailoring material properties for specific applications. The recent discovery of the twist-bend nematic phase (NTB) has sparked great interest in the scientific community, not only from a fundamental viewpoint, but also due to its potential for innovative applications. Here we report on the unexpected phase behaviour of a binary mixture of twist-bend nematogens. A binary phase diagram for mixtures of imino-linked cyanobiphenyl (CBI) dimer and imino-linked benzoyloxy-benzylidene (BB) dimer shows two distinct domains. While mixtures containing less than 35 mol % of BB possess a wide temperature range twist-bend nematic phase, the mixtures containing 55–80 mol % of BB exhibit a smectic phase despite that both pure compounds display a Iso–N–NTB–Cr phase sequence. The phase diagram shows that the addition of BB of up to 30 mol % significantly extends the temperature range of the NTB phase, maintaining the temperature range of the nematic phase. The periodicity, obtained by atomic force microscopy (AFM) imaging, is in the range of 6–7 nm. The induction of the smectic phase in the mixtures containing 55–80 mol % of BB was confirmed using polarising optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction. The origin of the intercalated smectic phase was unravelled by combined spectroscopic and computational methods and can be traced to conformational disorder of the terminal chains. These results show the importance of understanding the phase behaviour of binary mixtures, not only in targeting a wide temperature range but also in controlling the self-organizing processes.

Mesophase stability in binary mixtures of monotropic nematogens

Binary phase diagrams were constructed from laterally substituted methyl azo/ester derivatives, namely 4-(4″-substituted phenylazo)-3-methyl phenyl-4″-alkoxy benzoates (Ina–d). In this group of compound the unsubstituted and chloro-substituted derivatives possess the nematic phase monotropically, while the nitro and methyl analogues are enantiotropically nematogenic. The binary phase diagrams constructed were made once from the monotropic nematogens with each other, and another with the enantiotropic nematogens. In both the cases enantiotropic nematic phase was observed that covers a wide range of composition. The mesophase behaviour of all binary mixtures was investigated by differential scanning calorimetry (DSC) and polarised light microscopy (PLM). The nematic phase was exhibited in all binary mixtures. Independent of the alkoxy chain length, the entropy change, ΔSN–I of the N–I transition of pure components was found to vary irregularly with the anisotropy of polarisability (∆αX) of the polar substituent, X.

Induced Smectic Phase in a Binary Mixture of Nematogens

The phase diagram of binary system of nematogenic compounds 4-n-pentyl phenyl 4-n′-hexyloxy benzoate (ME6O.5) and 4-cyanophenyl 4-heptyl benzoate (CPHB) shows the presence of induced smectic phase. In the present paper, we report the results of DSC, refractive index, and density measurements of different binary mixtures of (ME6O.5 + CPHB) throughout the entire composition range. The density and refractive index values have been analyzed to obtain orientational order parameters. We have fitted the experimental order parameters with the theoretical values from McMillan's theory for mixtures having smectic phase, and Maier–Saupe theory for others having nematic phase only.

Orientational order and molecular parameters of two nematogenic mixtures from X-ray studies: confirmation of electro-optical results

This study entails the report of X-ray studies conducted on two nematogenic mixtures (code names ZLI 1221 and ZLI 1291) containing identical moieties: phenyl cyclohexane, biphenyl cyclohexane and cyclohexyl benzoate, but exhibiting different mesogenic ranges and properties. X-ray studies have been undertaken with a view to investigate the difference in the mesogenic behaviour of the said mixtures at the molecular level. The average apparent molecular length ‘l ’ and intermolecular distance ‘D’ of the two mixtures and their variations with temperature have been determined from X-ray data. The orientational order parameters ⟨P 2⟩ and ⟨P 4⟩ and their thermal variations have also been obtained. Comparison has been made with the ⟨P 2⟩ values obtained from X-ray study with those obtained from optical study. The average apparent molecular lengths of the two mixtures corroborate the findings regarding the relative molecular weights of the two samples obtained from optical and dielectric data.

Tracer colloids close to an isotropic–nematic domain interface with phase transition-induced solute transport

In mixtures of nematogens with isotropic solutes, the equilibrium solute concentration in the nematic phase of the medium is usually lower than in the isotropic phase. Under constant cooling this results in continuous solute transport across the domain interface during phase transformation, leading to a non-homogeneous solute concentration around the drop. In this case, the interaction of colloids with an approaching isotropic–nematic interface is influenced by the interaction with the transferred solute. In 4-pentyl-4-cyanobiphenyl (5CB) mixtures containing small amounts of octane or tetrachloroethylene, we visualized an attractive interaction between tracer colloids and nematic droplets within a range of several micrometers from the droplet surface. This attraction depends on the interaction of the colloid surface with the solute and requires droplet growth, i.e., a non-homogeneous solute concentration around the droplet. The colloid interaction with this concentration gradient is consistent with prediction from diffusiophoretic models.

Pretransitional behavior of dielectric permittivity on approaching a clearing point in a mixture of nematogens with antagonistic configurations of dipoles.

The results of studies of static dielectric permittivity epsilon(T) in the isotropic phase of 4-cyano-4-pentylalkylbiphenyl and n-p-methoxybenzylidene-p'-butylaniline (5CB-MBBA) mixtures are presented. 5CB and MBBA are nematogens with antagonistic permanent dipole moments configurations. An increase in MBBA concentration strongly decreases the pretransitional effect. However, a derivative analysis detected the existence of a pretransitional anomaly even if its weakness made a straightforward epsilon(T) fit impossible. The obtained anomalies can be well portrayed by fluidlike equations, isomorphic to that applied in critical binary mixtures. Every time the same value of the specific heat critical exponent alpha approximately 0.5 was obtained. A preliminary discussion of the influence of the permanent dipole configuration on the pretransitional behavior of dielectric permittivity was also possible.

Equilibrium Phase Properties of Polymer/Liquid Crystal Blends: Theory and Experiments

A theoretical formalism to calculate the phase diagrams of polymer/liquid crystal mixtures is presented. Mixtures involving linear and crosslinked polymers together with low molecular weight nematic and smectic-A liquid crystals are considered. The generalization to binary mixtures of nematogens is briefly shown. The method of calculating model phase diagrams of side chain liquid crystal linear and crosslinked polymers in either isotropic or nematic solvents is presented. In nematogen mixtures, the coupling parameter induces tremendous changes in the miscibility of both linear and crosslinked polymers with low molecular weight analogous or different liquid crystals. Applications of the theoretical formalism are given.

Refractive Index, Density and Distribution Function in Nematic Mixtures

Using refractive index and density data for the nematogenic mixtures Octyl benzoic acid (OBA) and Nonyl benzoic acid (NBA), orientational order and hence the distribution function has been determined. From this higher order parameter <P4> has been computed.

On the phase equilibria of nematic mixtures

The theory of Brochard et al. for mixtures of nematogens is applied to the case of crosslinked polymers with side chain liquid crystals and low- molecular-weight liquid crystals. Phase equilibria of those mixtures are analyzed by allowing the Flory–Huggins interaction parameter for isotropic mixture and the rubber elasticity parameters of the elastic network to be functions of the polymer volume fraction. A wide range of crosslinking density is covered showing a strong influence on the phase behavior of such compounds. The clearing temperature of nematic-isotropic transition of each nematogen is different and the cross-nematic interaction parameter is a geometric average of the parameters for pure nematogens. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 478–485, 2000

Fluorescence photobleaching recovery spectroscopy in a dye doped nematic liquid crystal

The in situ study of the diffusion dynamics of fluorescently tagged proteins or dextrants within cells or biologically relevant fluids is a customary practice in the field of biophotonics @1,2#. Normally, dyes which bind to specific macromolecules or proteins are employed as tracers to follow the particular dynamics of the latter. The dyes can be resonantly pumped by an external laser which is usually focused by a microscope objective. The fluorescence emitted by the tagged molecules can be collected by the same objective, to be subsequently filtered and detected. If photobleaching of the dyes takes place within the focal region of the objective, a decay of the fluorescence signal is observed in the early stages of laser illumination. The decay time produced by photobleaching depends on the input power, specific laser wavelength, and peculiarities of the absorption spectrum of the dyes. On the other hand, if the laser is turned off for a given period of time, a recovery of the fluorescence signal can be observed when the laser illumination is reinstated, for diffusion of new molecules takes place from the neighboring regions into the focal volume of the laser spot. In this manner, the diffusion process of the tagged molecules themselves can be studied. Diffusion of biomolecules is, in fact, one of the most basic an important mechanisms in cell biology @3#, and can be microscopically studied in this manner. The term fluorescence photobleaching recovery spectroscopy ~FPRS! has been coined as a general description of this method. The technique has different variations according to the details of the specific experiment. In particular, the geometrical aspects of the illuminating optics and the imaging method to monitor the fluorescence emission ~normal, confocal, etc.! determine the type of experiment under consideration. Very recently, the usage of two-photon scanning microscopy in addition to FPRS has been proposed @4# as a method for creating a flat, almost two-dimensional, distribution of photobleached molecules, allowing a better control of the initial conditions before diffusion takes place. Liquid crystals ~LC’s ! are complex fluids with intrinsic long range orientational order and intricate hydrodynamic properties @5#. Dye molecules can be diluted within a LC to take advantage, in this manner, of the intrinsic molecular order of the liquid crystalline phases, and become oriented along particular directions. The induced macroscopic order of the dyes was called guest-host interaction by Heilmeier, Castellano, and Zanoni, who observed it for the first time@6#. Since the absorption of the dyes is normally very anisotropic, dye doped LC’s are ideal systems where dichroism can be controlled by external parameters like temperature and magnetic or electric fields. Furthermore, the addition of dyes to nematic liquid crystals has been shown to be the source of several very interesting and complex nonlinear optical properties. In particular, the observation of enhanced optical torques @7,8#, unusual reorientation dynamics upon ultrashort laser illumination @9#, and permanent holographic recording of patterns @10# are a few incomplete examples of the rich optical properties of these systems. From the viewpoint of applications, dye doped LC’s seem to be promising substances for electro-optic devices with enhanced contrast ratios in the visible @11#. The diffusion process of dyes in LC’s has heretofore been poorly studied. In this paper, we shall concentrate on the study of dye diffusion through both oriented and unoriented nematic liquid crystal hosts. In particular, the application of FPRS, the effect of temperature, and a direct view into the diffusion process by means of digital imaging of the fluorescence emission will be demonstrated. Dye diffusion within an ordered nematic LC cell takes place in an anisotropic manner, which ensures the long term preservation of the dichroism. A direct view into the diffusion process of photobleached dye molecules will be shown by virtue of a twobeam FPRS technique. The interplay between photobleaching and diffusion will be explicitly shown, as well as the effect of temperature on a planar oriented LC cell where dye molecules are locally photobleached. Moreover, we show that Raman rather than fluorescence signals could be used to study the interdiffusion of transparent substances like binary mixtures of nematogens, where intrinsic fluorescence cannot be used. The paper is organized as follows: Section II presents the experimental setup for the forthcoming experiments, the sample preparation, a description of the interpretation of the data, and the experimental findings. The necessary theoretical background is also supplied in Sec. II, whenever it is relevant for the interpretation of the data. Finally, in Sec. III, a brief discussion is presented and a few conclusions are

Experimental and Theoretical ab Initio Study of the 13C−13C Spin−Spin Coupling and 1H and 13C Shielding Tensors in Ethane, Ethene, and Ethyne

Experimentally and theoretically (ab initio) determined CC spin−spin coupling tensors and 1H and 13C nuclear shielding tensors are reported for ethane (13C2H6), ethene (13C2H4), and ethyne (13C2H2). The experimental anisotropies of the CC coupling tensors, ΔJCC, for all these molecules, and also the combination JCC,xx − JCC,yy for ethene, were derived from sets of anisotropic couplings (Dexp) analyzed from the 1H and 13C NMR spectra of molecules partially oriented in liquid−crystalline environments. Both harmonic vibrations and structural deformations arising from the correlation of vibrational and reorientational motions were taken into account in the D couplings. The ab initio calculations of all the J tensors were performed using MCSCF linear response theory. The best calculated and experimental ΔJCC values (along with JCC,xx − JCC,yy for ethene) are found to be in good mutual agreement. Together with earlier work on the nJCC tensors in benzene, this study shows that the indirect contribution, 1/2JCCaniso, to experimental couplings between differently hybridized carbons is small and can generally be omitted. This means that the use of experimental DCC couplings in the determination of molecular order tensors and/or conformation does not introduce serious errors. The experimental determination of the 1H and 13C shielding tensors was based on the liquid crystal director rotation by 90° in mixtures of thermotropic nematogens with opposite anisotropy of diamagnetic susceptibility. Ab initio SCF and MCSCF calculations utilizing gauge-including atomic orbitals produce results in good agreement with experiments.

Pretransitional behaviour of nematogenic mixtures 2H NMR investigation

T. W. Cheung, S. Fan, G. R. Luckhurst and D. L. Turner, J. Chem. Soc., Faraday Trans., 1997, 93, 3099 DOI: 10.1039/A702282D

Phase diagrams of binary mixtures of biaxial nematogens

Abstract A mean field theory is used to describe nematic phases of binary mixtures of biaxial molecules. Using a general pseudopotential consistent with the D2h symmetry of the constituent particles, the theory is used to calculate the elements of the order tensors necessary to describe the orientational order in binary mixtures in both uniaxial and biaxial nematic phases. For a single component, the model only requires one parameter, r 2, a ratio of anisotropic interaction strengths, to predict the temperature dependence of the four order parameters. The temperature dependence of the orientational distribution functions is illustrated for both rod-like and plate-like molecules. For binary mixtures, three anisotropic interaction strengths, r 1, r 2, and r 3, are needed to calculate the order parameters of both components as a function of concentration and temperature. The free energy is evaluated to predict the phase stability of the mixture. By systematically varying the anisotropic interaction strengths...

Computer simulation studies of anisotropic systems. XVIII. Re-entrant phase separation in nematogenic mixtures of cylindrical and spherical particles

Addition of a solute composed of quasi-spherical molecules to a nematic liquid crystal is known to depress the nematic–isotropic transition temperature. A biphasic régime, consisting of coexisting nematic and isotropic phases, is also created at the transition. A molecular field theory of such mixtures, developed by Humphries and Luckhurst, predicts, in addition, the appearance of a re-entrant biphasic region following the nematic phase for a narrow range of compositions. This unusual re-entrant phase separation has not been observed for real nematogenic mixtures, presumably because the system freezes before the re-entrant phases can be formed. Here we report the observation of this biphasic régime for a model nematogenic mixture, formed from cylindrical and spherical particles, which was studied using the Monte Carlo technique of computer simulation. The particles are confined to the sites of a simple cubic lattice but still retain their rotational freedom; in consequence the mixture is unable to freeze in the conventional sense. The temperature variation of the heat capacity and the solute–solute radial distribution function reveal the predicted transition to the re-entrant biphasic régime. The internal energy and the second-rank orientational order parameter were also determined as a function of temperature. The predictions of the Humphries–Luckhurst theory are found to be in good qualitative accord with the results of the simulation.

A Van Der Waals Model for the Nematic Liquuid Crystal Mixtures

Abstract Phase diagrams and excess molar volumes of binary nematogenic mixtures are investigated in the frame of a van der Waals - type theory for systems made up of hard rectangular parallelepipeds. It is shown that the excess volumes can be positive or negative or can change sign depending on temperature and composition. In agreement with experiment the model predicts a coexistence of two nematic phases for mixture of rod-like and plate-like molecules.

N.M.R. investigations and the molecular field theory of nematic mixtures

Abstract N.M.R. measurements have shown that the mixture E5 can be described by a single order parameter. Based on molecular field theory the nematic-isotropic transition and the temperature dependence of the order parameter in the nematic phase are considered for a binary mixture of nematogens. Guided by the results of the N.M.R. measurements the binary mixture is treated as an effective medium characterized by a single order parameter. Soft attractive forces are taken into account as well as the excluded volume. The coexistence of nematic and isotropic phases in the phase transition region is discussed in detail.

Experimental Studies on a Terminally Non-Polar Reentrant Nematogenic Mixture

Abstract We present here the results of our x-ray, high pressure and dielectric studies on a reentrant nematogenic mixture of n-dodecyl-4-(4-ethoxybenzylideneamino)-α-methylcinnamate and 4-n-heptyloxyphenyl-4-(4-ethylcyclohexanoyloxy)benzoate, neither of which possesses a strongly polar end group. The layer spacing measurements show that the smectic A phase is probably of the partially bilayer type. While the pressure-temperature diagram of this mixture is similar to that observed for strongly polar reentrant systems, the dielectric results show some significant differences.

Dimethylmercury in mixtures of thermotropic nematogens

Abstract The geometry of dimethylmercury was studied in several mixtures of the liquid crystals ZLI 1167 and phase 4. The variation of the geometry is not very large but systematic changes can be detected as a function of the liquid crystal composition. The anisotropics, δ[sgrave], of the 1H, 13C and 199Hg shielding tensors were investigated by applying two methods based on mixing liquid crystals with opposite diamagnetic anisotropies and by applying various referencing methods. Apart from these, the anisotropy, δJ HgC, of the 199Hg–13C spin–spin coupling tensor was studied in four mixtures of these liquid crystals, and was found to vary with the solvent.