Cholesteric Esters Research Articles

Rheological properties of cholesteric liquid crystals as lubricant additives

Rheological properties of Cholesteryl n-valerate, Cholesteryl decanoate and Cholesteryl myristate which are esters of cholesterol have been studied. Phase transition temperatures and rheological parameters such as viscosity, elastic modulus G[Formula: see text], loss modulus G[Formula: see text] as functions of temperature, shear rate and time are investigated. In frequency sweep test, a higher transition crossover region has occurred for Cholesteryl myristate, whereas for Cholesteryl n-valerate a frequency-independent plateau prevailed for both the moduli. The occurrence of blue phase in Cholesteryl decanoate during temperature sweep measurements is an indication for the rheological support. The results for steady state have informed that cholesteric esters are having non-Newtonian flow behavior in their respective cholesteric phases. The power-law model has explained well the shear rate dependence of shear stress. A few practical applications of these esters as lubricant additives are discussed, too.

Cholesteric ester of p-nitrobenzoic acid: Mesomorphic properties, crystal and molecular structures

The mesomorphic properties of cholesteric ester of p-nitrobenzoic acid С34Н49NO4 were studied by differential scanning calorimetry and polarizing thermomicroscopy. Its thermal stability was determined using TGA data. The molecular and crystal structures were characterized by single-crystal X-ray diffraction. Crystals were monoclinic, а = 10.7023(4) A, b = 10.0995(4) A, с = 14.1563(6) A, β = 101.499(1)°, space group Р21, Z = 2. An extensive molecule contained three different structural moieties: p-nitrobenzoic acid moiety, a cholesteric core consisting of four conjugated rings of different conformation, and a hydrocarbon chain of six carbon atoms. A molecule was chiral and “left” with a torsion angle of 2.6°.

Chiral Phases of a Confined Cholesteric Liquid Crystal: Anchoring-Dependent Helical and Smectic Self-Assembly in Nanochannels

Chiral liquid crystals (ChLCs) allow a fundamental insight into the interplay of molecular chirality and the formation of macroscopic, self-assembled helices. They also exhibit unique optical properties, in particular huge polarization rotation, which is employed in a wide range of photonic technologies. Here, we present a study of linear and circular optical birefringence in combination with X-ray diffraction experiments on an archetypical ChLC, i.e., the cholesteric ester CE6, confined in cylinders of mesoporous alumina and silica with distinct polymer surface graftings resulting in normal or tangential wall anchoring. The unconfined ChLC exhibits a discontinuous, first-order isotropic-to-chiral nematic (cholesteric) phase transition with the formation of double-twist helices and a discontinuous cholesteric-to-smectic A transition. The thermotropic behavior of the confined ChLC, explored in a channel radii range of 7–21 nm, deviates substantially from bulk behavior. There is no isotropic state. In contrast, a chiral paranematic phase with a preferred arrangement of the ChLC at the channel wall is found. For normal anchoring, a radial-escape structure evolves upon cooling. The phase transition to the smectic phase is completely suppressed. For tangential anchoring, a large optical activity indicates a continuous paranematic-to-cholesteric transition with double-twist helices aligned parallel to the long axes of the cylinders. Upon cooling, these helical structures, known as basic building blocks of blue ChLC phases, transform in a continuous manner to a cylinder-aligned smectic A phase.

Phase polymorphism and thermal stability of new cholesterol thioesters derivatives

We report the fundamental properties of a series of new mesogens with cholesteric esters, cholesteryl 4-(4-alkoxybenzoylothio)benzoates CnH2n+1OC6H4COSC6H4COOch, where ch = cholesteryl, for n = 7–10, nOSBch in short. They share a common feature, which is a high temperature cholesteric mesophase. In addition, the n = 10 compound has a monotropic ferroelectric smectic C* phase. The chemical structure and purity of nOSBch were confirmed by standard methods: 1H nuclear magnetic resonance (NMR), 13C NMR, Fourier transform infrared spectroscopy and elemental analysis. Differential scanning calorimetry (DSC), polarising optical microscopy and transmitted light intensity were used to study the polymorphism of these mesogens. The substances were proved stable up to 240–270°C (thermal stability deduced from DSC scans). The crystal structures were determined from X-ray diffraction data. Possible conformational disorder as well as nearest-neighbours intermolecular interactions were inferred from quantum chemical calculations.

Dicholesteryl icosanedioate as a glass-forming cholesteric liquid crystal: properties, additive effects and application in color recording

A cholesteric ester dimer – dicholesteryl icosanedioate – was examined for color imaging. Cholesterol was introduced as one of the dopants which performed an important role reducing phase transition temperatures. The other dopants, 4,4′-dialkoxyazobenzene and 4,4′-dialkoxybiphenyl promoted formation of smectic domains in cholesteric phases which resulted in the expansion of the reflective band shift over the whole visible region. In particular, the photo-responsive dopant 4,4′-dialkoxyazobenzene allowed us to realize reversible RGB color tuning of the cholesteric film by both photon and thermal modes. The trans–cis photoisomerization led to a cholesteric pitch change which resulted in reflective color change, and recorded optical images that have been stable for more than 8 months in a glassy solid film state.

Thermal behavior of some cholesteric esters

Taking into account the importance of thermal stability in the liquid crystals field, the study presents thermal behavior of some cholesteric esters, which differ by the nature of the functional group attached to the cholesteryl unit and the connecting position of the nitro or amino functions to the aromatic ring. The cholesteric esters present liquid crystalline properties, with high melting and clearing points and may be used as intermediates in the synthesis of liquid crystals. Some other kinetic characteristics, such as reaction order (n), activation energy (Ea) and pre-exponential factor (lnA) have been also evaluated. The type of functional units adjacent to the aromatic unit determines thermal stability of the cholesteryl compounds. Groups with a powerful withdrawing effect induce a decreasing of the temperatures at which the material starts to lose mass. An increased thermal stability for the amino esters has been observed, probably because of some intermolecular hydrogen bonds formation.

Novel supramolecular hydrogen-bonded cholesteric mesogens: liquid crystalline, thermoptical and glass-forming properties

Liquid crystalline, thermoptical and glass-forming properties of a series of novel supramolecular hydrogen bonded cholesteric mesogens have been described. These assemblies were built via intermolecular hydrogen bonding between cholesteric esters containing pyridine groups and cholesterol containing carboxylic acid moieties. All hydrogen-bonded complexes exhibit well defined cholesteric phases over a range of temperatures depending upon the number of carbon atoms present in their alkyl chains. The formation of pure LC materials on 1 ∶ 1 complexation could be confirmed from the phase diagrams of the binary mixtures and infrared absorption spectra. By rapid cooling of the hydrogen bonded assemblies from their cholesteric temperature to 0 °C glassy liquid crystalline films maintaining helical molecular order were obtained. Different reflection colors of the cholesteric phase were selectively and repeatedly fixed by changing the starting temperature of rapid cooling.

Effect of Blending of Cholesteric Cellulose Esters on the Pitch Height

Butyric esters (BuPC) of hydroxypropylcellulose (HPC) with different degrees of substitution (DS) were obtained by the reaction of (2-hydroxypropyl)cellulose with butyryl chloride. Blends of these BuPCs were obtained from ternary solutions by removal of the com mon solvent. The blends displayed thermotropic choles teric mesophases and showed iridescent colors at ambient temperatures depending on the blend composition. The spectroscopic analysis of the blends showed systematic variations of the reflection colors with variations of the ratio of the blend components between the reflection colors of the pure blend components covering the range from 453-938 nm corresponding to pitch heights from 308 640 nm.

Visualization of enantiomers through deuterium NMR in cholesterics. Optimization of the chiral liquid crystal solvent

Abstract Thermotropic cholesteric liquid crystals made of mixtures of cholesteric esters and of a Δχ <0 nematic (ZLI2806) may be used as NMR solvents to visualize enantiomers. Proton decoupled deuterium NMR spectra of a racemic mixture show that the quality of the spectra strongly depends on the temperature and on the nature and the concentration of the ester. A relationship between the spectral resolution and the presence of textural defects in the mesophase is also discussed. The best results, both for spectral resolution and enantiomeric separation, were obtained with a 45/55 by weight mixture of cholesteryl butyrate and ZLI2806 at 368 K.

Investigation of the chirality dependence of thermal properties in chiral-racemic mixtures of the cholesteric ester CE6

Abstract A scanning adiabatic calorimetric technique has been used to study the thermal properties of the chiral and racemic liquid crystal CE6 and the phase diagram covering the cholesteric phase, the three blue phases and the isotropic phase. The purpose of this investigation is to study thermal properties of liquid crystals as a function of chirality, while all other parameters remain constant. Results for the temperature and the chirality dependence of the enthalpy and of the heat capacity are reported. The latent heats between the cholesteric phase and BPI and between the different blue phases change slightly as a function of the chirality. The total heat of transition at the isotropic phase boundary is independent of the chirality, but with decreasing chirality, we observe a large increase in the latent heat and, correspondingly, a decrease in the pretransitional contribution. These experimental facts are in qualitative agreement with the predictions of a Landau–de Gennes theory for blue phases.

Chemical shift imaging of atherosclerosis at 7.0 Tesla.

Chemical shift imaging (CSI) was performed on cadaveric atherosclerotic fibrous plaques, periaortic adipose tissue, and cholesterol standards using a 7.0 Tesla horizontal bore prototype imaging spectrometer. Proton spectroscopy of intact tissue and deuterated chloroform extracted samples was done at the equivalent field strength of 7.0 Tesla on a vertical bore spectrometer, including studies of temperature dependence and T2 relaxation measurements. Spectra obtained using CSI on the imaging magnet were comparable with those from the conventional vertical spectrometer. Fibrous plaques and adipose tissue had unique spectral features, differing in the ratios of their water and various fat components. Chloroform extractions revealed a typical cholesteric ester spectrum for the fibrous plaque in contrast to the triglyceride spectrum of the adipose tissue. These two tissues also had different T2 relaxation measurements of their major fat resonances, with fibrous plaques having a short T2 compared to adipose tissue (15.9 milliseconds vs. 46.2 milliseconds). Temperature dependence studies showed that spectral signal intensity of the fat resonance of the fibrous plaque increased while linewidth decreased with increasing temperature from 24 degrees C to 37 degrees C. Atherosclerotic lesions may be studied at 7.0 Tesla, and NMR parameters defined in the present study may be used for further studies at other magnetic field strengths.

Optical Rotatory Dispersion Measurements of the Liquid Crystalline Blue Phases in Thick Layers

Abstract The results of optical rotatory dispersion (ORD) measurements in liquid crystalline blue phases (BP) are reported for pure compounds and mixtures. They have been performed on polycrystalline samples (thick layers). Two features distinguish their spectra from those obtained from liquid single crystals: (1) The magnitude of the rotation is much smaller, and (2) no sharp anomalous ORD region is present. The spectra possess a weak, broad anomalous region. Their sign and wavelength dependence follows the sense and pitch of the cholesteric helix. ORD experiments with mixtures of the same cholesteric esters with a nematic substance yield results typical of well oriented samples. The spectra exhibit larger rotation angles and a significantly smaller anomalous region.

Synthesis of methacrylate and acrylate monomers of cholesteric esters via phase transfer catalysis

Synthesis of methacrylate and acrylate monomers of cholesteric esters via phase transfer catalysis

Some Novel Cholesteric Esters Incorporating the 1,4-Disubstituted Bicyclo(2.2.2)octane Ring

Abstract A chiral center has been introduced into different positions of one of the terminal alkyl chains of the known 4-alkylphenyl, 4-alkoxyphenyl and 4-alkyl-2-fluorophenyl 4-alkylbicyclo(2.2.2)octane-1-carboxylates1.2 to produce seven homologous series of esters possessing cholesteric phases. Several of the esters exhibit enantiotropic cholesteric phases of tight pitch at and above room temperature, whilst others exhibit longer pitch enantiotropic cholesteric phases (sometimes at room temperature) which reflect light in the visible region of the electro-magnetic spectrum and are thermochromic.

Structural transformations in polymer macromolecules with mesogenic groups in side chains

Polarization luminescence, viscometry and spectropolarimetry were used to study structural transformations in cholesteric ester macromolecules of polymethacroyl-ω-hydroxycarboxylic acids in polar and nonpolar solvents in a wide range of temperature. It was found that in macromolecules of these polymers under certain conditions nuclei of mesophase are formed, the interaction of which results in a conformation transition of sphere-globule type. The importance of the kinetic factor in forming nuclei of the mesophase was demonstrated. It was shown that the length of side chains-carriers of mesogenic groups and solvent viscosity influence the formation of globular structures. A study was made of the effect of the molecular weight of cholesteric esters of polymethacryloyl-ω-hydroxycarboxylic acids on parameters reflecting structural transformations.

Liquid-crystalline copolymers of cholesteric type

Results are presented of investigations into the structure and liquid-crystalline properties of copolymers of cholesteric ester of 15-methacryloyloxypentadecanoic acid with p-methoxyphenyl ester of p′-(2-methacryloyloxyethoxy)benzoic acid (copolymer I) and p-methoxydiphenyl ester of p′-(2-methacryloyloxyethoxy)benzoic acid (copolymer II). All copolymers above T g change into the liquid-crystalline state, which is characterized by spontaneous anisotropy, that may be “frozen” in a glassy matrix below T g. With a content of cholesterol-containing component of 12–24 mole% copolymer II forms a liquid-crystalline phase of cholesteric type recorded by the existence of selective light reflection. The wave length of maximum selective light reflection which is in the IR range, λ max remains constant below T g and on increasing temperature, is displaced in the long-wave range.

Determination of Ionic Mobility in Mesomorphic Materials

Ionic mobility has been measured in isotropic and mesomorphic state of the following cholesteric esters: cholesteryl oleate, cholesteryl myristate, cholesteryl pelargonate and cholesteryl erucate, ...

Thermotropic liquid‐crystalline polymers: A new type of mesomorphic macromolecular system

AbstractA new approach to the synthesis of thermotropic liquid‐crystalline (LC) polymers is described based on the polymerization of monomers containing various mesogenic groups attached to the backbone chain by flexible polymethylene spacer groups. Dozens of new LC polymers are described, including derivatives of polymethacryloyl L‐lysine, cholesteric esters of long‐chain aminocarboxylic acids, and other aromatic polymers. Phase transition temperatures, heats of fusion, and x‐ray structural analysis were evaluated. Polymers exhibiting spontaneous optical anisotropy in the glasslike, elastic, and fluid states are described, and schemes of molecular packing in the LC phase are proposed. We show the possibility of realizing smectic, nematic, and cholesteric types of structures in these thermotropic LC polymers. An LC structure for various comblike copolymers not having mesogenic groups at every monomeric unit was detected. The role of specific interactions in side groups and their mobility in allowing for the LC structure are discussed, and the necessity of having particular conformations of the macromolecules is shown to provide the LC structure in films made from various solutions.

Thermotropic liquid crystalline polymers. I. Cholesterol‐containing polymers and copolymers

AbstractAn approach to the creation of thermotropic cholesterol‐containing liquid crystalline polymers by the chemical binding of cholesterol molecules with side chains of comblike polymers is presented. This type of structure permits a decrease in the steric hindrances provided by the backbone chains for the purpose of realizing the liquid crystalline state. A number of new cholesteric esters of poly(N‐methacryloyl‐ω‐aminocarbonic acid)s (PChMAA‐n) with different side‐chain lengths (n = 2–11) as well as a series of copolymers of ChMA‐n with n‐alkylacrylates and n‐alkylmethacrylates have been synthesized. The experimental evidence of liquid crystalline structure formation in these polymers in glass, viscoelastic, and fluid states is discussed. Molecular and supermolecular structures of cholesterol‐containing comblike polymers have been studied and the model of macromolecular packing in the liquid crystalline state is proposed. It is shown that the existence of a layered order of side methylene groups together with ordering of cholesterol groups is necessary to the production of the liquid crystalline state in these polymers.

Thermotropic liquid crystalline polymers—A new type of mesomorphous macromolecular system

SYNOPSIS A new approach to the synthesis of thermotropic liquid-crystalline (LC) polymers is described based on the polymerization of monomers containing various mesogenic groups attached to the backbone chain by flexible polymethylene spacer groups. Dozens of new LC polymers are described, including derivatives of polymethacryloyl L-lysine, cholesteric esters of long-chain aminocarboxylic acids, and other aromatic polymers. Phase transition temperatures, heats of fusion, and x-ray structural analysis were evaluated. Polymers exhibiting spontaneous optical anisotropy in the glasslike, elastic, and fluid states are described, and schemes of molecular packing in the LC phase are proposed. We show the possibility of realizing smectic, nematic, and cholesteric types of structures in these thermotropic LC polymers. An LC structure for various comblike copolymers not having mesogenic groups at every monomeric unit was detected. The role of specific interactions in side groups and their mobility in allowing for the LC structure are discussed, and the necessity of having particular conformations of the macromolecules is shown to provide the LC structure in films made from various solutions. Low-molecular-weight liquid-crystalline (LC) substances are subjects paid continuing and careful attention by organic chemists, crystallographers, and other representatives of the chemical disciplines due to their unique complex of properties. structure, and practical applications. The situation is different in polymer chemistry; the first publications concerning the synthesis of thermotropic LC polymers were in the early 1970s. Today they number about 100 and increase every year, reflecting the interest of synthesis chemists, physical chemists, and physicists in such unusual, for classical polymer chemistry, substances. Experimental results obtained in the University of Moscow during last five to six years will be reviewed in this paper. These experiments led us to formulate a new principle of thermotropic LC polymer synthesis [ 1-71. Polymer systems which provide LC properties and have structures analogous to the “classical” organic liquid crystals could be divided schematically into three groups: