Cholesteric Liquid Crystalline Phase Research Articles

Optical properties of (acetoxypropyl)cellulose mesophases: factors influencing the cholesteric pitch

(Acetoxypropyl)cellulose (APC) forms a thermotropic cholesteric liquid crystalline phase, and with dibutyl phthalate (DBP) it also forms a lyotropic cholesteric phase. The reflection bands for the mesophases occur in the visible region, at wavelengths which depend on concentration and temperatures. The pitch of the cholesteric helicoidal structure is derived from measurements of the mean refractive indices and of the reflection band wavelengths for mesophase samples containing from 0 to 30% diluent at temperatures from ambient to 170°C. The pitch of the thermotropic mesophase increases with increasing temperature and with decreasing molar mass. The pitch of the lyotropic mesophase increases with increasing temperature and diluent content. Pitch values approach infinity at temperatures close to the clearing temperature of the mesophase, and no reversal in the sense of the pitch with temperature or diluent content was detected. The experimentally observed changes in pitch with composition and temperature are in reasonable agreement with the predictions of a recent theory for cholesteric mesophases composed of helical rod-like species. The average distance between chains in the mesophase is estimated from X-ray diffraction measurements, and hence the average angle of twist between neighbouring APC molecules may be found. The angle decreased from 2.2° for pure APC to 0.9° for a volume fraction of 0.73 APC in DBP.

Molecular and mechanical aspects of helicoid development in plant cell walls

AbstractThe view is presented that extracellular architecture in plant cell walls results from an interplay between molecular self‐assembly and mechanical reorientation due to growth forces. A key initial self‐assembly step may involve hemicelluloses. It is suggested that hemicelluloses may self‐assemble into a helicoid via a cholesteric liquid crystalline phase; the detailed molecular structure of hemicelluloses (stiff backbone, bulky side chains, and the presence of asymmetric carbon atoms) is shown to be consistent with cholesteric requirements for such self‐assembly. Since hemicelluloses are hydrogen‐bonded to the periphery of cellulose microfibrils, the cellulose could then itself become helicoidally arranged. Such ‘universal plywood’ structure is found in the walls of a wide variety of plants, and in several types of cell (including wood). The permanent effects of growth stresses on patterns seen in sections of helicoids are displayed by computer graphics plots, and the expected changes in stiffness are calculated.

Chemical composition and physical state of lipid deposits in atherosclerosis

The composition, morphology, and physical properties of lipids in atherosclerotic lesions from human aortas were studied in order to elucidate the factors for the accumulation of cholesterol and its esters in the vessel wall. Lesions were classified histologically into 3 groups: fatty streak, fibrous plaque, and advanced plaque. The relative lipid composition of the lesions was plotted on the phase diagram of the 3 major lipids: cholesterol, cholesteryl ester, and phospholipid. Early fatty streaks had compositions within the 2-phase zone with a cholesterol-phospholipid liquid crystalline phase and a cholesteryl ester oily phase. Advanced fatty streaks and fibro-fatty plaques fell within the 3-phase zone with excess free cholesterol. Advanced plaques also had an average lipid composition within the 3-phase zone, but with a larger excess of free cholesterol. From the lipid-chemical point of view there is a continuous progression from early fatty streaks through advanced fatty streaks and fibro-fatty plaques to advanced plaques. In fatty streaks the cholesteryl esters accumulate in the form of isotropic and anisotropic droplets. The latter are in the smectic liquid crystalline state with the molecules arranged in layers and have surfaces that are spherical and smooth. Fibrous and advanced plaques showed beside droplets also amorphous lipids and cholesterol monohydrate crystals. Some of the amorphous lipids were solid up to about 45°C and exhibited a smectic phase at cooling, indicating cholesteryl esters as the major component. The transition temperatures of high-melting cholesteryl esters, e.g. palmitate, are depressed by low-melting ones. Most of the triglycerides are present in the cholesteryl ester droplets and abolish the cholesteric liquid crystalline phase.

Chemical characteristics of cellulosic liquid crystals

Cellulosic derivatives form an interesting range of polymer liquid crystals. They belong to a class of mesophases in which the polymer chain is stiff or semiflexible and is chemically homogeneous. The cellulose backbone, a single-stranded homopolymer of β-linked 1,4-anhydroglucose units, contains three hydroxy groups per anhydroglucose unit which provide convenient sites for substitution reactions, leading to a wide variety of cellulose ethers and esters. Many of these derivatives form cholesteric liquid-crystalline phases in suitable solvents, and some derivatives with relatively large (but non-mesogenic) side chains form both lytropic and thermotropic liquid crystals.Published observations on the phase separation of cellulose-based polymers indicate that the mesophases form at critical volume fractions of polymer ranging from 0.3 to 0.5 for high molar mass samples at room temperature. The critical volume fractions for a given polymer and solvent decrease with increasing molar mass to approach these asymptotic values. The critical volume fraction increases with temperature. The nature of the solvent is also important; highly polar or acidic solvents generally favour mesophase separation at lower critical concentrations than simple organic solvents. Furthermore, whereas heavily substituted derivatives with long flexible side-chains form mesophases easily in a wide range of solvents, the more familiar derivatives with smaller substituents seem to form mesophases with specific solvents, but not with others.We have measured the critical concentrations of fractions of (acetoxypropyl)cellulose in dialkyl phthalate solvents over a large temperature range and compared the results with predictions of theories for the phase separation of freely jointed and worm-like chains. The results indicate that chain geometry and stiffness are the major factors controlling liquid-crystalline phase formation and that anisotropic inter-chain interactions play a minor role. Thus the chemical structure of polymer and solvent govern the phase separation indirectly in these systems, primarily through effects on the chain conformation.

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

Abstract 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 (Pt)-temperature (Tt) phase diagrams show the expected widening of the cholesteric range with Pt and Tt. 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.

Direct Nuclear-Magnetic-Resonance Measurements of Biaxiality in the Cholesteric Liquid Crystalline Phase

We report what we believe is the first experimental measurement of biaxiality in a cholesteric phase. This observation is made in a selectively deuterated nematic that was twisted by the addition of a chiral compound. A theoretically predicted biaxial order parameter (measured in terms of an asymmetry in the time-averaged deuterium quadrupole interaction) was found to be \ensuremath{\sim}${10}^{\ensuremath{-}3}$ for a pitch \ensuremath{\sim}3 \ensuremath{\mu}m and to increase both with decreasing pitch and as the isotropic phase is approached.

Cholesteric liquid crystalline phases based on (acetoxypropyl)cellulose

Cholesteric liquid crystalline phases based on (acetoxypropyl)cellulose

Liquid Crystals Composed of N-Acylamino Acids

Abstract Optical active N-acyl-L-glutamic acid (L-AGA) exists in two forms distinguishable by differential thermal analysis, infra-red spectroscopy, and X-ray diffraction. The X-ray diffraction patterns suggest that one form is more ordered than the other. As both forms have the same chemical structure and are interchangeable, implying that these are the polymorphic L-AGA, we denote the disordered form L-AGA-A (A: amorphous) and the other more ordered form L-AGA-C (C: crystalline). Data obtained by several methods imply that L-AGA-A is likely to have a parallel order along the long molecular axis with random spaces between the layers. On the other hand, L-AGA-C has a rigid crystalline structure. A new type of cholesteric liquid crystal was obtained by suspending L-LGA-A in specific organic solvents. L-AGA-C did not form any liquid crystalline phases when suspended in the same organic solvents which resulted in liquid crystals from L-AGA-A. The results strongly imply that the particular crystalline structural features are needed to form suspended cholesteric liquid crystalline phases in organic solvents.

LIQUID CRYSTALS COMPOSED OF N-ACYLAMINO ACID. I. CIRCULAR DICHROISM IN LIQUID CRYSTALS COMPOSED OF N-LAUROYL-l-GLUTAMIC ACID AND AROMATIC SOLVENTS

Abstract Amorphous powdered N-lauroyl-l-glutamic acid (LGA) soaked into aromatic solvents such as benzene showed birefringence under a polarizing microscope and a positive CD band. An achiral dye molecule dissolved in this system showed induced CD. From these results it is concluded that the LGA-benzene system is a cholesteric liquid crystalline phase.

The thermal transitions and structural properties of 5α-cholestan-3β-ol esters of aliphatic acids

5α-Cholestan-3β-ol esters of aliphatic acids undergo both enantiotropic and monotropic changes of state. Ten saturated and three unsaturated esters have been examined by differential scanning calorimetry and polarizing microscopy to determine transition temperatures, enthalpies, and entropies. The results are compared with an analogous series of cholesterol esters. All esters of evennumbered n-alkanoic acids from C 22 to C 20 melt from a crystalline state to an isotropic liquid. The crystalline state has been studied by X-ray powder diffraction. The C 8 to C 20 esters have progressively increasing crystalline melting transition temperatures from 76 to 99°C and possess similar X-ray powder diffraction patterns, suggesting that these compounds form an isostructural series. Esters of C 2, C 4, and C 6 acids exhibit polymorphism. Crystalline cholestanol oleate melts to an isotropic liquid, whereas cholestanol linoleate and linolenate fail to crystallize, even after several months at −20°C. Esters of the even-numbered saturated acids from C 4 to C 14 form monotropic cholesteric liquid crystalline phases. Esters C 10, C 12, and C 14 form smectic liquid crystalline phases. Cholestanol oleate, linoleate, and linolenate form both cholesteric and smectic mesophases. The lower smectic to cholesteric and cholesteric to isotropic transition temperatures of the cholestanol esters compared to the corresponding transition temperatures of the analogous cholesterol esters suggest that the Δ 5 double bond in cholesterol increases the thermal stability of the mesophases of cholesterol esters.

Helical Structure of Cholesteric Liquid Crystalline Phase

Helical Structure of Cholesteric Liquid Crystalline Phase

Chemistry and Applications of Liquid Crystals

AbstractApproximately 5% of all organic compounds are transformed at their melting point into liquid crystals—thermodynamically stable, anisotropic liquids which in contrast to isotropic melts appear turbid and are also known as mesophases. Such melts are classed as smectic, nematic, and cholesteric liquid crystalline phases, depending upon the arrangement of the constituent molecules. The discovery of numerous potential applications during the past ten years has awakened the study of liquid crystals from its former slumber as a physical curiosity and placed it in the limelight of the scientific stage. Uses in display systems for measured values and for computer and process data, as well as for remote controlled timetables, for windows of variable light‐transmission, etc., appear particularly promising. Not only black‐and‐white contrasts are now possible but also color production.