Showing papers in "Molecular Crystals and Liquid Crystals in 1974"
TL;DR: In this paper, the structure of the low temperature orthorhombic phases β1, β2 (indistinguishable on the basis of powder photographs) has been determined from a single crystal X-ray analysis, the most probable space group of these phases is Bb21 m, the mirror planes required of the molecules perpendicular to their length being obtained by disordering.
Abstract: The n-eicosane, C20H42: n-docosane, C22H46 system has been studied by X-ray crystallo-graphic and calorimetric techniques. The phase diagram exhibits no less than six distinct solid phases. The two terminal solid solutions, γ1 and γ2 are triclinic in keeping with the pure components. An orthorhombic phase, β0, exists at all compositions just below the solidus and has a centered (C or F) lattice. The structure of the low temperature orthorhombic phases β1, β2 (indistinguishable on the basis of powder photographs) has been determined from a single crystal X-ray analysis. The most probable space group of these phases is Bb21 m, the mirror planes required of the molecules perpendicular to their length being obtained by disordering.
110 citations
TL;DR: In this article, the authors compared calculated and experimental curves for 15 binary systems with 4,4′-di-n-alkoxybenzenes as components and proved that the agreement between theory and experiment is good.
Abstract: On the basis of the thermodynamics of ideal systems it is possible to calculate the melting curves of binary eutectic systems. We Compare calculated and experimental curves for 15 binary systems with 4,4′-di-n-alkoxybenzenes as components. Because of the occurrence of mixed solid crystals there are usually noticeable differences between calculated and experimental values in systems with neighbouring members of the homologous series. In most other cases the agreement between theory and experiment is good. A simple method for the theoretical estimation of eutectic points of multicomponent systems is described and proved.
69 citations
TL;DR: In this paper, the principal polarizabilities of nematic compounds with reference to the two approaches using either the Vuks formulae or the Neugebauer's relations are discussed.
Abstract: The calculation of the principal polarizabilities of nematic compounds with reference to the two approaches using either the Vuks formulae or the Neugebauer's relations is discussed. The polarizabilities and internal field constants have been calculated from the birefringence data in the nematic phase of seven compounds viz., (i) p-azoxyanisole, (ii) p-azoxyphene-tole, (iii) anisaldazine, (iv) p-methoxybenzylidene p-butylaniline, (v) 4-n-butyl-4 ′-ethoxy-α-chlorostilbene, (vi) 4-n-octyl-4′-etnoxy-α-chlorostilbene and (vii) mixture of the isomers of 4-methoxy-4′-butylazoxybenzene. The results support the validity of the Neugebauer's relations rather than the Vuks formulae. The isotropic polahzability and the polarizability α‖ parallel to the long axis of the molecules are accounted for on the basis that, in the case of the conjugated bonds immediately on either side and in between the benzene rings, the respective bond polarizabilities have values which correspond to their double bond polarizabili...
66 citations
TL;DR: The most powerful method for investigating the structure of the liquid crystalline phases is X-ray diffraction as discussed by the authors, which has led to some generally accepted structural models for several anisotropic mesophases.
Abstract: The phase equilibria of systems composed of a surface-active agent and water are usually complex, involving normal solutions, crystalline phases, and a variety of liquid crystalline phases. The most powerful method for investigating the structure of the liquid crystalline phases is X-ray diffraction. (A review on this subject has recently been given by Fontell1.) While these X-ray studies have led to some generally accepted structural models for several anisotropic mesophases there is still considerable controversy regarding the structure of the optically isotropic cubic mesophases which form in many binary and ternary soap systems. An important reason for this is that for many cubic phases the number of X-ray reflections obtained is insufficient for an unambiguous indexing of the diffraction pattern.
65 citations
TL;DR: In this paper, a discussion of dielectric properties of some nematic phenyl benzoates with the structure for n = 2, however, the sign depends on the para substituents in particular, replacement of an alkyl group by an alkoxy group shifts Δ∊ towards negative values.
Abstract: A discussion is given of the dielectric properties of some nematic phenyl benzoates with the structure For n = 2 the dielectric anisotropy Δ∊=∊‖– ∊⊥ is found to be positive. For n = 1, however, the sign depends on the para substituents In particular, replacement of an alkyl group by an alkoxy group shifts Δ∊ towards negative values. The frequency dependence of ∊‖ shows a relaxation at relatively low frequencies (kliz range), while ∊⊥ remains constant. The activation energy for this relaxation is 0.4 till 0.5 eV for all the compounds studied. In case of a positive static Δ∊ the relaxation leads to a change of sign of Δ∊ which is of interest for the study of electrohydrodynamic instabilities.
63 citations
TL;DR: In this article, the effects of different terminal rings on smectic properties of 4-MeO and 4-R.C6H4.N:CH.R4.
Abstract: New mesogens have been prepared and studied by differential thermal analysis and optical microscopy to elucidate the effects on mesomorphic properties of replacing a benzene ring by differnt hiterocyclic rings. Earlier workers have assessed the effects on nematic properties arising from such changes in the terminal groups of molecules of the type 4-MeO.C6H4.C6H4.N:CH.R4 (where R is phenyl, pyridyl or certain five-membered heterocylic rings). By studying, the corresponding n-octyl ethers, the relative effects of the different terminal rings on smectic properties have now been established. The compounds exhibit smectic polymorphism and the types of phase have been examined. Compounds of the type 4-R.CH:N.C6H4-C6H4-N:CH-R-4′ showed some differences in trends compared with those for compounds with only one terminal group, R. Other compounds, including two series of the type 4-X'C6H4-CO-O-C6H4-CH:CH.R-4′ (where X is methoxy or n-octyloxy and R is phenyl, pyridyl or diazinyl), have been examined and sh...
59 citations
TL;DR: In this article, the effect of piezc-electricity on deformation in a Schadt-Helfrich-cell is discussed, and the maximum angle of tilt γ m increases to almost π/2 even in a weak field.
Abstract: At rest between two metallized glass plates a nematic liquid shows a screw-like deformation pattern if one turns the plates by an angle ωm around an axis perpendicular to the plates. If a voltage U is applied, the molecules are tilted with respect to the electric field when U exceeds a threshold voltage. With increasing amount of twist ωm the maximum angle of tilt γ m increases to almost π/2 even in a weak field. Numerical results are given for the Schadt-Helfrich-cell, i.e. ωmλπ/2. The effect of piezc-electricity on deformation in a Schadt-Helfrich-cell is discussed.
57 citations
TL;DR: With the polarizing microscope, the authors examined thin sections and polished slices of the polymerized nematic and cholesteric mesophases and found that the textures they have observed are compatible with what one would expect to find in their liquid counterparts without the complication of surface effects.
Abstract: With the polarizing microscope, we have examined thin sections and polished slices of the polymerized nematic and cholesteric mesophases. We have found that the textures we have observed are compatible with what one would expect to find in their liquid counterparts without, however, the complication of surface effects.
49 citations
TL;DR: In this article, the conditions for the separate study of light generation in the plane-parallel anthracene plates and nonlinear fluorescence quenching due to exciton-exciton annihilation have been found.
Abstract: The fluorescence of a number of anthracene crystals has been studied over a wide range of nitrogen laser optical pumping. The conditions for the separate study of light generation in the plane-parallel anthracene plates and nonlinear fluorescence quenching due to exciton-exciton annihilation have been found. The threshold of generation I n (l) and quenching constant k ss have been found to be substantially dependent on the structural perfection of crystals. The values of I n (1) and K ss are reduced to 4.1019 cm−2 sec−1 and ∼ 10−9 cm3 sec−1 respectively in the most perfect subliminated plates. The decrese in the quenching constant is connected with the nontrivial influence of lattice defects on the process of exciton-exciton annihilation.
49 citations
TL;DR: In this paper, the effect of nonmesomorphic solutes of different sizes and shapes on nematic phase stability was investigated using a vacuum microbalance system which employs a Cahn RG Electrobalance to determine the weight of volatile solute absorbed by a known weight of nematic material at a given temperature and solute partial pressure.
Abstract: An experimental approach has been devised which permits the study of the effect of nonmesomorphic solutes of different sizes and shapes on nematic phase stability. The apparatus consists of a vacuum microbalance system which employs a Cahn RG Electrobalance to determine the weight of volatile solute absorbed by a known weight of nematic material at a given temperature and solute partial pressure. From such measurements, solute activity coefficients (γ2) were determined as a function of solute mole fraction (x 2) at several set temperatures (T) below the nematic-isotropic transition temperature (61.2°C) of the pure nematogen p-methoxybenzylidene-p'-n-propylaniline (MBPA). The solute probes studied were n-heptane, benzene and carbon tetrachloride. For all three systems the observed behavior was: In γ2 decreased sharply with increasing x 2 in the nematic region, became discontinuous at a mole fraction x2 corresponding to the solute induced transition point, and then decreased gradually (approaching ...
41 citations
TL;DR: In this paper, the visual detection between crossed polarizers of organic vapors on various liquid crystal films is described, and the most sensitive detector was found to be N-(p-methoxybenzilidine)-p-n-butylaniline (MBBA).
Abstract: The visual detection between crossed polarizers of organic vapors on various liquid crystal films is described. The liquid crystal is utilized as a smear on glass or as an impregnant in microporous polypropylene. The organic vapor induces a change of the liquid crystal from a birefringent mesophase to an isotropic liquid. The effect is generally reversible, i.e., removal of the film from the contaminated atmosphere restores the original state of the film. Non-selective detection is achieved easily in the ppm range. Of the liquid crystals examined, the most sensitive detector was found to be N-(p-methoxybenzilidine)-p-n-butylaniline (MBBA). Optical reflection from cholesteric materials, has been utilized previously for detection purposes. However, dependence on temperature and the need for instrumental monitoring to achieve high sensitivity do not currently allow the achievement of simple detector devices. In contrast, temperature effects and the need for instrumental monitoring are not as importa...
TL;DR: In this paper, it was shown that a suspension of microcrystals of chlorophyll-a (Chl-a) in a nonpolar medium may be electrodeposited as a film on a cathode.
Abstract: It is found that a suspension of microcrystals of chlorophyll-a (Chl-a) in a nonpolar medium may be electrodeposited as a film on a cathode. Spectroscopic properties of the film indicate that it is quite uniform and consists of randomly oriented microcrystals. Upon heating, the electrodeposited microcrystalline film is transformed to an alternate form of aggregated Chl-a. This new film appears to resemble very closely the Chl-a films previously obtained in monolayer work. Either film may be dissolved into monomer form and it is seen how Chl-a monomer is conserved during the complete process proceeding from electrodeposition to thermal conversion and the final dissolution. Furthermore, it is found how the oscillator strength over the red-visible region is the same for the dissolved monomer and the second type of film. The simple excited state resonance model for the spectrum of the latter is thus supported. The oscillator strength of the microcrystal film, by contrast, is about 40% greater. Such e...
TL;DR: In this paper, the authors indicate certain limitations to the elastic moduli and flexoelectric coefficients of liquid crystals in terms of their elasticity and elasticity properties, and their properties are discussed.
Abstract: Phenomenological and molecular considerations indicate certain limitations to the elastic moduli and flexoelectric coefficients of liquid crystals.
TL;DR: In this article, a theoretical and experimental study of the thermal properties of a well aligned nematic liquid crystal in a vertical temperature gradient is presented, where the authors show that due to the long time constant of the relaxation of the liquid crystal and the anisotropy of the heat conductivity, the value of the temperature gradient threshold for convective instabilities is strongly modified.
Abstract: We present a theoretical and experimental study of the thermal properties of a well aligned nematic Liquid Crystal in a vertical temperature gradient. Due to the long time constant of the relaxation of the director and to the anisotropy of the heat conductivity, the value of the temperature gradient threshold for convective instabilities is strongly modified as compared to the isotropic case. Experiments on planar MBBA films show a drastic reduction of the threshold. A detailed description of the shape and temperature distribution of the convective rolls is given.
TL;DR: In this paper, the authors measured the quasielastic width of nematic p-azoxyanisol at relatively small scattering vectors (K < 0.3A−1) and with a correspondingly high energy resolution (4·10−7 eV), using a backscattering neutron spectometer.
Abstract: Quasielasiic scattering experiments on nematic p-azoxyanisol have been performed at relatively small scattering vectors (K<0.3A−1) and with a correspondingly high energy resolution (4·10−7 eV), using a backscattering neutron spectometer. The observed quasielastic width can be interpreted in terms of self-diffusion, and the resulting diffusion constant (D = 4, 1.10−6 cm2/sec) agrees quite well with results from tracer experiments (Yun and Fredrickson). The quasielastic width measured by several other authors at relatively large K-values (several A−1) should be interpreted by rotational motions of the molecule with an estimated rotational diffusion constant of Dr = 1010 .. 10 11 sec−1.
TL;DR: In this paper, the Schroder-van Laar equations for predicting the liquidus temperature and the eutectic temperature and composition of binary mesophase systems were examined.
Abstract: Examination of a number of binary mesophase systems shows that the Schroder-van Laar equations for predicting the liquidus temperature and the eutectic temperature and composition are widely applicable. Additionally emperical correlations between the melting point and eutectic composition and temperature for binary mixtures having one common component are shown to be applicable to a considerable number of systems.
TL;DR: A review of the extensive studies on liquid crystals of polypeptides and the elucidation of their properties with particular stress on the electromagnetic orientation has been attempted, and the following hypotheses have been obtained:
Abstract: A review of the extensive studies on liquid crystals of polypeptides and the elucidation of their properties with particular stress on the electromagnetic orientation has been attempted, and the following hypotheses have been obtained:
TL;DR: In this article, the instabilities in AC fields are discussed for a planai layer of a nematic mixture of two phenyl benzoates. Due to a relaxation of ∊⊥ a reversal of the sign of the dielectric anisotropyΔ∊ occurs.
Abstract: The instabilities in AC fields are discussed for a planai layer of a nematic mixture of two phenyl benzoates. Due to a relaxation of ∊⊥ a reversal of the sign of the dielectric anisotropyΔ∊ occurs ...
TL;DR: The unit cell dimensions and space groups of the common n-alkyl esters of cholesterol (with the exception of the propanoate) have been determined by single crystal X-ray diffraction studies as discussed by the authors.
Abstract: The unit cell dimensions and space groups of the common n-alkyl esters of cholesterol (with the exception of the propanoate) have been determined by single crystal X-ray diffraction studies. No overall structural pattern is evident for this series but there are small groups of esters whose structures appear to form regular sequences. Using X-ray powder photography, the crystalline solid phases of the esters obtained from solution were compared with those obtained by cooling the melt. In the majority of cases there are no significant differences but for the mcthanoate, ethanoate, butanoate, pentanoate and dodecanoate there appears to be solid state polymorphism.
TL;DR: In this paper, the absorption spectra of oriented and amorphous naphthacene and pentacene films have been measured at high pressure, and the Davydov splitting of the 0-0 band in oriented films increase with increasing pressure at the rate of 37 cm−1/kbar for NPHTHACENE and 70 cm− 1/karp for pentACene.
Abstract: The absorption spectra of oriented and amorphous naphthacene and pentacene film have been measured at high pressure. The Davydov splitting of 0–0 band in oriented films increase with increasing pressure at the rate of 37 cm−1/kbar for naphthacene and 70 cm−1/kbar for pentacene. The 0–2 vibronic bands of both oriented films show the large red shifts compared with the 0–1 vibronic band. For amorphous films the Davydov splitting is not observed at atmospheric pressure. The pressure induced spectral red shifts in the amorphous state are slightly larger than those in the crystalline-state.
TL;DR: In this paper, the reliability of extrapolation of the transition curves has been questioned by some research workers and an attempt has been made under this investigation to arrive at some conclusive evidence in regard to reliability of the method.
Abstract: In binary systems, consisting of two structually similar components of which one is a liquid crystal, extrapolation of the transition curves should yield values for latent transition temperatures of the non-liquid crystalline substances. The reliability of this method of arriving at the latent transition temperatures has been questioned by some research workers. An attempt has been made under this investigation to arrive at some conclusive evidence in regard to the reliability of the method. Several binary systems with suitable Schiff's bases as one component and the well-known p-azoxyanisole or p-azoxyphenetole as the other liquidcrystal component have been studied from which a fair conclusion in favour of the dependability of the extrapolation method may be drawn. Yet, there cannot be a final word since some of the binary systems under study pose a difficulty in understanding their behaviour although discrcpancies observed have been accounted for in terms of polarity and non-ideality of the mix...
TL;DR: The absorption coefficient of all phases of p-methoxy benzylidene p-n-butylaniline was measured in the 80 to 400 cm−1 range.
Abstract: The absorption coefficient of all phases of p-methoxy benzylidene p-n-butylaniline was measured in the 80 to 400 cm−1 range. An analysics of the obtained data is given. There is evidence that there are two modifications (metastable and stable) in the solid phase featuring different absorption spectra. Pretransition effects were observed to accompany all phase transitions.
TL;DR: In this paper, a room-temperature mixture of nematic p-methoxy benzilidene p-n butyl aniline (MBBA) and cholesteryl nonanoate (CN) was used to investigate electrohydrodynamic instabilities in cholesteric liquid crystals with negative dielectric anisotropy.
Abstract: Electrohydrodynamic instabilities have been investigated in cholesteric liquid crystals with negative dielectric anisotropy. We have used a room-temperature mixture of nematic p-methoxy benzilidene p-n butyl aniline (MBBA) and cholesteryl nonanoate(CN). The ac electric field is applied parallel to the helical axis of the planar texture. The onset of a two-dimensional periodic pattern is optically observed at threshold. According to the excitation frequency, two regimes must be distinguished, namely a low frequency (conduction) regime and a high frequency (dielectric) regime. For both regimes, we give a full experimental analysis of the threshold voltage and of the period of the instability as a function of the various parameters (frequency, sample conductivity, cholesteric pitch, sample thickness). The dependence of the threshold conditions vs. frequency can be quantitatively accounted for by Hurault's theory in the low frequency regime. As far as the high frequency regime is concerned, no theory...
TL;DR: In this article, the liquid crystalline character of the plastic crystal mcsophases formed by non-amphiphilic globular molecules was investigated and the constitutional analogies of the nematic and smectic mesophases with fibrous and lamellar micelles in amphiphilic systems were recalled.
Abstract: Attention is drawn to the liquid crystalline character of the plastic crystals formed by “globular” molecules. The constitutional analogies of the nematic and smectic mesophases formed by non-amphiphilic “lath-like” molecules with the fibrous and lamellar mesophases formed by fibrous and lamellar micelles in amphiphilic systems are recalled and it is suggested that comparable constitutional analogies exist between the plastic crystal mesophases formed by non-amphiphilic globular molecules and the viscous isotropic cubic mesophases formed in micellar amphiphilic systems. It is suggested that the globular molecules with fairly free rotational motions and no long range orientational order which characterise the non-amphiphilic plastic crystal mcsophase are replaced in the amphiphilic viscous isotropic cubic mesophases by globular micelles again with fairly free rotational motions and no long range orientational order. In both these classes of rotational mesophases the details of the cubic lattice ad...
TL;DR: Differential thermal analysis to 2 kbar of zone-refined p-azoxyanisole in hydrostatic apparatus has corroborated the results of Hulett and of Robbcrecht at lower pressures.
Abstract: Differential thermal analysis to 2 kbar of zone-refined p-azoxyanisole in hydrostatic apparatus has corroborated the results of Hulett and of Robbcrecht at lower pressures—initial slopes ≲ 48 deg k...
TL;DR: In this paper, spin-labeled fatty acids are used to investigate a lyotropic mesophase in which the amphiphilic molecules are arranged in cylindrical rods and a homogeneous ordering of the cylinders is obtained by sucking the liquid crystalline phase in glass capillaries.
Abstract: Spin labeled fatty acids are used to investigate a lyotropic mesophase in which the amphiphilic molecules are arranged in cylindrical rods. A homogeneous ordering of the cylinders is obtained by sucking the liquid crystalline phase in glass capillaries. This considerably simplifies the epr spectra. The lipid molecules in the cylinders undergo fast rotations around their long axes with a correlation time of approximately 10−10 sec and slower rotations around the axis of the cylinder. The latter effect is responsible for distinct differences between the spin label spectra of cylinder and bilayer phases. The hydrocarbon chains in the cylinders are flexible and the order parameter decreases from the polar surface towards the interior of the cylinders. The microviscosities of both phases are about equal and amount to approximately 0.05 poise.
TL;DR: In this article, the anisotropic electrical conductivity of MBBA (p-methoxybenzylidene-p-n-butylaniline) containing tetrabutylammonium tetraphenyl-boride (henceforth denoted as AB) was investigated.
Abstract: An investigation was made of the anisotropic electrical conductivity (σ‖ and σ⊥) of MBBA (p-methoxybenzylidene-p-n-butylaniline) containing tetrabutylammonium tetraphenyl-boride (henceforth denoted as AB where A+ is the tetrabutylammonium cation and B− is the tetraphenylboride anion). Alignment parallel and perpendicular to the applied electric field was accomplished by means of a 6000 Gauss magnetic field. The measurements were made at 22°C in a frequency range of 5,000 to 20,000 Hertz and at a sequence of nominal concentrations of AB up to about 10−4 moles/liter. The DC angularaverage conductivity σ(= 1/3 σ‖ + 2/3 σ1) was obtained by extrapolating plots of σ versus the square of the frequency to zero frequency. The experimental measurements fit apparently quite well by a simple dissociation model, AB ≠ A+ + B− with the results From these results we obtain for the mobility sums
TL;DR: In this article, it was shown that the requirement that the basic unformly-oriented state of equilibrium of a nematic liquid crystal be stable to small disturbances leads to Parodi's relation between the viscosity coefficients, provided the material aligns in shear flow.
Abstract: It is shown that the requirement that the basic unformly-oriented state of equilibrium of a nematic liquid crystal be stable to small disturbances leads to Parodi's relation between the viscosity coefficients, provided the material aligns in shear flow. This relation was first derived from Onsager reciprocal relations.
TL;DR: In this article, the surface tension and density of azoxyanisole were measured using a modified Wilhelmy technique which allowed continuous temperature scanning was used to make surface tension measurements.
Abstract: p-Azoxyanisole has a nematic liquid crystal phase in addition to the normal isotropic liquid phase. In line with the rapid increase in interest in liquid crystals in recent years, its properties have been the subject of several investigations. As with nearly all initial explorations, however, these studies have raised questions as well as provided answers. Barrall, Porter and Johnson1 measured specific heat as a function of temperature, using differential scanning calorimetry and reported an unexpected knee in the liquid-liquid transition peak. Neumann and Sell2 studied the surface tension and density of azoxyanisole. A modified Wilhelmy technique which allowed continuous temperature scanning was used to make surface tension measurements. Temperature scanning density measurements were made by measuring the buoyancy of a glass bead suspended in the liquid from an electrobalance. Their results showed a step decrease at the liquid-liquid transition in both surface tension and density on heating. A s...
TL;DR: In this article, the Munn-Siebrand theory of charge transport in anthracene has been applied to a number of aromatic hydrocarbon crystals belonging to the space group P21/a (Z=2).
Abstract: The calculation of two-center transfer integrals (TIs) has been critically examined and closed form analytical expressions have been obtained for them. It has been found that the TI between similar molecules (0,0,0) and (l,m,n) [molecular center located at la+mb+nc] is not the same as that between (0,0,0) and (l,m,n). This necessitates a reformulation of the band theory of charge transport in organic semiconductors. The revised theory has been applied to a number of aromatic hydrocarbon crystals belonging to the space group P21/a (Z=2). In view of the Munn-Siebrand theory of charge transport in anthracene, an attempt has been made to predict the likely mechanism of charge transport (coherent or hopping) in various directions in these crystals. Effect of the molecular size on semiconduction has also been discussed.