Liquid Crystalline Substances Research Articles

Thermotropic liquid crystalline polymers, 3. Comb‐like polymers with side chains simulating the smectic type of liquid crystals

AbstractThe syntheses of a number of new comb‐like polymers are described, which contain mesogenic groups as side branches presenting models of low‐molecular liquid crystals of smectic type. The resulting polymers are capable to form an enantiotropic liquid crystalline phase, which may be defined as smectic, according to the terminology used for low‐molecular liquid crystalline substances. The thermodynamic limits of the liquid crystalline state in these polymers were determined, which made it possible to define the liquid crystalline state as a thermodynamically stable phase with spontaneously appearing anisotropy of properties (in particular, the optical anisotropy). The characteristic feature of the structure of these polymers was found to be the layer order of side groups providing — in combination with the packing of the mesogenic groups — the possibility of liquid crystalline properties. Only the mesogenic groups take part in the formation of the crystalline packing in these polymers, whereas the methylene chains are in the amorphous phase together with the backbone chain.

Dielectric reorientation of smectic C, F and I phases

AbstractAs shown for 4 liquid crystalline substances also smectic F and smectic I phases may be reoriented by an electric field similar to a smectic C phase. The threshold voltage of the effect was determined approximately. For one substance the tilt angle of the smectic C and I phase could be measured by means of the dielectric reorientation.

One-dimensional model with rotational and liquid-crystalline phase transitions

A one-dimensional system of hard-rod particles with rotational-like internal degrees of freedom is considered. Particles interact with each other through the infinite-range, infinitely weak attractive Kac potential, and through a nearest-neighbor short-range potential. The latter depends also on the internal states of the interacting particles. Exact results for thermodynamic properties and for some correlation functions are obtained. It is found that the considered system exhibits several first-order transitions between phases with different “rotational structure,” i.e., with different ordering with respect to the internal degrees of freedom. The calculated equation of state seems to suggest that in the solutions of liquid-crystalline substances in neutral solvents there may exist regions in which the coefficient of thermal expansion is negative—an effect similar to that well known in liquid water.

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:

Mesomorphic Properties of Some α-Methyl Cinnamic Acids and their Esters

Abstract Two homologous series of liquid crystalline substances have been synthesized and their mesomorphic properties studied. These are (i) trans-p-n-alkoxy-α-methyl cinnamic acids and (ii) trans-p-n-alkoxy-α-methyl-p'-cyanophenyl cinnamates. The esters are colourless and fairly low melting materials of high positive dielectric anisotropy. The effect of the α-methyl group on the mesomorphic thermal stability of both the homologous series is discussed and compared with other materials of similar structure. The nematic-isotropic transition temperatures in ascending series of the esters show an initial decrease and then a gradual increase, which is not a commonly observed phenomenon for homologous series of liquid crystalline substances.

A review on the specific volume of liquid crystals

The temperature variation and the pretransitional effects of specific volume of liquid crystalline substances have been briefly discussed in this article. Perhaps no such review exists in literature as the measurements on specific volume are not generally published independently and the observations are very scattered. It is also concluded on the basis of the measurements of various authors that mesophase-isotropic transition is a true first order phase transition and pretransitional effects in specific volume occur markedly only on the low temperature side of the transition which can be best explained by Maier- Saupe theory.

Dynamic Viscoelastic Properties of Cholesteric Liquid Crystals

Abstract Propagation of ultrasonic shear waves at frequencies from 2.4 to 18 MHz was studies in three 18-carbon unsaturated fatty acid esters of cholesterol, cholesteryl obleate (CO), cholesteryl linoleacte (CL), and cholesteryl linolenate (CLn), and three mixtures of the esters, 80% CL-20% CO, 95% CL-5% CO, and 80% CLn-20% CO. The oleate and linoleate esters are of biological importance because they comprise about 70–80% of the cholesteryl esters contained in atherosclerotic plaque. A shear wave pulse superposition technique was used to obtain the complex shear mechanical impedance and, hence, the dynamic shear viscosity and dynamic shear rigidity. The reflectance technique permits the study of liquid crystals whose long axes are oriented, in the plane of shear, with respect to the shearing direction of the ultrasonic wave. Falling ball viscosity measurements using a Hoeppler precision viscosimeter were made as a function of temperature from 35 tc 55°C. Shear wave measurements of dynamic shear rigidity and dynamic shear viscosity are shown as functions of temperature. This work represents the first measurement of both the dynamic viscosity and rigidity in a liquid crystalline substance. In the smectic phase, a dynamic shear rigidity is exhibited in all systems studied. The shear mechanical impedance of cholesteryl linoleate is found to be independent or orientation of the long axis of the molecule with respect to the direction of shear in agreement with the prediction of Brochard which is based on Leslie's hydrodynamic theory of the cholesteric state. The dynamic shear viscosity is found to be dependent on the shear rate. This may be explained in terms of a relaxation in the short range order present in the isotropic phase of both nematic and cholesteric compounds, as recently discussed by De Gennes.

Birefringence and polymorphism of liquid crystals

The double refraction of 32 liquid crystalline substances and their dependence on temperature and wavelength has been studied. The measurements were made on uniaxially oriented liquid crystalline layers using the principle of Abbé's double prism. Such optically uniaxial layers are obtained from the nematic states and the smectic modifications A and B. The smectic modification D proved to be optically isotropic. The smectic modification C could not be obtained uniaxially. The double refraction at the phase transition between two liquid crystalline modifications changes discontinuously and the several liquid crystalline modifications show a characteristic temperature dependence of the refractive indices.

Biaxial Liquid Crystals

Convergent light observations have been made on three liquid crystalline substances with nematic and smectic phases. The nematic phase was observed to be uniaxial as expected, but smectic $C$ was found to be biaxial. The optic axial angle $2V$ for smectic $C$ was found to be on the order of 20\ifmmode^\circ\else\textdegree\fi{} for all three compounds. Also, the tilt angle for smectic $C$ is nearly 45\ifmmode^\circ\else\textdegree\fi{} for all three liquid-crystal systems. Anisotropy of the degrees of order is suggested as a partial explanation for the biaxial character of smectic $C$.

Heat Capacity and Enthalpy of Transition of Aromatic Liquid Crystals

Abstract Adiabatic calorimetry was applied to some liquid crystalline substances. Especial reference is made for the first time to the transition between smectic modifications of different types. The enthalpy of transition, including pre-and post-transition effects, is 2100 joules/mole for the smectic B to smectic A transition of ethyl p-(4-ethoxybenzylideneamino-)cinnamate, and only 100 joules/mole for the smectic C to smectic A transition of di-n-dodecyl 4,4′-azoxy-α-methylcinnamate.

Positron Annihilation in Liquid Crystals

Positron lifetimes have been measured, as a function of temperature, in the following liquid crystalline substances: cholesteryl benzoate, cholesteryl stearate, cholesteryl myristate, and p-azoxyanisole. The value of τ2 was observed to vary at the melting point of each compound and at smectic transitions. No change in τ2 occurred at nematic isotropic and cholesteric isotropic transitions. Possible explanations for the behavior of τ2 are suggested.

The Isolation and some properties of liquid crystalline substances from solanaceous plants infected with three strains of tobacco mosaic virus

Tobacco mosaic was the first disease shown to be caused by a filter-passing virus, and Beijerinck (1898) suggested as its cause a “contagium vivum fluidum", Since then many other theories have been advanced, but there has been little positive evidence to indicate whether the virus more nearly resembled organisms such as small bacteria or chemical molecules such as the larger proteins. Recently, however, Stanley has isolated from tobacco (1936a) and tomato (Stanley and Loring 1936) plants suffering from mosaic a protein which he describes as crystalline and as possessing the properties of the virus. When susceptible plants were inoculated with this protein at a dilution of 10 -9 they developed typical symptoms of the disease. The protein was obtained from infective sap by repeated precipitation with 40% saturated ammonium sulphate solution, and by adsorption on and washing from celite. The “crystals” described by Stanley were small needles produced by precipitation with acid ammonium sulphate. A number of statements in Stanley’s earlier paper (1935 more especially those dealing with the nitrogen content and the serological activity, made us doubt the purity of his product, and preliminary experiments with methods similar to those used for the preparation of suspensions of potato virus “X” (Bawden and Pirie 1936) gave us products with much higher precipitation end-points with antisera than those claimed by Stanley. We have now exchanged material with Dr. Stanley and find no gross differences in the activities of our respective products. We have found, however, that by further purification the protein in neutral solution can be obtained in liquid crystalline states. Also, as will be shown later, there are considerable differences in the chemical descriptions given of the virus protein; some of these differences have already been resolved, and others presumably will be by future work.

Visible Mesomorphic Fibres of Tobacco Mosaic Virus in Juice from Diseased Plants

IN NATURE of December 19 last there appeared an interesting letter by Bawden, Pirie, Bernal and Fankuchen under the heading ” Liquid Crystalline Substances from Virus-infected Plants”. I have arrived at conclusions similar to some of those recorded in the above letter, and the following, to some extent complementary, observations may be of interest.

Liquid Crystalline Preparations of Cucumber Viruses 3 and 4

IN a previous note1, we showed that liquid crystalline substances infective at high dilutions could be isolated from solanaceous plants infected with three different strains of tobacco mosaic virus. We have been unable to transmit these strains to cucumber plants, and other workers have found cucumber to be immune to tobacco mosaic virus. From cucumber plants infected with cucumber viruses 3 and 42, we have now isolated nucleoproteins with a similar chemical composition and similar in many of their properties to those obtained from solanaceous plants infected with tobacco mosaic virus ; they are, however, rather less stable and more difficult to isolate.