Calorific Effect of the Magnetically Induced Phase Transition in Induced Chiral Nematic Systems

The study of the calorific effect of the magnetically-induced phase transition in doped nematic phases is described. The systems examined consisted of PCB as a nematic host phase and S-N-1-phenylethyl-4n-alkylbenzamides as non-mesogenic optically active dopants. The calorific effect was estimated indirectly from the temperature dependence of the magnetic-field-threshold of the helical unwinding and of the reverse process; i.e. the helical winding. It appears that it is the ratio of the length of an optically active dopant and that of the nematic host which determines the temperature dependencies of such parameters as the helical pitch, microscopic twisting power, and latent heat. A comparison with the magnetocaloric effect in nematic liquid crystals¹ is also made.     

Thermodynamics of Solutions of Various Nonmesomorphic Solutes in the Nematic and Isotropic Phases of 7CB

Thermodynamic results from gas-liquid chromatography, (GLC) are reported for thirty two nonmesomorphic solutes, at infinite dilution, in the nematic and isotropic phases of p-n-heptyl-p'-cyanobiphenyl (7CB). The solutes fall into five categories; normal alkanes, branched alkanes, alkenes, hexadienes and aromatics. The results are examined within the framework of statitical thermodynamic models. The observed trends are interpreted on the basis of solute structure (e.g. shape, size, flexibility, polarity and polarizability). The results conclusively show that the presence of one or more double bonds in the solute structure increases its compatibility with the nematic and isotropic phases of the liquid crystalline solvent. This effect is more drastic when the double bonds are conjugated. Other implications of this study include shape selective GLC separation of geometric isomers and an examination of the effect of solute structure on the stability of the nematic phase at finite solute concentrations.     

Dielectric Behaviour of the Nematic Phase of 4-n-Pentyloxyphenyl 4-(3-Bromo-4-n-nonyloxybenzoyloxy)-3-ethyl-benzoate (PBNEB)

Due to the high viscosity of 4-n-pentyloxyphenyl 4-(3-bromo-4-n-nonyloxybenzoyloxy)-3-ethyl-benzoate (PBNEB) the dielectric reorientation processes take place in the kHz and MHz range respectively. We observed two different relaxation ranges in the nematic phase from 10 Hz to 10 MHz if the measuring field is parallel to the director and one relaxation range for the perpendicular orientation. Dielectric measurements at 6.7 GHz demonstrate that at this frequency ϵ′∥ and ϵ′⊥ can be regarded as high-frequency limit. The experimental values are discussed from the point of view of the Meier-Maier theory.     

High Pressure Studies of the Phase Transition Enthalpies of the Liquid Crystal EBBA up to 2.5 kbar

The liquid crystal p-Ethoxy-benzylidee-p-n-butylaniline(EBBA) exhibits one nematic-liquid isotropic and two solid-nematic coexistence lines.

With a high pressure micro calorimeter based on differential scanning calorimetry (DSC) previously described the phase transition enthalpies of these three phase transitions have been measured in the pressure range 1 bar to 2.5 kbar. Numerical data are presented for the transition temperatures, the enthalpy changes, the entropy changes, and the volume changes for both solid-nematic transitions as well as for the nematic-liquid isotropic transition.

Additionally the effect of impurities on the nematic-liquid isotropic transition enthalpy at normal pressure is discussed.     

On the Transition between Cholesteric and Nematic Phases in Magnetic Field

Singularities at the critical lines bounding the cholesteric and nematic phases on the magnetic field versus temperature plane are investigated. Critical exponents, α and γ, of the specific heat and magnetic susceptibility are found both unities. By regarding the inverse of pitch of the cholesteric structure as an order parameter, it is found that the exponents β and δ are zero and infinity, respectively, and these exponents satisfy the scaling relations, α + 2β + γ = 2 and α + β (δ + 1) = 2. In relation to the discommensurate charge density wave in layer structure materials, it is noted that the transition from the nematic to cholesteric phase occurring with the change of magnetic field and/or temperature is interpreted as a condensation process of solitons.     

Anisotropic Molecular Orientation of Micellar Nematic Phase in a Binary Mixture of Two Nonmesogenic Compounds

We report the results of our studies on the optical and thermal properties of two nonmesogenic compounds, namely, Cetyl-dimethyl-ethylammonium bromide (CDEAB) and Glacial acetic acid. The mixture exhibits very interesting schilieren texture of micellar nematic phase, SmA, SmC*, and SmE phases, respectively, at different concentrations of CDEAB sequentially when the specimen is cooled from its isotropic phase. The order parameter (S) of the micellar nematic phase is estimated with the help of temperature dependence of optical anisotropy from the measured values refractive index and density data. The temperature variation of order parameter of the experimental curve is very well fitted with the Mayer–Saupe theoretical curve. X-ray studies have been discussed. The formation of above phases has been confirmed by optical and DSC studies.     

On the Non Linear Behaviour of Nematic Liquid Single Crystal Elastomers (NLSCE) Under Biaxial Mechanic and Electrical Force Fields

A continuum theory has been developed to give account of the three characteristic zones appearing in the “semisoft” behaviour of nematic single crystal elastomers under the effect of biaxial mechanic and electrical force fields considering higher order coupling terms. The analysis is based on a model which takes into account two coexistent coupled components showing a preferred direction. Two cases are analysed according to the type of input: stress and displacement. Bifurcation phenomena are predicted in the first case. Sress-strain curves show the characteristic features experimentally observed. The case where shear terms in the strain are considered is approximately analysed.     

Nature of the Maximum Microsegregation of Binary Alloys from Peritectic Phase Systems

A value of the distribution coefficient is of a significant importance for the microsegregation phenomenon. Moreover, the nature of the microsegregation is due to a given phase system, and the Sb–30 Sn alloy has been set as an example to show the maximum microsegregation which can form during solidification of this alloy. A new relation, supplementary to the Scheil equation has been proposed.     

Chain Ordering in the Nematic Phase of 40ba

From the intensity profile of the X-ray diffraction pattern in the nematic phase of 40BA it is inferred that the alkoxy chains have an orientational order.     

On the temperature Dependence of the orientational Order Parameter in the Nematic Phase of Cyanophenyl Behzoates

The orientational order parameters fot two liquid crystal materials, 4-cyanophenyl 4-butylbenzoate and 4-cyanophenyl 4-pentylbenzoate, have been derived by measuring the change in the refractive index as function of temperature. The order parameters are compared with MaierSaupe theory, and the sharpness of the transitions has been shown using the Haller's plot.     

TSD of Liquid Crystals in the Supercooled Nematic Phase

MBBA and EBBA are used to apply the techniques of thermally stimulated depolarization (TSD) to liquid crystals in their glassy state. The orientational mobilities of the butyl and methoxy resp, ethoxy group show significant differences compared to the crystalline phases. A small TSD band in MBBA at 69.5 K and in EBBA at 64.5 K probably comes from relaxations of the butyl group. A ten times stronger TSD band at 117.5 K resp. at 124 K is due to relaxations of the methoxy respect to the ethoxy group. Using the partial heating technique and the thermal sampling technique a detailed analysis of this band allows more insight into the distribution of reorientation enthalpies and relaxation times.     

Modern Classification of Electrohydrodynamic Instabilities in the Nematic Phase

Physical models for various electrohydrodynamic instabilities are reviewed and an up-to-date classification is presented.     

On the Problem of the Mechanism of Binary Acoustic Action on a Homeotropic Structure of Nematic Liquid Crystals

Crystallography Reports - A universal model of the distortion mechanism of a homeotropic nematic liquid crystal layer under joint action of the periodic shear and compression has been verified and...     

New Finding of the Glass Transition Phenomenon in Surfactant Gel Phase: The Binary System of Water and Dioctadecyldimethyl-Ammonium Chloride

Differential scanning calorimetry on the two-component system of dioctadecyldimethylammonium chloride (DOAC)–water was carried out in the temperature region from 80 °C down to -100 °C. As the result, a new type of glassy state was found around at -40 °C for the supercooled complete gel phase.     

Nematic Behavior of 5OCB in a Dielectric Medium at Phase Transition Temperature ‐A Statistical Analysis

A statistical analysis has been carried out to determine the configurationally preference of a pair of 5OCB (4′‐n‐pentyloxy‐4‐biphenylcarbonitrile) molecules with respect to translatory and orientational motions. The CNDO/2 method has been employed to evaluate the net atomic charge and atomic dipole components at each atomic centre of the molecule. Modified Rayleigh‐Schrodinger perturbation theory along with multicentered‐multipole expansion method has been employed to evaluate long‐range intermolecular interactions, while a ‘6‐exp’ potential function has been assumed for short‐range interactions. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration in vacuum and in a dielectric medium at phase transition temperature using Maxwell‐Boltzmann formula. It has been observed that the probabilities are redistributed in dielectric medium and there is considerable rise in the probabilities of interactions, although the order of preference remains the same. On the basis of stacking, in‐plane and terminal interaction energy calculations, all possible geometrical arrangements of the molecular pairs have been considered. The results are discussed in the light of experimental as well as other theoretical observations.     

Study of the Smectic a Nematic Transition in Octyl and Nonyl Cyanobiphenyl

A new DSC method is used to study a smectic A to nematic transition. The measurement of the height of the DSC transition peak gives the order of the transition. The smectic A to nematic transition of octyl cyanobiphenyl was found to be second order while the same transition was found to be first order for nonyl cyanobiphenyl.     

Anisotropic Surface Order in the Isotropic Phase of Nematic Liquid Crystals

A spinning-drop technique was used to study the thermal variation of surface tension of two nematic liquid crystals at the interface with an isotropic fluid. A sharp increase in interfacial tension was observed in the vicinity of the nematic-isotropic transition. The interfacial tension-temperature characteristics, unlike monotonically decreasing dependence found in most isotropic fluids, showed regions of positive slope, both in the isotropic and anisotropic phases. A positive slope in the isotropic region of the liquid crystal is interesting since it implies that excess order has developed at the surface while the buk interior remains isotropic.     

Phase Transition Studies in Some 4-n-Alkyl-N-(4-n-Alkyloxy-Benzylidene)-Anilines

The results of phase transition investigations in two homologous series of 4-n-alkyl-N-(4-n-alkyloxy-benzylidene)-anilines, viz., 60. m and 70. m compounds using thermal microscopy, differential scanning calorimetry, density and refractive index measurements are reported. The nature of the order of various phase transitions and the pre-transitional effects in the fluctuation dominated non-linear regions in the vicinity of IN, IA and NA transitions are discussed in the light of the available data on other nO. m compounds.     

The Behaviour of Contact Angles of Wetting for Liquid Crystalline Drops on Glass Near the Phase Transition

The experimental values of contact angles of wetting for pazoxyanisole - glass in the nematic and isotropic phase have been obtained. At the point the phase transition, a change in the value of contact angle of wetting is observed. The values of adhesion energy, have been calculated for different orientation of molecules with respect to the glass support.

It is well-known /I/that contact angles of drops on solid supports enable an information on interfacial energy γ_SL to be obtained. A change in the structure of liquid results in alteretion of the contact angle as it a has been shown recently for polymer solutions /2/. It seem to be quite possible that orientational ordering of liquid crystals has to affect the value of contact angles of wetting. Nevertheless, this value was not found to very upon phase transition from the isotropic to liquid crystalline state /3/.

We studied the behaviour of contact angle of wetting for a drop of a classical liquid crystalline compound p-azoxyanisole (PAA) in its nematic and isotropic phases.