Showing papers in "Molecular Crystals and Liquid Crystals in 1969"

On Molecular Structure and Physical Properties of Thermotropic Liquid Crystals

Abstract: Some models for the molecular structures of liquid crystals including a ferroelectric liquid are discussed. It is shown that a smectic liquid with a layered structure, in which the preferred orientation of the molecular axis is inclined against the layers, has some similarities with a nematic liquid. Such a tilted smectic liquid may also have a twisted modification corresponding to a twisted nematic or cholesteric liquid crystal. Attention is drawn to an optically isotropic texture of a cholesteric liquid. A proposal is made for its molecular structure in which a fairly regular, lattice-like arrangement of alignment singularities is assumed. Twist waves in nematic liquids are regarded and numerical estimations are made for their wave velocity and damping in p-azoxyanisole. It is concluded that the damping is too strong for an observation of such waves.

Guest-Host Interactions in Nematic Liquid Crystals

Abstract: new electro-optic effect based on guest-host interactions in nematic liquid crystals is described. The cooperative alignment of a nematic liquid crystal in an electric field is used to orient “guest” pleochroic dye molecules (molecules whose absorption spectrum is a function of the molecular orientation with respect to the polarization of the incident light). Electronic color switching was achieved using fields of the order of 10 kV/cm (dc through audio) with a variety of dyes. A detailed study of the optical density changes as a function of field strength and dye concentration for one guest-host system is also presented.

Solubilization in Micelles and Mesophases and the Transition from Normal to Reversed Structures

Abstract: The relationship between various kinds of solubilization and the formation of mesophases is elucidated. In aqueous solutions of association colloids there are three processes by which mesomorphous phases form: (i) the separation of the micellar substance in mesomorphous form on reduction of the water content, (ii) the separation of the micellar substance at lower concentrations, from the c.m.c. upwards, as a result of the changes that the micelles undergo on solubilization of foreign amphiphilic and lipophilic substances, and (iii) below the c.m.c. through the formation of aggregates between the solubilizate and the molecules or ions of the association colloids. The last of these processes is typical of the solubilization of fatty acids and liquid monohydric alcohols. In systems of sparingly soluble amphiphilea that do not give micellar aqueous solutions but take up water with swelling, mesophases often form through this “solubilization” of water, a process that is often promoted by the presence ...

Influence of Electric Fields on the Molecular Alignment in the Liquid Crystal p-(Anisalamino)-phenyl Acetate

Abstract: The effects of electric fields on the molecular alignment in the liquid crystal p-(anisalamino)-phenyl acetate are discussed The behavior of this material is similar to that of p-azoxyanisole in that the ordering which is normally observed shows an alignment with the molecular axis preferring a direction parallel to the field at low audio frequencies, and perpendicular to the field for frequencies of a few hundred kHz Results comparing the effectiveness of electric and magnetic fields for producing molecular alignment are presented which support a theory presented earlier to explain the ordering in p-azoxyanisole at audio frequencies This theory involves the anisotropy associated with the electrical conductivity Results are also presented which show that the effectiveness of dc electric fields is comparable to that for low audio frequencies A few comments are made concerning recent work employing electric fields to produce ordering in liquid crystals for NMR studies

Continuum Theory of Liquid Crystals of Nematic Type

Abstract: We describe continuum theory now being used to describe macroscopic behavior of liquid crystals of nematic type. Two types of predictions are discussed. The first concerns the propagation of orientation waves. The second area is viseometry, including effects of simple electromagnetic fields. We discuss some unusual size effects predicted when such fields are absent.

Long Range Order and Thermal Fluctuations in Liquid Crystals

Abstract: Some recent applications of the Oseen-Zocher-Frank elastic theory of nematic and cholesteric liquid crystals are reviewed (1) The effects of a static magnetic field H are calculated in detail In particular, for a cholesteric system, the spiral structure is distorted by the field and the period increases with H, up to a critical field Hc at which a nematic phase is expected Hc is inversely proportional to the pitch of the unperturbed helix, and should be observable in systems with long periods A cholesteric → nematic transition of this type has in fact been qualitatively observed (2) The optical striations which are observed when a cholesteric liquid is inserted in a region of variable thickness have been shown by Cano to correspond to discontinuities of the torsion The transition region between regions of different pitch has been analyzed in terms of a “disclination line” normal to the helical axis (3) In a nematic liquid crystal the long wavelength thermal fluctuations of orientation can

NMR Relaxation in Adamantane and Hexamethylenetetramine: Diffusion and Rotation

Abstract: The ultraslow-motion NMR technique has been used to measure translational diffusion coefficients in the plastic crystal adamantane; The diffusion coefficients in the brittle crystal hexamethylenetetramine (HMT) were too low to be measured by this technique; they are smaller than those for adamantane by a factor of 10−4. It is claimed on the basis of Weertman's theory that this difference in diffusion coefficients accounts for the difference in plastic flow properties between the two materials. For rotation in β-adamantane, α-adamantane, and HMT the respective jump times are:

A Theory of the Cholesteric Mesophase

Abstract: A theory of the macroscopic twist which is a characteristic feature of the cholesteric liquid crystal phase is proposed. This is based on the presence of anharmonicity in the forces resisting the relative twist of neighboring planes of molecules and views the macroscopic twist as the rotational analog of thermal expansion. On the basis of a simple mode for the harmonic part of the forces, the temperature dependence of the peak reflectance wavelength observed by Fergason et al. is well explained.

Singlet Exciton Fission and Tri plet-Tri plet Exciton Fusion in Crystalline Tet racene

Abstract: The dominant radiationles decay channel in crystalline tetracene at 300 °K is a fission of an excited singlet into two triplet excitons with a rate constant γS = 1.5 × 10−12 ± 5% cm3 -sec−...

The Structure of p-Azoxybenzenes in Magnetic Fields

Abstract: -A systematic X-ray diffraction study of the 10 homologues of n-azoxybenzenes has been performed. The structures of nematic and smectic mesophases as a function of the strength of the magnetic field and the temperature have been established.

The Smectic Phase of Lipid-Water Systems: Properties Related to the Nature of the Lipid and to the Presence of Net Electrical Charges

Abstract: The lamellar phase of the lipid-water systems, when studied as a function of the water content, displays properties that belong to one of the following classes, according to the nature of the lipid. (a) The lamellar phase takes up an almost indefinite amount of water intercalated between the lipid layers (lipids bearing net electrical charges). (b) The maximum amount of intercalated water is rather small, and the water in excess stays as a separate phase (lipids devoid of net charges). (c) The amount of water in the lamellar phase is limited: further addition of water induces a transition to another liquid-crystalline phase (soaps and detergents). It is shown here that if small amounts of ionizable lipids, either anionic or cationic, are incorporated to a non-ionizable lipid, the lamellar phase takes up very large amounts of water, EW in the lipids of class (a): this phenomenon is not observed if the added lipids are an equinormal mixture of anionic and cationic species. These experiments stress ...

Interfacial Effects on the Viscosity of a Nematic Mesophase

Abstract: The effects of interfacial orientation on the viscosity of nematic p-azoxyanisole have been studied. Viscosity was measured in four glass capillary tube viscometers with diameters ranging from 78μ to 516μ. The surfaces of the capillaries were subjected to two standardized pretreatmenta—acid cleaning or acid cleaning followed by rubbing with a fiber—in order to produce different interfacial orientations. Both treatments resulted in non-Newtonian viscosity behavior and a dependence of viscosity on tube diameter not in agreement with that predicted for a homogeneous fluid. However, it was found that the treatment of the interface had a profound effect on viscosity. Non-Newtonian viscosity and diameter effects were much more striking in the caw of interfaces subjected to acid cleaning only. The significance and implications of these findings is discussed.

Continuum Theory of Cholesteric Liquid Crystals

Abstract: This paper discusses a continuum theory which has recently been proposed for liquid crystals of the cholesteric type. As in earlier continuum theories, the liquid crystal is regarded as an incompressible liquid with a preferred direction at each point, described by a unit vector. Solutions of the equations are investigated for shear flow between two flat plates, one at rest, and the second moving parallel to the first with a constant velocity. It is found that the theory predicts non-Newtonian behaviour, and that secondary flow and temperature variations occur. The predictions are compared with available experimental data, and one finds agreement in that a uniform apparent viscosity is predicted at large shear rates.

Advances in Synthesis and the Role of Molecular Geometry in Liquid Crystallinity

Abstract: Although publications concerning advances in synthesis and the role of molecular geometry in liquid crystallinity have not been numerous since the last conference in 1965, I have nevertheless had to be somewhat selective in the material which I have chosen to review in the available time. Broadly speaking, I have selected those areas of study which have given rise to new results which can be logically related to past experience in the field of molecular structure and the properties of liquid crystals. To this end, I have divided the lecture into three parts, of which the first is largely introductory.

Interaction Between UV Radiation and Cholesteric Liquid Crystals

Abstract: Mixtures of cholesteryl iodide and cholesteryl bromide with cholesteryl nonanoafce were exposed to UV radiation and the resultant color shifts were measured as a function of exposure and composition. The formation of images by this process will be described, and the presiding mechanism will be briefly discussed.

Mesomorphism of Homologous Series

Abstract: Problems associated with the molecular interpretation of mesomorphic behavior within a given or between different homologous series and possible effects of impurities on mesomorphic properties are considered. The temperatures and enthalpies of phase transitions were determined for the first twenty cholesteryl esters and compared with the results of corresponding thiocholesteryl esters. The investigated Compounds had a purity of 99 mole percent or better. With two exceptions, the thiocholesterols exhibited higher clearing temperatures, but generally lower heats of clearing. Smectic phases were observed for choleateryl octanoate and higher members, while for thiocholesterol these phases started with the heptanoate. The study of ω-phenyl cholesteryl esters (first 8 members) showed that all members exhibit cholesteric mesophases except the acetate. The values of heats of clearing lie on two well separated curves, one for odd and the other one for even values of chain length. The corresponding ω-pheny...

Lyotropic Mesomorphism in Lipid-Water Systems

Abstract: When a physical chemist looks at living matter, he sees watery bits and fatty or lipid bits and, in spite of the old saying that oil and water don't mix, there seems to be a lot of mixing and peculiar mixing at that; both lipids in water and water in lipids. Nor is this so strange because the model of lipid phase and water phase lying in contact without any interaction would seem to be a singularly useless one for the complex co-ordinated activities which we call life. Nevertheless the triglycerides are such inert lipids, which is a useful property as depot fats,1 but one which requires pancreatic lipolysis for intestinal absorption; that is, conversion to lipids which do interact with water and aqueous bile salt solutions.

A Quasi-Chemical Lattice Treatment of Rod-like Molecules. Application to the Nematic-lsotropic Transition

Abstract: A quasi-chemical treatment is applied to a face-centered cubic lattice system containing N0 holes (or “solute molecules”) and Nx interacting rods of length-to-breadth ratio x. The resulting partition function is then maximized with respect to the numbers of rods in each of the six allowed directions on the lattice. Two stable “phases” are obtained: a completely aligned “phase”, more stable at lower volume fractions of holes ν0, and an isotropic “phase”, more stable at higher ν0's. When alignment is not favored energetically, the relative stabilities of these two “phases” are temperature independent, but when alignment is favored, this is no longer the case and a first-order aligned → isotropic transition is observed. When the dependencies of the transition temperature and enthalpy of transition on the segment interaction energies of the system, on ν0, and on x are determined, it is found that our simple model is able to predict a number of important aspects of the behavior of nematogenic substanc...

Optical Studies on Grandjean Planes in Cholesteric Liquid Crystals

Abstract: Optical investigations were performed to prove the idea of variable pitch in the Grandjean structure of cholesteric liquid crystals. According to this model, the helical structure is deformed in such a manner that with decreasing thickness of the wedge-shaped layer the pitch of the screw decreases, while the number of turns remains constant. At the next “step”, however, the pitch gets large again and the number of turns decreases by a half. From measurements on the Grandjean structure with several derivatives of cholesterol, the values of the pitch are determined as a function of temperature and found to be in good agreement with values obtained from the wavelength at maximum reflection. This is considered as a confirmation of the model used, in particular of the assumption that the gap width is essentially equal to an integer multiple of the half pitch and the number of turns changes by one half at the optically observed discontinuities.

Separation de Racemiques Nematiques en Deux Antipodes Cholesteriques

Abstract: Mesomorphic compounds which may exist as enantiomers were prepared: namely (a) alkoxy-biphenylcarboxylic acids having one ssymmetric center in the ether chain (b) arylidene-p-aminocmates eaterified with an alcohol containing an asymmetric center In these two series we observed nematic phases in the cases of racemic derivatives and cholesteric phases in the cases of enantiomers Some compounds show two mesomorphic phases of the same kind The different transitions are determined by microscopic observations and by calorimetric measurements Phases diagrams between enantiomem are presented: these are charactoristic of solid solutions

Orientation Studies on Fluorobenzenes in Nematic Liquid Crystals by NMR

Abstract: The orientation of C6H6, 1,2-, 1,3-, and 1,4-C6H4F2, 1,3,5-C6H3F2, 1,2,4,5- and 1,2,3,5-C6H2F4 and C6F6 in nematic liquid crystalline solvents has been determined by 1H and 19F magnetic resonance measurements. Most of these substances have been studied in two solvents, in 4,4′-di-n-hexyloxyazoxybenzene and in a mixture (3:2) of 4-heptanoyloxy- and 4-hexanoyloxy-4′-ethoxyazobenzene. Orientation parameters (S-values) of the solute molecules are given and their free energies are approximately determined as functions of the orientation. The fluorobenzenes show some correlation in their free energies and in their S-values that are theoretically expected by regarding only dispersion forces and assuming localized contributions from the substituted bonds. The results confirm that the influence of permanent electrical dipole moments is relatively small. Although the orientation in the two solvents is similar, a close comparison shows that there are differences, which are assumed to be due to specific inte...

The Polymorphism of the Smectic Liquid Crystal

Abstract: The presence of one nematic and two smectic mesophases of p-anisolaminoeinnamate has been confirmed by the methods of differential, thermal and X-ray analysis. The scheme of phase changes is similar to the scheme of H. Arnold and H. Sackmann. Some difference lies in the anticoincidence of temperature of the transformation of the smectic phase B into solid.

NMR Spectra of Molecules Oriented In a Lyotropic Mesophase

Abstract: A lyotropic liquid crystalline phase has been used as an orienting matrix for NMR studies. The solvent and orientation properties of this phase are somewhat different from those of thermotropic nematogens. Spinning samples can be used facilitating the recording of well-resolved spectra suitable for analysis. The spectrum of ethanol has been recorded and analyzed in full by an iterative computer procedure. It is shown that the preferred orientation of ethanol in the mesophase is such that the C—C—O plane is perpendicular to the magnetic field. Proton and deuteron magnetic resonance rotation studies have also been performed and are discussed in the light of the high resolution results.

Mixed Mesomorphism in Binary Systems Forming Smectic-Nematic Phases

Abstract: A number of binary systems have been investigated with p-n-dodecyloxybenzal-p-n-butoxyaniline—a mesomorphic component exhibiting both smectic and nematic phases—and other non-liquid crystalline solutes. When the transition temperatures of such binary systems are plotted against the mole per cent composition of the liquid crystalline component, two types of curve are obtained: (1) A usual curve where the transition lines are depressed regularly and (2) a rising curve exhibiting only a smectic phase and a maximum. The latter type of curve indicates an enhancement in the smectic thermal stability while the former shows a decrease in the mesomorphic thermal stability during the course of mixed liquid crystal formation; the range of temperature over which the nematic phase exists increases at the cost of the smectic mesophase. It is observed that in such a binary mixture, if at least the first few compositions chosen melt to give the isotropic liquid at a temperature higher than the mesomorphic-isotro...

Electric-Field Induced Phase Change in Cholesteric Liquid Crystals

Abstract: Optically-negative cholesteric liquid-crystal films can be transformed to an optically-positive state by applying high d-c electric fields. A phase transformation occurs in which the helicoidal cholesteric structure is converted to either a planar smectic, or a linear nematic, structure. The threshhold field for this transformation has been studied as a function of sample thickness, temperature and composition. Bulk fields are responsible for the phenomenon. It is inferred that the compositional dependences are due to size factors and internal molecular dipole moments.

Lattice Defects in Organic Crystals

Abstract: Since the last review of this topic at the 1969 Brookhaven Symposium the principal efforts in this area have been directed towards the characterization of both point and line defects and the preparation of low defect content samples for physical and chemical examination.

The Liquid Crystal Phase of Poly-γ-benzyl-L-glutamate in Solution and in the Solid State

Abstract: : Concentrated solutions and solid films of Poly-gamma-benzyl-L-glutamate were examined using proton magnetic resonance, X-ray diffraction and optical microscopy. The unusual findings can be correlated with the cholesteric-like liquid crystal phase known to exist in concentrated solutions of this polypeptide. The continuity in these observations on passing from the liquid to the solid state is interpreted in terms of the existence of a helicoidal structure in the solid state of PBLG derived from that found in the liquid crystal phase. (Author)

Photo-Enhanced Electron Emmision from Alkali Metal Contacts into Anthracene

Abstract: Dark- and photo-current measurements are used to study trap characteristics and optical de-trapping processes in anthracene samples provided with alkali and alkaline-earth metal contacts. The spectral response of the photocurrent yield in the entire photon-energy range studied (0.95–3.4 eV) originates from optical excitation of electrons from a single discrete set of traps which is continuously filled by electron injection from the active metal contact. The traps are 0.95 eV deep and are present with a density of about 1013 cm−3. De-trapping appears to occur by direct photoexcitation into the conduction band at long wavelengths and by interaction of occupied traps with triplet and singlet excitons at shorter wavelengths. The spectral yield curve exhibits considerable structure which can be advantageously used to study the various electronic-vibronic states in anthracene.

The Viscosity of Mesophases Formed by Cholesteryl Myristate

Abstract: This investigation provides the first reported viscosity measurements on cholesteryl myristate. Viscosities were measured as a function of both temperature and shear over temperatures corresponding to the isotropic liquid, the cholesteric mesophase, and the smectic mesophase. The myristate ester was chosen for study because its multiple phase transitions have been previously shown to be reproducible. The phases also persist over relatively long temperature ranges. The viscosity measurements were made at lower shear rates than previously reported for esters of cholesterol by using a Weissenberg Rheogoniometer. These are the first cone-and-plate viscosity measurements reported for any esters of cholesterol. The shear rates in this cone-and-plate viscometer are homogeneous and variable. The results show large breaks in viscosity behavior at the independently-measured thermodynamic transitions between the crystal, smectic, cholesteric, and isotropic states. Viscosities for the isotropic state are New...

Structure of Liquid Crystalline Phases from Amorphous Block Copolymers

Abstract: We have studied the structure of liquid crystalline phases of A-B block copolymers by small-angle X-ray scattering. We have found three types of structure: lamellar, hexagonal and cubic. We have also shown the influence on the structural type and structural parameters of the following factors: concentration and nature of the solvent, molecular weight and composition of the copolymer.