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Showing papers in "Molecular Crystals and Liquid Crystals in 1979"


Journal ArticleDOI
TL;DR: An improved treatment of the lattice model proposed in 1956 is presented for a system of hard rods, with axial ratio x, dispersed in a diluent as mentioned in this paper, where the equilibrium distribution of orientations of the rodlike particles in a phase with nematic order is derived and explicitly taken into account.
Abstract: An improved treatment of the lattice model proposed in 1956 is presented for a system of hard rods, with axial ratio x, dispersed in a diluent. Interparticle forces, apart from repulsions on contact, are deliberately ignored within the scope of the present paper. The equilibrium distribution of orientations of the rodlike particles in a phase with nematic order is derived and explicitly taken into account. Thermodynamic properties in the asymptotic limit of high degree of order reduce approximately. to those obtained previously. Calculations carried out for equilibrium between anistropic (nematic) and isotropic phases yield ν′x/νx = 1.4653 for the ratio of volume fractions in the respective phases in the limit x → ∞. The product xνx, expressing the combined covolume of solute species in the isotropic phase at coexistence is 7.89 in the same limit. The calculated critical value of the axial ratio for coexistence of the two phases in the neat liquid is xcrit, = 6.417. Results are compared with thos...

268 citations


Journal ArticleDOI
TL;DR: In this paper, the orientation-dependent energy between a pair of segments is taken to be proportional to cos2 λij, where λj is the angle between the principal axes of their polarizability tensors, assumed to be cylindrically symmetric with respect to the molecular axis.
Abstract: A system of rigid, inpenetrable, rodlike molecules subject to orientation-dependent mutual attractions is treated by extension of the theory presented in the preceding paper. This energy is formulated, for a system at constant volume, by considering interactions between pairs of segments in contact, rather than in terms of interactions between entire molecules. The orientation-dependent energy between a pair of segments is taken to be proportional to cos2 Ψij, where Ψij is the angle between the principal axes of their polarizability tensors, assumed to be cylindrically symmetric with respect to the molecular axis. A characteristic temperature T∗ measures the intensity of these interactions. The orientational energy of the system as a whole is of the form derived by Maier and Saupe. The orientation distribution with respect to the domain axis and the partition function are formulated with T∗ and the axial ratio x as parameters, Steric effects of molecular shape asymmetry, embodied in x, are of for...

204 citations


Journal ArticleDOI
TL;DR: The concept of inherent tilt inherent to all A phases accounts for the observed differences between layer thickness d and molecular length l of four quite different compounds, without any need for assuming additional tilt, interdigitation, or kinking of chains.
Abstract: Even though the average molecular direction in smectic A phases is perpendicular to the smectic planes, most molecules in these phases have rather large tilt angles because the orientational order parameter S in all smectic A phases is significantly less than unity. This molecular tilt inherent to all A phases accounts for the observed differences between layer thickness d and molecular length l of four quite different compounds, without any need for assuming additional tilt, interdigitation, or kinking of chains. From the differences between d and l , S values from 0.78 to 0.84 are obtained, which yield average molecular tilt angles from 20° to 17°. It is predicted that all smectic A phases with monomolecular layers will follow the same pattern. The concept of “inherent tilt” should apply to all orthogonal smectic phases, thermotropic as well as lytropic.

128 citations


Journal ArticleDOI
TL;DR: In this article, the authors developed a model based on the effect of the decrease of the effective viscosity in the presence of the back flow induced by the rotation of n and taking place as a convective roll pattern.
Abstract: In the course of experiments using the Freedericksz transition on a planar nematic film in a magnetic field perpendicular to the limiting plates, we have observed the transient existence of a periodic roll structure very similar to published observations by Carr, and apparently similar to Williams domains. The present paper develops a model based on the effect of the decrease of the effective viscosity in the presence of the back flow, induced by the rotation of n and taking place as a convective roll pattern. Direct observations and order of magnitude estimates of the wavelength of the roll and of the time constant for the development of the rolls agree with the model. We also addressed ourselves with the problem of distortion in an initially well aligned bulk material placed in a field perpendicular to n and find that inertia effects can also lead to a roll structure in the problem.

89 citations


Journal ArticleDOI
TL;DR: In this article, the dominant interactions between electron-electron and electron-phonon coupling inside the cation-radical chains were analyzed and it was shown that the metallic behavior at high temperatures and the occurrence of a Peierls distorsion below 100 K arise mainly from a competition between electron electron and electron phonons.
Abstract: In the course of investigations of organic metals we have investigated the cation-radical series TMTTF2-X′ with X′ = BF4, ClO4, PF6, SCN and Br. On this isomorphous series a large range of physical properties have been studied: electrical conductivity, magnetic susceptibility, EPR, optical reflectivity and specific heat. In correlation with crystallographic results, which indicate the existence of TMTTF diads, the dominant interactions are analyzed. It results that the metallic behavior at high temperatures and the occurrence of a Peierls distorsion below 100 K arise mainly from a competition between electron-electron and electron-phonon coupling inside the cation-radical chains

89 citations


Journal ArticleDOI
TL;DR: In this article, the authors compared liquid crystals of lecithin/ethylene glycol and water systems using X-ray data and optical microscopy and found that the lecitin/water system showed shorter interlayer spacings.
Abstract: Liquid crystals of the lecithin/ethylene glycol and the lecithin/water systems were compared using X-ray data and optical microscopy. The ethylene glycol system showed shorter interlayer spacings a...

81 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the voltage dependence of the capacitance of a twisted nematic liquid crystal layer just above threshold and present experimental results obtained over a range of temperatures with the nematic mixture E7.
Abstract: The magnetic field dependence of the capacitance of a twisted nematic layer has been analysed by Shtrikman et al (1) who found only moderate agreement between their theory and the experimental results of Gerritsma et al (2). Here we investigate the voltage dependence of the capacitance of a twisted nematic liquid crystal layer just above threshold and present experimental results obtained over a range of temperatures with the nematic mixture E7. Agreement between theory and experiment is satisfactory at all temperatures.

71 citations


Journal ArticleDOI
TL;DR: In this paper, a smectic C liquid crystal exhibiting positive dielectric anisotropy was oriented with the director parallel to the glass substrates, and when an electric field was applied normal to the director direction, a Dielectric reorientation (Freedericksz transition) occurred.
Abstract: A smectic C liquid crystal exhibiting positive dielectric anisotropy was oriented with the director parallel to the glass substrates. When an electric field was applied normal to the director direction a dielectric reorientation (Freedericksz transition) occurred. Dissolving a dichroic dye in the smectic C phase and applying an electric field, a guest host effect similar to nematic phases was observed. From the dielectric reorientation the tilt angle of the smectic C phase was determined at different temperatures. The tilt angles obtained by the electrooptical measurements agree sufficiently with the values measured by x ray investigations.

70 citations


Journal ArticleDOI
TL;DR: In this paper, the authors studied the pretransitional behavior of a series of mesomorphic compounds, the alkyl cyanobiphenyls (CNρρC n H 2n + 1) for n = 5-12, and showed that the presence of the smectic A phase changes markedly the pret-ransitional behaviour, and gives rise to two distinct pret-transitional regions.
Abstract: The pretransitional behaviour of a homologous series of mesomorphic compounds, the alkyl cyanobiphenyls (CNρρC n H 2n + 1) for n = 5–12, has been studied as a function of temperature by light scattering in the isotropic phase. The higher homologues, n = 8–12, exhibit a smectic A phase that becomes increasingly important at the expense of the nematic phase for increasing n and for n = 10 and 12 no nematic phase is observed. We have shown from light scattering and differential scanning calorimetry measurements that the presence of the smectic A phase changes markedly the pretransitional behaviour, and gives rise to two distinct pretransitional regions. The first region is for temperatures greater than ∼3°C above the clearing temperature Tc where the systems exhibit a typical pretransitional behaviour adequately described by the phenomenological Landau-de Gennes model of a second order phase transition with an intervering first order transition at Tc . The constants of this model for this region are...

66 citations


Journal ArticleDOI
Robert J. Cox1
TL;DR: In this article, the role of the dye structure and the liquid crystal mixture with respect to factors such as order parameter, absorption spectra and solubility is considered with regards to the application in electrooptic displays.
Abstract: The guest-host interactions of pleochroic dyes dissolved in nematic liquid crystals has been reviewed, particularly with respect to the application in electrooptic displays. The role of the dye structure and the liquid crystal mixture is considered with respect to factors such as order parameter, absorption spectra and solubility.

58 citations


Journal ArticleDOI
TL;DR: A number of esters and other compounds containing a trans-1,4-substituted cyclohexane ring system have been prepared, and the liquid crystal thermal stabilities of these new mesogens are discussed in relation to those of their analogues containing the 1,4phenylene ring as discussed by the authors.
Abstract: A number of esters and other compounds containing a trans-1,4-substituted cyclohexane ring system-have been prepared, and the liquid crystal thermal stabilities of these new mesogens are discussed in relation to those of their analogues containing the 1,4-phenylene ring. The observed effects are discussed in terms of a possible thermal energy absorption by the cyclohexane ring in causing minor conformational changes. Other physical properties of these new mesogens are also discussed with particular reference to their potential in applications. In relation to electro-optical display devices, such properties as birefringence, dielectric anistropy, viscosity etc., are important parameters, and the new materials have been assessed in this context as possible additives to the 4-n-alkyl- and 4-n-alkyloxy-4′-cyanobiphenyls for producing a favorable combination of physical parameters. Also reported is a liquid crystal mixture which has a 100°C nematic range and which contains only materials which are ali...

Journal ArticleDOI
TL;DR: In this article, serious problems arise with the usual classification and proposed mechanisms for polymorphic transitions in crystals, and it is shown that serious problems arises with the traditional classification and mechanism.
Abstract: It is shown that serious problems arise with the usual classification and proposed mechanisms for polymorphic transitions in crystals. On the basis of the investigation of polymorphic transitions i...

Journal ArticleDOI
TL;DR: In this article, a complex mesomorphic polymorphism is shown in a hexa-n-dodecanoate of triphenylene, and evidence of three different mesophases are given.
Abstract: A complex mesomorphic polymorphism is shown in a hexa-n-dodecanoate of triphenylene. Both by calorimetric measurements and optical textures, evidence of three different mesophases are given. Two of these transitions are practically second order ones.

Journal ArticleDOI
TL;DR: The phase diagram of the system 8 CBP/CBOOA indicates that the pure components show this effect too as discussed by the authors, and X-ray data on layer thickness and transition enthalpies are presented.
Abstract: Binary mixtures of some terminal non-polar liquid crystals with the two terminal polar liquid crystals 4-cyano-4′-n-octyloxy-biphenyl (8 CBP) and 4-cyano-benzylidene-4′-n-octyloxyaniline (CBOOA) exhibit reentrant nematic phases. The phase diagram of the system 8 CBP/CBOOA indicates that the pure components show this effect too. X-ray data on layer thickness and transition enthalpies are presented.

Journal ArticleDOI
TL;DR: In this article, a simple phenomenological theory is presented for the surface or interface of nematic liquid crystals in terms of the surface order parameter μu, which is based on the existence of an easy axis at the surface.
Abstract: A simple phenomenological theory is presented for the surface or interface of nematic liquid crystals in terms of the surface order parameter μu. The theory is based on the existence of an easy axis at the surface, so that the surface order parameter is not only a function of a director but also the easy axis. Taking into account the concept of an easy axis field, the temperature dependence of pretilt angle can be expressed both for the free surface and the interface between liquid crystals and solid substrate. Two typical experimental results of temperature dependent angle obtained by Bouchiat and Langevin-Cruchon, and Toda, Mada and Kobayashi are compared with the theory, and good agreements are obtained qualitatively between them.

Journal ArticleDOI
TL;DR: In this paper, the phase behavior of some linear polyphenyls was investigated and shown to be phase-fluorouracane-like in the presence of liquid crystals and liquid crystals.
Abstract: (1979). Phase Behavior of Some Linear Polyphenyls. Molecular Crystals and Liquid Crystals: Vol. 49, No. 7, pp. 207-209.

Journal ArticleDOI
TL;DR: In this paper, a series of thermotropic liquid crystalline polymers has been synthesized, which have incorporated a mesomorphic moiety, 4-alkoxyphenyl 4′-alkoxybenzoate, into the polymer backbone.
Abstract: A new series of thermotropic liquid crystalline polymers has been synthesized. These materials have incorporated a mesomorphic moiety, 4-alkoxyphenyl 4′-alkoxybenzoate, into the polymer backbone. T...


Journal ArticleDOI
M. Delfino1
TL;DR: The application of optical second harmonic generation (SHG) to the study of non-centro-symmetry in 71 α-amino acids, 22 dipeptides, 16 proteins and 5 viruses was reported in this article.
Abstract: The application of optical second harmonic generation (SHG) to the study of non-centro-symmetry in 71 α-amino acids, 22 dipeptides. 6 tripeptides. 16 proteins, and 5 viruses is reported. All α-amino acids except α-glycine, all peptidcs containing at least one enantiomeric α-amino acid residue, and all proteins and viruses are found to be non-centrosymmetric. Of the 22 racemic α-amino acids investigated, 19 form centrosymmetric crystalline racemates. DL-alanine and DL-norleucine form non-centrosymmetric racemates whereas m-glutamic acid does not form racemic crystals except when hydrated. These results lead us to suggest that non-centrosymmetry and its consequences (i.e., allowable piezoelectricity and possible pyroelectricity) is an essential and universal property of all biological systems. Furthermore, the concept of polarization homeostasis (i.e., biomolecules in vivo have a macroscopic equilibrium polarization in which the polarization ≠ 0) is proposed as a basic biological phenomenon and sho...

Journal ArticleDOI
TL;DR: The diffuse-cone model as mentioned in this paper was developed on the basis of the existence of orientational disorder in smectic phases, and it is valid for all A and C phases.
Abstract: The diffuse-cone model, developed on the basis of the existence of orientational disorder in smectic phases, is valid for all smectic A and C phases, and also, with appropriate modifications, for all other smectic phases. The fundamental angular parameter is not the optical tilt angle τ, but the preferred molecular tilt angle θm. This explains why τ does not follow the (TA-C-T)0.35 dependency predicted by De Gennes whereas θm does. The diffuse-cone model also explains why the tilt angle obtained from measurements of the smectic layer thickness will in general not be equal to τ, and thus invalidates arguments based on such differences which were used to support a particular structural model.

Journal ArticleDOI
TL;DR: The temperature variation of refractive indices (ne, ne), birefringence (δn), density (ρ) and order parameter (S) of two liquid crystals, namely, N-(p-hexyloxybenzylidene)-p-toluidine (HBT) and N-(...
Abstract: The temperature variation of refractive indices (ne, ne), birefringence (δn), density (ρ) and order parameter (S) of two liquid crystals, namely, N-(p-hexyloxybenzylidene)-p-toluidine (HBT) and N-(...

Journal ArticleDOI
TL;DR: The analysis of the time behaviour of the transient current flowing through the sample allows the estimation of the mean mobility of anions and cations in the nematic liquid crystal as mentioned in this paper.
Abstract: The nematic liquid crystal is considered as a weak electrolyte. The analysis of the time behaviour of the transient current flowing through the sample allows the estimation of the mean mobility of anions and cations. The electrohydrodynamic flows are avoided thanks to the application of low voltages. For p-octyloxyphenyl p'-pentyloxybenzoate the measured values of mobility are = 0,9 · 10−6 cm2 V−1 s−1 and = 1,3 · 10−6 cm2 V−1 s−1 at 326K. The temperature behaviour of mobility and conductivity is described by identical activation energies: 0.48 eV and 0.16 eV in nematic and isotropic phase respectively.

Journal ArticleDOI
Nguyen Huu Tinh1, H. Gasparoux1
TL;DR: Some derivatives of an homologous series of 4-aIkoxy-benzoyloxy-4′-cyanotolanes are synthesized in this article, and two of them exhibit a stable nematic reentrant phase at atmospheric pressure.
Abstract: Some derivatives of an homologous series of 4-aIkoxy-benzoyloxy-4′-cyanotolanes are synthesized. Two of them exhibit a stable nematic reentrant phase at atmospheric pressure.

Journal ArticleDOI
I. C. Lewis1, C. A. Kovac1
TL;DR: Vorlander as discussed by the authors studied the melting behavior of various polyphenyl compounds and reported that both p-quinquinquephenyl and p-sexiphenyl exhibited liquid crystalline behavior.
Abstract: Vorlander,1 studying the melting behavior of various polyphenyl compounds, reported that both p-quinquephenyl and p-sexiphenyl exhibited liquid crystalline behavior. Based on these early qualitativ...

Journal ArticleDOI
TL;DR: In this paper, the synthesis and mesomorphic properties of several new series of liquid crystalline materials are described, including diacetylenic structures, 4,4′-di-n-acyloxydiphenyldi-acetylene and unsymmetrical 4n-alkoxy.
Abstract: The synthesis and mesomorphic properties of several new series of liquid crystalline materials are described. These include two novel diacetylenic structures, 4,4′-di-n-acyloxydiphenyldi-acetylene and the unsymmetrical 4-n-alkoxy, 4′-cyanodiphenyldiacetylene, and the analogous 4-n-alkoxy,4′-cyanotolanes and stilbenes. A discussion of the effect of structure on transition temperatures is included.

Journal ArticleDOI
TL;DR: The dielectric spectrum of n-heptyl-cyanobiphenyl (7CB) and 7OCB was reported over a frequency range up to 18 GHz and over a temperature range well into the isotropic phase as mentioned in this paper.
Abstract: The dielectric spectrum of n-heptyl-cyanobiphenyl(7CB) and n-heptoxy-cyanobiphenyl(7OCB) are reported over a frequency range up to 18 GHz and over a temperature range well into the isotropic phase. Two relaxation processes are observed in the isotropic phase of both compounds, as well as in the nematic phase (when measurements are made parallel to the nematic director). Possible mechanisms for the two relaxation processes are discussed.

Journal ArticleDOI
TL;DR: In this paper, a double-peaked distribution function was obtained for the central ring librations in the high temperature phase of the PTP and diffraction data (neutron as well as x-ray) collected in the low temperature phase has confirmed this hypothesis.
Abstract: At 300 K crystalline para-terphenyl (PTP) may be considered to be planar, but the librational amplitude of the central ring is unusually high (16°). This behaviour has been explained by the occurrence of a double well potential for the central cycle which occupies at random one of the two possible positions and jumps continuously from one minimum to the other one.1 When the temperature is lowered, a superstructure appears: an antiferro-reorientational ordering results from the stabilization of the central ring in one of the two possible wells.2 A recent treatment of the diffraction data (neutron as well as x-ray) collected in the high temperature phase has confirmed this hypothesis: a double-peaked distribution function was obtained for the central ring librations.3

Journal ArticleDOI
TL;DR: The second order phase transition at 195 K in bis (toluene sulfonate) diacetylene polymer crystals has been studied by optical reflection and transmission, Raman and far-infrared spectroscopy.
Abstract: The second order phase transition at 195 K which occurs in bis (p-toluene sulfonate) diacetylene polymer crystals has been studied by optical reflection and transmission, Raman and far-infrared spectroscopy. Order parameter fitting to the data suggests that at low temperatures the transition has a two dimensional character while closer to the transition it appears to be three dimensional. There are indications of a broad transition region which might be expected for a predominately two dimensional phase transition in which fluctuations dominate the behaviour of the system. Analysis of new X-ray crystallographic data gives some support to a structural model in which the transition would have a substantial two dimensional character.

Journal ArticleDOI
TL;DR: The structure and dislocation pattern in a lamellar liquid crystal from water and polyoxyethylene dodecyl ether were studied for the effects of solubilization of large amounts of hydrocarbon using low angle X-ray diffraction and optical microscopy as discussed by the authors.
Abstract: The structure and dislocation pattern in a lamellar liquid crystal from water and polyoxyethylene dodecyl ether were studied for the effects of solubilization of large amounts of hydrocarbon using low angle X-ray diffraction and optical microscopy. The X-ray data showed extremely small changes of interlayer spacings with added aromatic hydrocarbons indicating them to be located within the amphiphilic layer of surfactant and water. Addition of the aliphatic hydrocarbon gave an increase of the lamellar distance equal to or even in excess of the one for non-penetrative addition indicating a change in the direction of the amphiphilic substances. The lamellar liquid crystal containing only water and surfactant showed a pattern of enhanced presence of focal conic dislocations with increased surfactant concentration for tetraoxyethylene dodecyl ether. For pentaoxyethylene dodecyl ether with a hydrophilic character this trend was observed after enhancing the lipophilic part by addition of hydrocarbon.

Journal ArticleDOI
TL;DR: In this article, the polymorphism of 4 cyanobiphenyl-4′-nalkoxy benzoate esters was investigated and a reentrant nematic phase was observed for the n = 8 and n = 9 homologues.
Abstract: The polymorphism of the 4 cyanobiphenyl-4′-n-alkoxy benzoate esters is investigated and a reentrant nematic phase is observed for the n = 8 and n = 9 homologues. X-Ray studies confirm this fact and show that in the reentrant phase the cybotactic groups are SA-like at high temperature near the SA phase while they appear SC-like at lower temperatures.