Showing papers in "Molecular Crystals and Liquid Crystals in 1973"
TL;DR: In this article, the authors discuss the nature of the order in the main smectic phases (A, C, and B) together with the related phase transitions, and suggest that the B phase may correspond to a plastic crystal, or to a system of weakly coupled solid layers: to discriminate between these two models, acoustical studies using shear waves appear to be most promising.
Abstract: We discuss the nature of the order in the main smectic phases (A, C, and B) together with the related phase transitions. (1) The B phase may correspond to a plastic crystal, or to a system of weakly coupled solid layers: to discriminate between these two models, acoustical studies using shear waves appear to be most promising, (2) the transitions A N, C N, C A (where N stands for nematic) may be of second order in certain cases, which will allow for a number of interesting experiments. The C A transition should be very similar to the λ transition of helium, and some critical exponents which are not accessible in helium could be measured here. The N A transition is similar to the onset of superconductivity in a metal. A bend (or twist) deformation tends to suppress the A phase just as a magnetic field destroys superconductivity. In the N phase the twist and bend elastic constants should show pretransitional anomalies. The C N transition is more complex. A tentative calculation suggests...
244 citations
TL;DR: This article observed alignment of nematic, cholesteric and smectic liquid crystals in directions depending on the orientation of submicroscopic, parallel grooves in adjacent glass and fused quartz surfaces.
Abstract: We have observed alignment of nematic, cholesteric and smectic liquid crystals in directions depending on the orientation of submicroscopic, parallel grooves in adjacent glass and fused quartz surfaces. The alignment can be explained on the basis of elastic strain energy in the bulk of the liquid crystal that would be increased if the long axes of the molecules were aligned by the surface but were not approximately parallel to the groove direction. We have also observed alignment parallel to grooved surfaces but skewed with respect to the grooves, apparently when long molecules become attached to the surface in a direction that causes a competing torque on the director of adjacent liquid crystal molecules.
243 citations
TL;DR: The role of surface topography and surface chemistry has been distinguished through the use of known surfactans, intentionally deformed substrates, and electron microscopy, and the roles of surface chemistry have been distinguished: substrates with surface energy lower than the liquid crystal surface tension cause homeotropic alignment; otherwise alignment is parallel to the substrate plane, following any long-range order.
Abstract: Chatelain hypothesized that orientation of liquid crystals by interaction with the substrate is due to an adsorbed layer of fatty contaminants, but substrate topography has also been postulated as a causal factor. Experiments with chemically cleaned surfaces rubbed without contamination now confirm the importance of the impurity layer, which has been investigated using Auger spectrometry. Through the use of known surfactans, intentionally deformed substrates, and electron microscopy, the roles of surface topography and surface chemistry have been distinguished: substrates with surface energy lower than the liquid crystal surface tension cause homeotropic alignment; otherwise alignment is parallel to the substrate plane, following any long-range order, e.g., grooves, present on the surface.
176 citations
TL;DR: In this paper, the structure of spherical droplets of nematic and cholesteric materials floating in an isotropic liquid was experimentally investigated by microscopic observation with polarized light.
Abstract: The structure of spherical droplets of nematic and cholesteric materials floating in an isotropic liquid is experimentally investigated by microscopic observation with polarized light. In nematic droplets either a star configuration or a bipolar configuration have been observed depending on the nature of the isotropic liquid. Cholesteric droplets adopt a spiral shaped structure whenever the radius of the droplet is much larger than the helical pitch. Changes in optical patterns induced by a magnetic field are analysed.
159 citations
TL;DR: In this paper, the actual position of the system of liquid crystalline modifications is discussed, which is based on the relations of miscibility of the liquid crystal modifications and on their texture.
Abstract: We deal with the actual position of the system of liquid crystalline modifications. This system is based on the relations of miscibility of the liquid crystalline modifications and on their texture...
150 citations
TL;DR: In this article, a combination of polarization microscopy, using a hot stage, and X-ray diffraction analysis was used to study disodium chromoglycate (DSCG) molecules.
Abstract: The system has been studied by a combination of polarization microscopy, using a hot stage, and X-ray diffraction analysis. The sequence of condensed phases occurring at room temperature with increasing concentration of disodium chromoglycate (DSCG) is: (i) a mesophase N; (ii) a mesophase M; (iii) crystalline solid solutions SS. A variety of evidence has shown that the molecule of DSCG retains an essentially planar configuration throughout the system. The mesomorphic phases are optically negative, indicating a parallel, or approximately parallel, arrangement of the molecular planes. (A parallel arrangement of these planes in the SS solid solutions had been previously indicated by a preliminary X-ray analysis, which pointed to there being only one molecule in the unit cell; and in conformity with such an arrangement the crystals were found to be optically negative.) The transformations between the N and M phases and between the M and SS phases take place pseudomorphically, and on reversal the orig...
148 citations
Abstract: A novel experimental procedure combining the measurement of (i) the flow velocity and (ii) the change in birefringence induced by laminar flow in a magnetic field has been used to determine the complete set of the five independent shear-viscosity coefficients of nematic liquid crystals The anisotropy of the magnetic susceptibility is obtained as well Results are reported for p'-methoxybenzylidene-p-n-butylaniline (MBBA), p-n-hexyl-oxybenzylidene-p'-aminobenzonitrile (HBAB), and a 1:1:1-molar mixture of HBAB with p-n-butoxybenzylidene-p'-aminobenzonitrile and p-n-octanoyl-oxybenzylidene-p'-aminobenzonitrile
126 citations
TL;DR: In this article, the structure and properties of disclination lines and disclination points in nematic and cholesteric liquid crystals are reviewed, and an equivalence between the directorfield of nematics and electrostatic or magnetostatic fields in two-dimensions is pointed out.
Abstract: Structure and properties of disclination lines and disclination points in nematic and cholesteric liquid crystals are reviewed. An equivalence between the directorfield of nematics and electrostatic or magnetostatic fields in two-dimensions is pointed out. Dislination lines can be classified in singular and non-singular lines. Disclination points correspond always to singular solutions. They are often associated with non-singular lines. Experimental observations of disclinations, of surface patterns in cholesterics, and of cholesteric textures are described. Structural features of special cholesteric “focal-conic” and “fan” texture are discussed in terms of the vectorfield defined by the twist axis.
123 citations
TL;DR: In this paper, the microscopic theories which have been proposed for lyotropic liquid crystals are studied, in particular in connection with the model gas of hard rods, and a series of experimental tests are proposed which would allow determination of the coefficients of the Onsager expansion; available experimental data are reviewed.
Abstract: The microscopic theories which have been proposed for lyotropic liquid crystals are studied, in particular in connection with the model gas of hard rods. For sufficiently long rods most theories are equivalent to that of Onsager; for shorter rods no theory seems to have a very good claim to accuracy. The theory of Flory is shown to be untrustworthy. A series of experimental tests is proposed which would allow determination of the coefficients of the Onsager expansion; available experimental data are reviewed.
113 citations
TL;DR: The history of our field of interest is more clearly discernible when time periods are chosen which are covered by characteristic summarizing papers Such papers can serve as the landmarks when a rather complex development is to be described briefly So I shall report on the following periods: First period, was from the middle of the 19th century until 1908, comprising the discovery of liquid crystals, and work of predominantly descriptive character as mentioned in this paper.
Abstract: The history of our field of interest is more clearly discernible when time periods are chosen which are covered by characteristic summarizing papers Such papers can serve as the landmarks when a rather complex development is to be described briefly So I shall report on the following periods: First period, was from the middle of the 19th century until 1908, comprising the discovery of liquid crystals, and work of predominantly descriptive character Monographs, covering this period, are those of Lehmann (1) (1904), Schenck (2) 1905, and Vorlander (3) (1908) The originals of all the older monographs are presented and held available for inspection Second period, was from 1908 to 1922 This is the time of the first theoretical work and the foundation of systematology of liquid crystals We must name E Bose,(4) M Born,(5) F Stumpf(6) (1918), G Friedel(7) (1922), and again D Vorlander(8) (1924) and O Lehmann(9) (1922) Third period, was from 1922 to 1933 This was perhaps the most fruitful pe
94 citations
TL;DR: The Onsager theory for the liquid crystal phase transition of a gas of hard rods is known to be accurate if the rods are long enough as mentioned in this paper, but it is not quantitatively accurate for L/D < 100.
Abstract: The Onsager theory for the liquid crystal phase transition of a gas of hard rods is known to be accurate if the rods are long enough. In order to quantify the necessary length, the first correction term (the third virial coefficient) to the Onsager theory is estimated numerically. On the basis of a study of the behavior of this function (for the cases L/D = 10, 20, 40, and 100), a model function which approximates its angular dependence is proposed. This is used to estimate the corrections to the predictions of the Onsager theory arising from the finite length of the rods, in both isotropic and ordered phases. It is concluded that the Onsager approximation is not quantitatively accurate for L/D < 100.
TL;DR: In this article, a single-domain sample of cholesteric liquid crystals and of the recently reported twisted smectic C phase may have identical optical properties for light incident in the direction of the axis of twist.
Abstract: Single-domain samples of cholesteric liquid crystals and of the recently reported twisted smectic C phase may have identical optical properties for light incident in the direction of the axis of twist. A single Bragg-reflection band exists for either type of liquid crystal in that case. However, with obliquely incident light the twisted smectic C sample would show additional Bragg-reflection bands at optical frequencies intermediate between the bands that would appear in both samples. Examples of computed spectra are given showing the differences that appear as the angle of incidence departs from normal.
TL;DR: In dielectrically positive mixtures of the nematic N4 with cholesteryl-chloride, an unusual texture transition was observed which can be applied in a storage effect as discussed by the authors.
Abstract: In dielectrically positive mixtures of the nematic N4 with cholesteryl-chloride an unusual texture transition was observed which can be applied in a storage effect. The variation of several characteristic parameters of these mixtures with concentration and temperature was measured.
TL;DR: In this paper, a survey of electric alignment in liquid crystals is given, emphasizing electro-optical effects and the physics behind them, as well as a discussion of the relationship between electric alignment and liquid crystals.
Abstract: A survey is given of electric alignment in liquid crystals, emphasizing electro-optical effects and the physics behind them.
TL;DR: Temperatures and heats of transition are presented for four binary nematic mesophases; p-azoxyanisole plus p-zoxyphenetole, p, p' -n-dipro-poxyazoxybenzene, p. p'-n-dihexyloxy-azoxy benzene as mentioned in this paper.
Abstract: Temperatures and heats of transition are presented for four binary nematic mesophases; p-azoxyanisole plus p-azoxyphenetole, p, p' -n-dipro-poxyazoxybenzene, p, p'-n-dibutoxyazoxybenzene and p, p'-n-dihexyloxy-azoxybenzene. The phase diagrams were shown to be of the simple eutectic type with the formation of ideal comesophase solutions permitting prediction of the eutectic composition and temperature. The heats of transitions can be estimated with reasonable accuracy.
[...]
TL;DR: In this paper, the authors present observations on the structure and dynamics of these walls at magnetic fields above the Freedericks transition of a nematic slab, where the molecules can rotate by two different ways, giving rise to adjacent domains separated by a wall.
Abstract: At the Freedericks transition of a nematic slab, the molecules can rotate by two different ways, giving rise to adjacent domains separated by a wall. We present observations on the structure and dynamics of these walls at magnetic fields above the transition. The local distortion through the wall agrees with theoretical prediction. The ellipticity of a wall surrounding a closed domain allows to determine some elastic constant ratios. If the magnetic field is decreased to the Freedericks critical field, the thickness and the velocity of the walls are observed to diverge respectively as (H - Hc )−1/2 and (H - Hc )−1. From the velocity of the walls we can deduce the twist viscosity γ1 in planar geometry, and a renormalized viscosity γ including backflow corrections in homeotropic geometry.
TL;DR: In this paper, light reflectivity measurements show that the molecules of MBBA lie parallel to the nematic-isotropic interface, that their orientations in the absence of any external field are correlated in the plane of the interface over distances greater than the optical wavelength and allow a study of the dynamics of the effect of a magnetic field on these molecules.
Abstract: Light reflectivity measurements show that the molecules of MBBA lie parallel to the nematic-isotropic interface, that their orientations in the absence of any external field are correlated in the plane of the interface over distances greater than the optical wavelength and allow a study of the dynamics of the effect of a magnetic field on these molecules. The spectrum analysis of the light scattered by the interface yields the value of the surface tension. This experimental value is used to estimate the correlation length in the isotropic phase.
TL;DR: In this article, the authors investigated the orienting influence of micro-groove of variable density using different substrate materials and different liquid crystals, and some theoretical considerations concerning the aligning effect on liquid crystals of surface deformations were presented.
Abstract: The usual way to align a liquid crystal layer homogeneously is rubbing the substrate, wherby micro-grooves are produced. We investigated the orienting influence of micro-grooves of variable density using different substrate materials and different liquid crystals. Some theoretical considerations concerning the aligning effect on liquid crystals of surface deformations are presented.
TL;DR: In this paper, a second-order character has been postulated for the mesophase transition of N-(2-hydroxy-4-methoxybenzylildene)-4′-butylaniline(2,4-OHMBBA) with a purity of 99.393 mole % was measured between 13 and 375 K.
Abstract: The heat capacity of N-(2-hydroxy-4-methoxybenzylildene)-4′-butylaniline(2,4-OHMBBA) with a purity of 99.393 mole % was measured between 13 and 375 K. The Melting point T m was 314.30 K, and the enthalpy and the entropy of fusion were 22.405 kJ mol−1 and 71.425 J K−1 mol−1, respectively. The temperature of transition from nematic to isotropic liquid was 33.65 ± 0.1 K, and the enthalpy and the entropy changes due to this mesophase transition were 887.4 ± 4.9 J mol−1 and 2.690 ± 0.015 J K−1 mol−1, respectively. A second-order character has been postulated for the mesophase transition. By cooling the nematic liquid crystal at a rate of − 12.2 K min−1 a frozen-in state of the stable nematic phase, i.e. a glassy state, was realized. The glass transition temperature Tg was 204 K and the heat capacity jump at T g was Δ C = 107 J K−1 mol−1. The activation enthalpy for the glass transition was estimated to be Δ H = 75 kJ mol−1. The residual entropy of the glassy state at O K was determined to be 12.69 ± 0...
TL;DR: In this paper, a method for calculating the anisotropic internal field constants and the ratio of αe to αo, even if density data are not available, has been presented for six cinnamates which exhibit both smectic A and B mesophases.
Abstract: In the smectic A and B mesophases, the polarization field is anisotropic and the extraordinary and ordinary refractive indices n e and n o are related to the effective polarizabilities αe and αo and the internal field constants γe and γo by the Neugebauer's relations. The Neugebauer relations lead to the Born relation which is valid to a high degree of accuracy particularly for the long wavelengths. The paper presents a method for calculating the anisotropic internal field constants and the ratio of αe to αo, even if density data are not available. The birefringence data reported by Pelzl and Sackmann for six cinnamates which exhibit both smectic A and B mesophases are discussed in terms of the theory and results are presented with regard to the internal field constants and the ratio of αe to αo at different temperatures. Their variations are explained as due to mainly the rotational isomerism exhibited by the alkyl and alkoxy endgroups in the different compounds.
TL;DR: In this paper, concentrated suspensions of filamentous bacterial viruses form cholesteric liquid crystals and the twisted nematic phase shows form optical rotation, where the pitch of the twisted layer is comparable to the wavelength of light, Bragg reflection from the sheets is observed.
Abstract: Concentrated suspensions of filamentous bacterial viruses form cholesteric liquid crystals. Under appropriate conditions, spherulites are observed. The twisted nematic phase shows form optical rotation. In dry gels, where the pitch of the twisted layer is comparable to the wavelength of light, Bragg reflection from the sheets is observed.
TL;DR: In this paper, 35 compounds of structural formula where n and m take on values from 1 to 7 and 4 to 8 respectively were synthesized by means of differential scanning calorimetry and thermal microscopy.
Abstract: We have synthesized(1) 35 compounds of structural formula where n and m take on values from 1 to 7 and 4 to 8 respectively. (2) We have studied the phase behavior and identified many of the mesomorphic phases of these compounds by means of differential scanning calorimetry(3) (DSC) and thermal microscopy(4) (TM)
TL;DR: In this paper, a series of liquid crystalline materials comprising substituted phenyl 4-benzoylioxybenzoates were prepared and compared with the more symmetrical analogs derived from hydroquinone or terephthalic acid which have been reported in the literature.
Abstract: A series of liquid crystalline materials comprising substituted phenyl 4-benzoylioxybenzoates were prepared. These compounds may be viewed as derivatives of the unsymmetrical molecule, p-hydroxybenzoic acid. The mesomorphic properties of these new compounds were compared with the more symmetrical analogs derived from hydroquinone or terephthalic acid which have been reported in the literature. The crystal-to-mesophase transition temperatures for the unsymmetrical materials were significantly lower than those of the corresponding symmetrical materials were significantly lower than those of the corresponding symmetrical derivatives, wheras the mesophase-to-isotropic transition temperature varied only slightly for R=R'. The stability of the mesophse (N I transition) for the unsymmetrical materials fell between those of the corresponding hydroquinone and terephthalic acid derivatives, as anticipated from electronic considerations. The resluting decrease in the crystal-to-mesophase transition temperat...
TL;DR: In this paper, the averaged optical anisotropy of a liquid single crystal of the nematic type (MBBA) is studied and a homeotropic layer is enclosed by two circular glass plates, one of them rotating against the other.
Abstract: Shear flow in a liquid single crystal of the nematic type (MBBA) is studied. A homeotropic layer is enclosed by two circular glass plates, one of them rotating against the other. Between crossed polarizers one observes a system of concentric dark rings. The averaged optical anisotropy is found to be a pure function of the product of shear velocity and layer thickness. This agrees with the continuum theory of Leslie. The constants k 11/k 33, λ1/λ2, and (λ2 − λ2)/k 33 are determined.
TL;DR: In this paper, the effect of terminal alkyl and alkoxy substituents on mesomorphic properties of p-alkylphenyl p-ALKylbenzoates, p-koxyphenyl p -alkylbenzosates, and palkoxymyroxymethylp-alkoxybenzoate was studied.
Abstract: The effect of terminal alkyl and alkoxy substituents on the mesomorphic properties of p-alkylphenyl p-alkylbenzoates, p-alkylphenyl p-alkoxybenzoates, p-alkoxyphenyl p-alkylbenzoates, and p-alkoxyphenyl p-alkoxybenzoates was studied. It was found that, with relatively few experiments, one could make reasonable predictions as to the mesomorphic thermal properties of a particular series of compounds. Although, as is well known, reliable predictions of the crystal-to-mesophase transition temperature based on molecular structure cannot be made, predictions of the mesophase-to-isotropic transition temperature can be made once several members of a series have been prepared. The ability of terminal alkyl and alkoxy substituents to promote liquid crystallinity in the phenyl benzoate system was evaluated. The ester central linkage was also compared with other central linkages in regard to mesomorphic potential.
TL;DR: In this paper, the hydrodynamic motion induced by the rotation of the molecular axis in a nematic liquid crystal (backflow) was studied, which can reduce strongly some effective viscosity coefficients.
Abstract: -We study the hydrodynamic motion induced by the rotation of the molecular axis in a nematic liquid crystal (backflow). This effect can reduce strongly some effective viscosity coefficients.
TL;DR: In this paper, the Frank planar solution is stable above a critical ratio r 0/R of the fiber radius to the capillary radius, when a glass fiber is inserted along the cylinder axis.
Abstract: In order to verify experimentally some recent calculations on screw disclination lines in nematics we have observed with a polarizing microscope, in a meridian section, nematic filled capillary tubes with homeotropic boundary conditions, with or without inserted fibers. In the simple case with no fiber we observe non-singular S = + 1 lines and the point singularities associated with them. The lines of force of the directors are visualized by the flickering of the fluctuations of the molecules and correspond well to the shape predicted theoretically. When a glass fiber (which also imposes a perpendicular orientation on the nematic) is inserted along the cylinder axis, the Frank planar solution is stable above a critical ratio r0/R of the fiber radius to the capillary radius. This instability appears experimentally by slightly displacing the tube. With two fibers, S = − 1 lines are created which appear to have no singular points.
TL;DR: In this article, the average intermolecular distance (D) between neighboring parallel molecules in the isotropic, nematic and smectic A phases, the apparent length (l) of the molecules in C6H4, C7, and C2H5 was calculated from X-ray photographs of some 4-alkoxybenzal-4′-ethylanilines.
Abstract: From X-ray photographs of some 4-alkoxybenzal-4′-ethylanilines (CnH2n+1O—C6H4—CH=N—C6H4—C2H5; n = 5, 6, 7) we have calculated the average intermolecular distance (D) between neighboring parallel molecules in the isotropic, nematic and smectic A phases, the apparent length (l) of the molecules in the isotropic phase, and the thickness (d) of the smectic layers. All D values for the smectic and nematic phases are between 4.94 and 5.03 A. In the isotropic phase, the slopes of the D versus t plots for C6 and C7 (0.0024 and 0.0028 A/°C, respectively) are approximately equal to those reported earlier, but for C5 the slope is somewhat larger (0.0038). The slopes of the D versus t plots for the nematic phase are of the order of 0.01 A/°C, and for the smectic phase of C7 the slope is about half of this; for the nematic phases the slope appears to increase with decreasing length of the stability range. All l versus t plots were definitely concave. Extrapolation of the curves to the N-I point yielded l valu...
TL;DR: In this article, a model for the structure of the smectic B modification of two liquid crystals (EBAC and TBBA) was proposed, which is similar to the model proposed in this paper.
Abstract: We proposed recently a model for the structure of the smectic B modification of two liquid crystals (EBAC and TBBA). These two compounds only differ by the fact that the molecules are perpendicular to the smectic planes in EBAC and tilted in TBBA.
TL;DR: In this paper, the average orientations of a great number of aromatic molecules in a nematic mixture of cholesteryl chloride and laurate at 30°C have been determined by absorption polarization studies.
Abstract: “Compensatcd” nematic mixtures of cholesteryl derivatives are well suited as anisotropic solvents for optical experiments in the ultraviolet. The average orientations of a great number of aromatic molecules in a nematic mixture of cholesteryl chloride and cholesteryl laurate at 30°C have been determined by absorption polarization studies. Our attempts to correlate the elements of the Saupe order matrix of these molecules with their physical properties yielded the following results: For some elongated molecules a linear relation holds between the average orientation of the long molecular axis and the molecular dimensions, while large deviations from this correlation are observed for molecules with easily polarizable substituents. The order parameters are directly related to the principal solute polarizabilities of the solute molecules which can be determined with good accuracy from the order matrix elements. We conclude from these results that repulsive forces do not contribute considerably to the...