Showing papers in "Molecular Crystals and Liquid Crystals in 1971"
TL;DR: In this article, the authors assume that the local state of order in the isotropic phase is a symmetric traceless tensor Qαβ, proportional to the anisotropic part of a tensor property such as the magnetic susceptibility.
Abstract: We assume that (1) the local state of order in the isotropic phase is a symmetric traceless tensor Qαβ, proportional to the anisotropic part of a tensor property such as the magnetic susceptibility; (2) the free energy may be expanded in powers of Qαβ and of its gradients. This allows a unified description covering the anomalous magnetic birefringence, the intensity of light scattering, and the properties of the nematic/isotropic interface. For a cholesteric, although the optical rotation is huge in the ordered phase, we predict that it should not be anomalous just above the transition point Tc. We also investigate the dynamics of fluctuations of Qαβ, and discuss the flow birefringence, the frequency width of the Rayleigh scattering, and the attenuation of ultrasonic shear waves, in terms of 3 viscosity coefficients.
766 citations
TL;DR: In this paper, the dispersion energy between two molecules is calcualted taking into account not only the dipole-dipole interaction but also the dipolesquadraupole interaction.
Abstract: The molecular properties of optically active molecules are used to extend the molecular statistical theory of the nematic phase in order to explain the helical structure of the cholesteric phase. In order to account for these properties the dispersion energy between two molecules is calcualted taking into account not only the dipole-dipole interaction but also the dipole-quadraupole interaction. The calculated twist thas the right order of magnitude. The theory moreover explains the twisting power of optically active solute molecules in a menatic solvent and also the concentration dependence of the induced twist angle. A consequence of this theory is that the helical structure only exists, if the distribution of orientations around the long molecular axes is not rotationally symmetric; one may expect this to be the case for e.g. planar molecules.
157 citations
TL;DR: In this paper, the properties of lattice models and continuum models of a hard rod liquid with and without attractive energies are presented, and the theory is of the molecular field variety in that short-range-order is neglected.
Abstract: Calculations of the properties of some lattice models and continuum models of a hard rod liquid with and without attractive energies are presented. The theory is of the “molecular field” variety in that short-range-order is neglected. Both the long-range-order parameter and the density are varied to obtain the state of minimum Gibbs free energy at given pressures and temperatures. Among the properties reported are expansivity, compressibility and specific heat. Each of these displays pretransition effects qualitatively similar to those seen on the low temperature side of nematic liquid crystal-isotropic liquid phase transitions. There are important quantitative discrepancies however. It appears that the discrepancies are due more to the approximate nature of solutions than to the idealizations of the models. The “near critical” nature of liquid crystal phase transitions cannot be well described in terms of long-range-order alone. One gross error of the hard-rod models can, however, be remedied. T...
134 citations
TL;DR: A theory of melting with application to liquid crystals in which both translational and orientational order parameters are taken into account is developed in a manner similar to that of Kirkwood and Monroe as discussed by the authors.
Abstract: A theory of melting with application to liquid crystals in which both translational and orientational order parameters are taken into account is developed in a manner similar to that of Kirkwood and Monroe.
114 citations
TL;DR: In this article, the effects of branching the alkyl chain have been investigated using alkyls groups containing three, four, five and six carbon atoms, and the effect of having a group other than methyl as the end unit of a n-alkyl chain has been examined by studying the liquid cry...
Abstract: Five homologous series (methyl to n-decyl) of n-alkyl 4-p-substituted-benzylideneaminocinnamates with p-substituents = CN2, NO2, MeO2 MeCO.O and Ph have been prepared and the liquid crystalline properties established. The relative thermal stabilities of the mesophases of the esters with different p-substituents are in keeping with established orders of terminal group efficiency, but in certain of the series the trends in transition temperatures with increasing alkyl chain length give rise to novel smooth curve relationships. The effects of branching the alkyl chain have been investigated using alkyl groups containing three, four, five and six carbon atoms. The effects of a methyl substituent on the first carbon of the alkyl chain are particularly marked, nematic properties suffering most dramatically. The behaviour is discussed in terms of intramolecular steric effects. The effects of having a group other than methyl as the end unit of a n-alkyl chain have been examined by studying the liquid cry...
92 citations
TL;DR: In this paper, a more general approach is outlined for arbitrary angles of incidence, and the two principal dielectric constants are determined from the angles of total reflection measured with a refrectometer.
Abstract: Selective light reflection at the plane structure of a cholesteric liquid crystal is treated by the theory of de Vries(1) for normal incidence only. A more general approach is outlined for arbitrary angles of incidence. Conditions for the borders of the reflection bands are given. Also the cases of ordinary total reflection are included. Using the derived formula, the two principal dielectric constants are determined from the angles of total reflection measured with a refrectometer. Their values are used to calculate ther eflection spectrum of a layer of finite thickness at normal incidence which is compared with an experimental spectrum. In agreement with the experimental curve the calculated spectrum shows a central band with a nearly rectangular shape flanked by a series of maxima and minima due to interferences, the positions of which depend strongly on the dielectric constants.
85 citations
IBM1
TL;DR: In this paper, the energy of interaction between two asymmetric molecules is considered and a generalization of the Maier-Saupe model of a nematic liquid is proposed, where the exact state of minimum internal energy is found to be biaxial.
Abstract: The general form of the energy of interaction between two asymmetric molecules is considered. This leads to a generalization of the Maier-Saupe model of a nematic liquid. The exact state of minimum internal energy of the system is found to be biaxial. In the molecular field approximation the consistency relations yield two distinct ordered states, a uniaxial state and a biaxial state. It is shown that with decreasing temperature the fluid will have two successive transitions according to the scheme: isotropicuniaxial orderbiaxial order.
78 citations
TL;DR: In this paper, the microscopic textures and the relations of miscibility suggest that two of these modifications can be assigned to the group smectic A and C. Their characteristics require the establishment of two new groups of modified pyrimidine with the arbitrary symbol Smectic F resp. G.
Abstract: 2-(4-n-pentylphenyl)-5-(4-n-pentyloxyphenyl)-pyrimidine has been prepared. By microscopic investigation, differential scanning calorimetry, dilatometric measurements and X-ray investigation, four different smectic modifications have been stated. The microscopic textures and the relations of miscibility suggest that two of these modifications can be assigned to the group smectic A and C. The other two smectic modifications cannot be assigned to any of the five known groups of smectic modifications. Their characteristics require the establishment of two new groups of smectic modifications with the arbitrary symbol smectic F resp. G.
69 citations
TL;DR: In this paper, the effect of hydrostatic presssure on phase transitions in PAA up to 3 kbar was studied and the different phases (solid, nematio and isotropic) were characterized by their NMR spectra.
Abstract: We report the study of the effect of hydrostatic presssure on the phase transitions in PAA up to 3 kbar. The different phases (solid, nematio and isotropic) are characterized by their NMR spectra.
64 citations
TL;DR: In this article, the dispersion equations for shear waves propagating in a nematic liquid crystal were derived from the Ericksen and Leslie's hydrodynamic equations, and the complex refection coefficient of a shear wave at a solid-nematic liquid-crystal interface is related to effective viscosity coefficients depeding on the orientation of the molecules with respect to the polarization of the wave.
Abstract: Dispersion equations for shear waves propagating in a nematic liqid crystal are derived from the Ericksen and Leslie's hydrodynamic equations. The complex refection coefficient of a shear wave at a solid-nematic liquid crystal interface is related to effective viscosity coefficients depeding on the orientation of the molecules with respect to the polarization of the wave. Some experimental results relative to paramethoxybenzilidene-butylanilin are presented.
64 citations
TL;DR: In this article, the refractive indices of anisaldazine, CH3OC6H4CH:NN:CHC4H4OCH3, have been measured in the crystalline, nematic and isotropic phases and the orientational order parameter in the mesophase has been evaluated by the application of the Vuks formula.
Abstract: The refractive indices of anisaldazine, CH3OC6H4CH:NN:CHC4H4OCH3, have been measured in the crystalline, nematic and isotropic phases and the orientational order parameter in the mesophase has been evaluated by the application of the Vuks formula. The curve for the order parameter versus the relative temperature (Te-T) is nearly parallel with those for p-azoxyanisole and p-azoxyphenetole and lies approximately midway between them.
TL;DR: A theory of the rotatory power of thin films of cholesteric liquid crystals was developed in this article, and the results are in qualitative agreement with the theory both inside and outside the region of reflexion.
Abstract: A theory is developed of the circular dichroism and rotatory power of thin films of cholesteric liquid crystals. It is an extension of the treatment for infinitely thick specimens discussed in a previous paper. The therory predicts that the circular dichroism of a thin film plotted as a function of wave-length should exhibit a principal maximum accompanied by subsidiary maxima, and that the rotatory dispersin should be anomalous. Experimental circular dichroism and rotatory dispersion curves are presented for cholesteric cinnamate, cholesteryl-2-propyn-1-yl carbonate and spectratherm. The results are in qualitative agreement with the theory both inside and outside the region of reflexion. The rotatory dispersion curves are also in accord with the de Vries equation outside the region of reflexion.
TL;DR: In this paper, the dielectric relaxation versus temperature and resistivity anisotropy of a room temperature nematic, the p-methoxy benzylidene p-n-butylaniline (MBBA), were measured.
Abstract: Measurements are presented on the dielectric relaxation versus temperature [0–10 MHz] and resistivity anisotropy of a room temperature nematic, the p-methoxy benzylidene p-n-butylaniline, (MBBA), pure or doped with organic ions.
TL;DR: In this paper, the mesomorphic behavior of the first sixteen members of the cholesteryl w-phenylakanoates was investigated and it was shown that the enthalpies and entropies exhibit a pronounced odd-even behavior.
Abstract: The determination of mesomorphic behavior was extended to cover the first sixteen members of the cholesteryl w-phenylakanoates. Microscopy and scanning calorimetry were used to identify the mesophases and to measure transition temperatures and associated latent heats. The clearing temperatures fall on two distinct curves: a high temperature branch occurring for odd number of C-atoms of the acid chain and a low temperature branch occurring for even numbers of C-atoms. The difference in clearing points of neighboring homologues decreases from about 100°C for low members to a few degrees for high members of the series. Contrary to this, the smectic-cholesteric transition temperatures show no such odd-even effect, although their enthalpies and entropies do exhibit a pronounced odd-even behavior. The enthalpies and entropies do exhibit a pronounced odd-even behavior. The enthalpies and entropies of the cholesteric-isotropic transitions show an odd-even effect where the odd chain length branch has the ...
TL;DR: The first optically active smectic liquid crystal to consist of only one compound and to permit supercooling to room temperature was reported in this article, which is bis-(p-6-methyl octyloxybenzylidene)-2-chloro-1, 4-phenylenediamine.
Abstract: The discovery is reported of the first optically active smectic liquid crystal to consist of only one compound and to permit supercooling to room temperature. The compound is bis-(p-6-methyloctyloxybenzylidene)-2-chloro-1, 4-phenylenediamine.
TL;DR: Pitch versus composition data in a binary mixture of a cholesteric and a room-temperature nematic are used to demonstrate that molecular influence in pitch determination is more linear when credited according to weight percent than when credited to mole % as mentioned in this paper.
Abstract: Pitch versus composition data in a binary mixture of a cholesteric and a room-temperature nematic are used to demonstrate that molecular influence in pitch determination is more linear when credited according to weight percent than when credited according to mole %. It is also shown that the effective rotary power of cholesteryl iodide is influenced by its environment to the extent that it exhibits the chirality of the other constituent in a binary mixture.
TL;DR: In this paper, the role of fluid flows is discussed in connection with their importance for domain formation and turbulence occurrence in p-azoxyanisole and di-n-heptoxy-azobenzene.
Abstract: The results of simultaneous electrical and optical investigations in p-azoxyanisole and di-n-heptoxy-azobenzene are presented. The role of fluid flows is discussed in connection with their importance for domain formation. The mechanism of domain formation and turbulence occurrence is found to be different for d.c. and very low frequencies than it is for higher frequencies. For strong electric fields the behaviour of azoxyanisole is very different from that of the azo compound, a fact attributed to the different sign of the dielectric anisotropy.
TL;DR: In this paper, the possibility of preparing stereospecific polymers from monomers organized in a liquid crystalline medium was investigated and the tacticity of the polymers was studied by infrared and NMR spectroscopy.
Abstract: The possibility of preparing stereospecific polymers from monomers organized in a liquid crystalline medium was investigated. Methacryloyloxybenzoic Acid (I) was polymerized in a smectic and in a nematic mesophase. Cetyloxybenzoic acid (II) was used as the smectogen and heptyloxybenzoic acid (III) as the nematogen. The molar ratios of II and III to I were 1/1 or 1/0.5. All the materials were very carefully purified. The mesophase transition temperatures were determined with the usual techniques. Benzoyl peroxide was the polymerization initiator in the nematic phase and for the polymerization in the smectic state, thin films deposited on freshly cleaved mica plates were subjected to UV radiation. The tacticity of the polymers was studied by infrared and NMR spectroscopy. The carbonyl region showed two IR absorption bands, one at 1690 and the other at 1740 cm−1, the former predominant in the isotactic and the latter in the syndiotactic polymers. Tacticities were computed from NMR spectra of poly (m...
TL;DR: In this paper, a gas-liquid chromatography (GLC) approach is proposed for determining the degree of order, S, in a nematic mesophase, which is based on the assumption of a two phase model and on exact thermodynamic arguments, a set of simultaneous equations is derived for evaluating S from GLC-determined solute infinite dilution activity coefficients and partial molar excess enthalpies.
Abstract: Owing to thermal motion, molecular orientation is never entirely complete in a nematic mesophase. The commonly used methods of obtaining a measure of the extent of parallelism in this mesophase are ESR and NMR. Proposed here is a gas-liquid chromatography (GLC) approach for determining the degree of order, S. Our working model regards the nematic phase as composed of a fraction of completely aligned molecules and a fraction of completely isotropie molecules. Further, solute molecules dissolved in the nematic state are assumed to distribute themselves between these two arrangements. Based on the assumptions of this “two phase” model and on exact thermodynamic arguments, a set of simultaneous equations is derived for evaluating S from GLC-determined solute infinite dilution activity coefficients and partial molar excess enthalpies. From meassurements on two nematogenic substances, P-azoxyanisole and 4, 4′-dihexoxyazoxybenzene, S values are obtained which are independent of the nature of the solute ...
TL;DR: In this paper, the deuteron resonance from D2O molecules exhibit quadrupole splitting in the region of ∼0 to 11 kHz, and the origin of this splitting and its dependence on system composition and temperature has been investigated in both two and three component systems, containing different amphiphiles, such as n-octylamine, the corresponding hydrochloride and hydrobromide, nonylphenol polyethylene glycol ethers, n-Octanoic acid and n-decanol.
Abstract: Lyotropic liquid crystals of the lamellar type have been studied by means of deuteron and proton magnetic resonance. The deuteron resonance from D2O molecules exhibit quadrupole splitting in the region of ∼0 to 11 kHz. The origin of this splitting and its dependence on system composition and temperature has been investigated in both two- and threecomponent systems, containing different amphiphiles, such as n-octylamine, the corresponding hydrochloride and hydrobromide, nonylphenol polyethylene glycol ethers, n-octanoic acid and n-decanol. It has been shown that the quadrupole splitting can give information concerning the rate of deuteron exchange between water and amphiphilic molecules. Effects of solubilization of hydrocarbons on the quadrupole splitting have also been investigated. High resolution proton resonance signals have also been observed and the width and shape of these signals are discussed. Some of the mesophases investigated can be aligned by means of a magnetic field of ca. 14 kilog...
TL;DR: In this paper, the focal length of the liquid crystal lens system responsible for visible domain pattern was measured and the resistance change associated with this electrohydrodynamic mode was quantitatively interpreted in terms of the director's spatial distribution which was predicted by Helfrich's theory of conduction induced alignment.
Abstract: AC electric fields are known to induce a visible domain pattern in certain nematic liquid crystals. We have measured the focal length of the liquid crystal lens system responsible for this visibility. We have also measured the resistance change associated with this electrohydrodynamic mode. These results can be quantitatively interpreted in terms of the director's spatial distribution which is predicted by Helfrich's theory of conduction induced alignment.
TL;DR: In this article, the temperature dependence of the Raman low frequency spectra of the three solid polymorphic phases of p-dichlorobenzene has been used to prove that the anomalous γ-phase is reversible at atmospheric pressure.
Abstract: The temperature dependence of the Raman low frequency spectra of the three solid polymorphic phases of p-dichlorobenzene has allowed us to prove that the so-called “anomalous” γ-phase is, in fact, the actual stable low temperature modification (below 279 K): we have shown that the transformation (α, γ) is reversible at atmospheric pressure. Our results and previous ones obtained under pressure by others lead us to deduce the existence of a triple point between the three solid phases of p-dichlorobenzene around room temperature and at pressures of the order of 500 bars.
TL;DR: In this article, it was shown that the instabilities in nematic liquid crystals (cellular flow in domains, turbulence) arise by the same mechanism as similar phenomena in isotropic liquids.
Abstract: Electrohydrodynamic instabilities in nematic liquid crystals (cellular flow in domains, turbulence) arise by the same mechanism as similar phenomena in isotropic liquids. In agreement with this vie...
TL;DR: The temperature dependence of diffusion coefficients in PAA, anisalazine, 4,4′-diheptyloxy-azoxybenzene, anhidrous Na-palmitate and Na-Palmitate-water mixtures has been studied by quasi-elastic scattering of cold neutrons and by the variable gradient proton spin echo method as discussed by the authors.
Abstract: The temperature dependence of the diffusion coefficients in PAA, anisalazine, 4,4′-diheptyloxy-azoxybenzene, anhidrous Na-palmitate and Na-palmitate-water mixtures has been studied by quasi-elastic scattering of cold neutrons and by the variable gradient proton spin echo method.
TL;DR: In this paper, the quadrupole and proton spin-lattice relaxation times in selectively deuterated terephthalbis-(4-amino-fluorobenzene) and other compounds were measured.
Abstract: Measurements of the quadrupole and proton spin-lattice relaxation times, T 1, in selectively deuterated terephthalbis-(4-amino-fluorobenzene)as well as proton relaxation in other compounds are comp...
TL;DR: In this paper, the authors show that the electrical conductivity of quinolinium is very high at room temperature (σ = 100 Ω−1 cm−1)l and the magnetic susceptibility is temperature independent from about 50 °K to 300 °K.
Abstract: Previous electrical,(1) optical(2) and magnetic susceptibility(3) data of the complex anion-radical salt quinolinium (TCNQ)2 have been interpreted in terms of a model in which the close face to face stacking of TCNQ molecules allows the formally unpaired electron (one unpaired electron per two TCNQ molecules) to be completely delocalized over the entire TCNQ array producing a strongly degenerate electron gas. Specific evidence for this model comes from several sources: the X-ray data for other 1 : 2 complexes (e.g. triethylammonium (TCNQ)2 (4) tetraphenylphosphonium (TCNQ)2 (5)) show a face to face stacking of TCNQ with a closer than van der Waals separation between the molecular planes (no X-ray data are available for the quinolinium salt); the electrical conductivity for this complex is very high at room temperature (σ = 100 Ω−1 cm−1)l; the magnetic susceptibility is temperature independent from about 50 °K to 300 °K which has been interpreted as being due to Pauli paramagnetism(3) and the EPR ...
TL;DR: The coefficient D for self-diffusion in nematic p-azoxyanisole has been determined from measurements of the energy width of quasi-elastically scattered neutrons.
Abstract: The coefficient D for self-diffusion in nematic p-azoxyanisole has been determined from measurements of the energy width of quasi-elastically scattered neutrons. The directional dependence of D was...
TL;DR: In this paper, the birefringence of the smectic high temperature modification of the anhydrous thallium soaps independece on temperature and at different wavelength was studied.
Abstract: The birefringence has been studied of the smectic high temperature modification (neat phase) of the anhydrous thallium soaps independece on temperature and at different wavelength. The measurements were carried out on uniaxially orientated liquid crystalline layers on the principle of Abbe's double prism. The lower homologues exhibit negative double refraction which decreases with increasing chain length and decreasing temperature. In the middle members of the homogous series a change in the sign of double refraction is observed. The higher members exhibit positive birefringence, which increases with increasing alkyl chain length and with decreasing temperature. An attempt will be made to to explain this behaviour on the basis of the structure of the smectic modification of the thallium salts.
TL;DR: In this article, a thermal imaging system using cholesteric liquid crystals has been designed and built which utilizes the extreme temperature sensitivity of cholesteryl oleyl carbonate (CHO) carbonate.
Abstract: A thermal imaging system using cholesteric liquid crystals has been designed and built which utilizes the extreme temperature sensitivity of cholesteryl oleyl carbonate. Theoretical considerations for the design and the operating conditions of the device are given. The experimental results indicate that the limitations on such a device are primarily due to the finite film thickness required for scattering by the cholesteric liquid (on the order of ten micrometers). This system was able to image 0.2°C with a limiting resolution of one line pair per millimeter. Improved lighting for direct viewing of the liquid crystal film should result in a sensitivity of about 0.02°C.
TL;DR: The recent upsurge of activity in the field of liquid crystals is undoubtedly a prelude to the synthesis of now mesomorphic compounds of varied structure as mentioned in this paper, which are characterized typically by two parameters; the structure of the various phases, e.g. smectic, nematic, etc., and by the transition temperatures between these phases.
Abstract: The recent upsurge of activity in the field of liquid crystals is undoubtedly a prelude tothe synthesis of now mesomorphic compounds of varied structure. Liquid crytals are characterized typically by two parameters; the structure of the various phases, e.g. smectic, nematic, etc., and by the transition temperatures between these phases.