High Pressure Investigations of the Phase Transitions in Cholesteryl Pentanoate and Cholesteryl Hexanoate

At high pressures too cholesteryl n-penta-noate (CH-5) and cholesteryl n-hexanoate (CH-6) exhibit only a cholesteric liquid crystalline phase. Our light reflection measurements up t o 2600 bars yield no indication of a pressure induced smectic phase which was suggested by Tikhomirova and Ginzberg' for CH-5 at about 850 bars and 120°C. The Obtained pressure (P_t)-temperature (T_t) phase diagrams show the expected widening of the cholesteric range with P_t and T_t. In the case of CH-5 the marked crystallization curve besides a melting curve could be explained by the transformation of the cholesteric into a metastable crystalline solid phase.     

Molecular Packing Coefficients of the Homologous Series Cholesteryl Alkanoates

The properties of cholesterol derivatives are important owing to the involvement of these compounds in a myriad of industrial and biological research areas such as thermography, display devices and the formation in atherosclerosis of arterial deposits containing cholesteryl esters. Molecular structural studies of mesogens in the crystalline state have been carried out in search of a better understanding of the physical properties which lead to mesophase formation. It has been demonstrated previously for a homologous series of nematogens¹ how the molecular packing coefficients of the solid phase vary with increasing chain length and how this throws light upon the variation of other physical properties. Here, we extend this treatment to the cholesteryl alkanoates. Detailed crystal structure analysis is available for only one member of this series: cholesteryl tetradecanoate, which was studied by Craven and de Titta.² Prior to this, unit cell dimensions and space groups has been obtained for 13 members by Barnard and Lydon.³ They found the crystallography of these compounds to be complex. No overall homology of crystal structure was apparent and at least eight different types of crystal structure occur.     

Order Parameter and Refractive Index of Cholesteryl Linoleate

Refractive indices of cholesteryl linoleate are determined at different temperatures and order parameters have been calculated in smectic and cholesteric phases.     

The Crystalline state and the Mesophases of Cholesteryl Oleyl Carbonate

The preparation of pure crystalline cholsteryl oleyl carbonate is described. The melting point, the mesophase transition temperatures, and the enthalpy of the melting process are recorded, and a method of maintaining cholesteryl oleyl carbonate in a pure state over extended periods of time is suggested.     

Optical Rotatory Power in the Isotropic Phase of Four Cholesteryl Esters

Recent theoretical work by numerous authors has indicated that pretransitional properties of the isotropic phase are highly dependent on the chirality of the nearby ordered phase. To test this aspect of the theories, we present results on the optical rotatory power in the isotropic phase for a series of four cholesteryl esters of varying chirality. The most important finding is that the separation between the first and second order transition temperatures decreases with an increase in chirality. However, the results of a recent experiment which claimed to present evidence for strong coupling between the structural modes of the ordered phase are not substantiated by the new measurements reported here.     

Investigation of the Cholesteryl Oleate Mesophases by X-Ray Diffraction

The following phase transitions occurring in the cholesteryl oleate were investigated by X-ray diffraction: solid-isotropic liquid, isotropic liquid-cholesteric, cholesteric-smectic.

The sample purity was 98%. Strong pretransitional effects were observed at the solid-isotropic liquid phase transition and at the cholesteric-smectic phase transition.

At this last transition the longitudinal coherence length ξ_∥ appears to diverge as the critical temperature is approached in the cholesteric phase, whereas the transversal coherence length ξ_τ increases at the critical temperature in the smectic phase in an abrupt way. It seems from the temperature dependence analysis of the angular position of the low angle peak that the smectic phase is a smectic A phase and that a progressive melting of chains occurs at the temperature increases.     

Photoreaction of Cholesteryl trans-Cinnamate in Mesomorphic States

Abstract

The photoreaction of cholesteryl trans-cinnamate in K Br matrix was studied in the solid, mesomorphic and isotropic liquid states at 25?225°C, and compared with the solution reaction in n-hexane. Cholesteryl trans-cinnamate was converted into a dimer on irradiation at wave-length longer than 300 nm at these phases. The photoirradiation in the solution led to a cis-isomer at 25?30°C. The initial rate of the dimerization at the mesophase was found to be higher than that at the isotropic liquid phase. The photoreaction in the solid took place at a much slower rate than in the other states. These suggest that some kinds of ordering besides the mobility of the molecules of the cinnamate enhance the rate and affect the course of the reaction on irradiation in comparison with the reaction in the isotropic solution.     

Pitch Behavior of the Cholesteric Helix at the Phase Transition Point Cholesteric/Smectic A up to 2500 Bars. Study of Some Cholesteryl n-Alkanoates

Experimental evidence is given, for the first time, for a finite pitch of the cholesteric helix at the phase transition point cholesteric/smectic A (Ch/S_A). This finite pitch changes into an infinite one at a given point on the phase equilibrium line Ch/S_A. Therefore this point is called the pitch infinity point (PIP).

With increasing n-alkyl chain length of the investigated cholesteryl n-alkanoates the pressure coordinate of the PIP is shifted to higher values up to a chain length of 13 C atoms. For the alkanoates with 13,15 and 17 C atoms the same PIP pressure of about 1000 bars was determined.

The Ch/S_A phase equilibrium line of the cholesteryl esters could at best be followed up to 2600 bars. The respective transition enthalpies were measured at normal pressure and the corresponding changes in volume calculated by means of the Clausius-Clapeyron equation.     

Some Peculiarities of Cholesteric-to-nematic Transition in Cholesteric Liquid Crystal Mixtures

The cholesteric-to-nematic (CN) transition in a binar compensated mixture consisting of 59% cholesteryl chloride and 41% cholesteryl pelargonate has been investigated. The peculiarities of temperature and field dependences of the CN-transition were determined by a diffraction method for a case when the sample thickness is greater than the cholesteric pitch. It is shown that the electric field induced cholesteric-to-nematic transition may occur through the distortions periodic in the full pitch of the cholesteric structure.     

Twist Grain Boundary Phases in the Binary Mixtures of Smectic and Cholesteryl Compounds

Abstract

The binary mixture of Terephthal-bis-p-n-hexadecylaniline (TB16A) and cholesteryl nonanoate (CN) exhibits I - N* - TGB _A - TGB _c*- Sm A - Sm I - Sm F - Sm G - K phases in the concentration region between 95 to 98% of TB16A at higher temperature range. The 90% of TB16A exhibits I - N* - TGB _A - TGB _c - Sm A - Sm I - Sm F - K phases. The mixtures of concentration between 50 to 80% of TB16A exhibits I - Sm A - Sm I - K phases. The frustrated blue phase is also observed in the mixtures of lower concentration of TB16A in CN at higher temperature. Optical, DSC and X-ray studies were carried out for the identification of the different phases. The phase diagram of the Chen-Lubensky model in this context is discussed.     

Growth Kinetics of Cholesteric Spherulites Under Electric Fields

The growth kinetics of cholesteric spherulites of cholesteryl laurate-cholesteryl caprilate mixture (50: 50% by weight) is investigated. It is show that the nucleation is bidimensional and the growth is determined by the orientational relaxation of the molecules near the interface. The same growth mechanism holds when the nucleation was followed under d.c. electric fields; in this case the growth rates were lower.

The electric field effects on growth kinetics can be explained by considering both changes in critical supersaturation and changes in some coefficients associated with dissipative effects during relaxation.     

Electric-Field Enhancement of Fluorescence Intensity of Cholesteric Liquid Crystals Containing Aromatic Chromophores

Fluorescence from cholesteric liquid-crystalline mixture containing cholesteryl 3-(l-pyrenyl)propanoate was measured under dc and ac electric field applied parallel to the helix axis. The excimer fluorescence of the pyrenyl chromophore increased and decreased its intensity reversibly upon turning the electric field on and off. Similar electric field effects were observed for such chromophores as tetracene, perylene, and 9,10-diphenylanthracene doped in the cholesteric mixture. The electric-field effect was observed only for fluorescence transitions which show left-handed circularly polarized fluorescence, indicating that only transitions which are polarized perpendicular to the optic axis in a quasi-nematic layer exhibit the effect. The experimental results were found to be intimately connected to the onset of the Helfrich periodic distortion of a planar structure.     

Electro—Optic and Dielectric Studies in Cholesteric Liquid Crystal Mixtures

In binary mixtures of cholesteryl chloride and cholesteryl alkanoates, the threshold field for the cholesteric to nematic transformation depends on the length of the alkanoate ester chain. The threshold field also depends on the pitch, the dielectric anisotropy and the twist elastic constant of the mixtures. This paper describes the experiments performed to establish the role of these three parameters in the observed variation of threshold field with alkanoate ester chain length in such binary mixtures.

The temperature dependence of threshold field, dielectric anisotropy and pitch has been studied. From these experiments the relative response of the various mixtures to applied electric fields has been established. In most of the mixtures, temperature ranges over which pretransitional effects do not predominate, have been identified. Twist elastic constants of the mixtures in these temperature ranges have been calculated. It has been shown that both the intrinsic response of the mixtures to applied electric fields, and the twist elastic constants do not show a regular variation with chain length of the alkanoate ester tail. Some observations suggest that mixtures containing non-smectogenic alkanoates show different temperature dependence of threshold field as compared to those containing smectogenic alkanoates.     

Behaviour of Ultrasonic Velocity in Cholesteric Liquid Crystals

Ultrasonic velocities (V) have been determined, employing a fixed path double crystal interferometer, in three cholesteric liquid crystals, namely cholesteryl propionate, cholesteryl laurate and cholesteryl myristate in their isotropic and anisotropic phases including the region of the phase transition. The variation of specific volume (v) is also studied in the same temperature range by a special dilatometer constructed for this purpose. Anomalous behaviour of ultrasonic velocity is observed near the isotropic-cholesteric phase transition in all three compounds. In the two polymesomorphic liquid crystals, namely cholesteryl laurate and cholesteryl myristate, contrary to the ultrasonic behaviour of cholesteryl stearate, prominent velocity dips are observed at cholesteric-smectic transition temperatures. The parameters adiabatic compressibility (β_ad) and molar sound velocity (R) are estimated and they are found to exhibit sudden jumps at cholesteric-smectic and isotropic-cholesteric transitions. The thermal expansion and temperature co-efficient of compressibility are found to show abnormal increase near the phase transition indicating the existence of large-magnitude pre-transitional effects near the phase transition. A comparative study of the ultrasonic behaviour of six aliphatic esters of cholesterol has shown that the magnitude of the velocity dip observed at the isotropic-cholesteric transition increased with increase of molecular weight and only cholesteryl acetate shows deviation.     

The Effect of Rhodamine 6G on Phase Transitions in Cholesteryl Ester Mixtures

Compensated cholesteric mixtures were the subject of a whole range of studies.^1-6 Due to their characteristic properties (the existence of a temperature at which compensation occurs and the dependence of the critical field on temperature), the orientation of guest molecules by compensated cholesteric hosts⁷ shows some particular features in comparison to the orienting mechanisms acting within nematic liquid crystals^8-15 or within chiral nematic mixture.^16,17 When the colour of sample varies due to the influence of the electric field and temperature the guest dye contributes to it as well the compensated cholesteric host.

In this paper we report the modification of compensated cholesteric mixture properties due to the introduction of Rhodamine 6G as guest.     

Infrared Spectroscopic Studies of Homologous Cholesteric Liquid Crystals

Abstract

Infrared (500–2500 cm^?1) spectra of cholesteryl alkyl carbonates (methyl, ethyl and oleyl) have been examined in the solid (cholesteryl methyl carbonate and cholesteryl ethyl carbonate) and liquid crystalline (cholesteryl oleyl carbonate) states. An attempt has been made to correlate the variation in the intensities and frequency shifts with the molecular structures. The shift in the carbonyl frequency in the case of cholesteryl oleyl carbonate may be correlated with the affect on the coplanarity with benzene ring.     

Helical Sense and Pitch of Cholesteric Liquid Crystals

The rotatory sense of a circularly polarized light transmitted through a cholesteric liquid crystal cell was studied by measuring the retardation of a birefringent plate placed upon the cell. Furthermore, the dependence of helical pitch on composition is described for various binary cholesteric and nematic-cholesteric mixtures. The helical sense of eleven cholesterics was determined by the experimental results. As a result, it is shown, for example, that cholesteryl chloride has a right-handed helical structure and cholesteryl propionate has a left-handed helical structure.     

Optical Effects and Molecular Ordering in Spherulites

The optical behaviour of spherulite textures exhibited by mixtures of cholesteryl acetate and cholesteryl ethyl carbonate is investigated. The observed optical effects arise due to a helicoidal arrangement of the molecules along the radial directions of the spherulites, the long axes of the molecules being oriented transverse to the radial directions. The helicoidal nature of the structure is strikingly demonstrated by two sensitive interference experiments wherein sinusoidal interference fringes are observed. The observed results are in agreement with the theory developed for the interference effects.     

High Pressure Studies on Mesomorphic and Polymesomorphic Transitions

The paper presents the following results: (1) pressure induced mesomorphism in p-p'-n-butylazobenzene and biphenyl p-ethyl benzoate. (2) the monotropic nematic-isotropic transition in trans-p-n-propoxy-x-methyl cyanophenyl cinnamate becoming enantiotropic under pressure, (3) suppression of the cholesteric mesophase in cholesteryl nonanoate and the location of a solid-cholesteric-isotropic triple point at ～ 2.85 kbar. confirming an earlier observation of Keyes. Weston and Daniels on the same compound, (4) a measurement of dT/dp for the solid-smectic A, smectic A-cholesteric and cholesteric-isotropic transitions in cholesteryl myristate and a comparison with the values evaluated from the Clausius-Clapeyron equation using the known latent heat and volumetric data.