Showing papers in "Molecular Crystals and Liquid Crystals in 1972"
Abstract: A formalism based on cluster expansion for non-uniform fluid is found useful for the discussion of various thoeries of liquid crystals. Using the general formalism the equations which dete...
327 citations
TL;DR: In this article, the problem of a nematic liquid crystal in an electric field is solved, and numerical solutions for various values of parameters are given for various parameters of the liquid.
Abstract: The problem of a nematic liquid crystal in an electric field is solved. Effects due to conductivity of the liquid have been neglected. Numerical solutions are given for various values of parameters. The solution is compared with experimental data taken from the literature. A computer program has been developed to do a non-linear least-squares fit of experimental data with the solution given in the present paper.
247 citations
IBM1
TL;DR: In this paper, a simple method for measuring the birefringence and indices of refraction of nematic liquids with an accuracy comparable with or superior to previously reported means is described.
Abstract: We wish to report on a simple method for measuring the birefringence and indices of refraction of nematic liquids with an accuracy comparable with or superior to previously reported means.(1–3) The method is exemplified in measurement of the temperature dependences of the indices of refraction of p-methoxybenzylidene p-butylaniline (MBBA) at two wavelengths.
156 citations
TL;DR: In this article, the X-ray patterns of two new smectic modifications (smectic D and Smectic E) are investigated, and the D-modification is potically isotropic.
Abstract: The X-ray patterns of two new smectic modifications (smectic D and Smectic E) are investigated. The D-modification is potically isotropic. A cubic structure is presumed. In the E-modification the pattern exhibits three sharp outer rings besides the inner irng, which points to a higher order compared withthe A, B and C-Phases.
102 citations
TL;DR: In this paper, the Frank equation was used to predict a lattice of χ disclinations at the surface of the sample, the distance between the χ's being related to the half pitch.
Abstract: When a large pitch cholesteric is held between parallel glass plates a distance d apart, near the isotropic transition, a characteristic striped pattern appears The distance between stripes is about the half pitch p oo/2 (providing d > p oo) and the texture has no measurable optical rotatory power It does not appear if d ≤ p oo Assuming that the molecules lie in planes perpendicular to the glass plates and twist uniformly in the interior of the sample about an axis parallel to the glass plates, we have solved the Frank equation(1) when tangential (or homoeotropic) boundary conditions are applied The solution predicts a lattice of χ(2) disclinations at the surface of the sample, the distance between the χ's being related to the half-pitch We have found that this configuration is not stable within a distance p o/4 (d∼p o) of the disclination and we suggest that this results in the splitting of the χ into a λ and a τ(2) This latter configuration appears to adequately explain the experimentally
99 citations
TL;DR: The optical properties of a thin film of liquid crystal, with one of its surfaces bounded by the isotropic liquid of the same compound, reveal the existence of point defects in the liquid crystal as mentioned in this paper.
Abstract: The optical properties of a thin film of liquid crystal, with one of its surfaces bounded by the isotropic liquid of the same compound, reveal the existence of point defects in the liquid crystal. Particular sample geometries are examined in which the detailed structure of the optical patterns seen with polarized light can be analyzed to develop a qualitative picture of the structure of the liquid crystal layer near the defects. Changes in the optical pattern induced by an electric field are also analyzed. The observations indicate that the defects are point disclinations at the sirface of the liquid crystal, of the kind recently described by deGennes, rather than line disclinntions emerging from the liquid crystal.
90 citations
TL;DR: In this paper, the influence of an electric field on nematic liquid crystals is investigated for several orientations of the liquid crystal, and a theoretical treatment of the normal deformation that arises from the interaction of the electric field with the dielectric anisotropy is given.
Abstract: The influence of an electric field on nematic liquid crystals is investigated for several orientations of the liquid crystal. We give a theoretical treatment of the normal deformation that arises from the interaction of the electric field with the dielectric anisotropy. Our experimental deformations show relatively good agreement with the calculated values. Some anomalous deformations were also investigated. Conditions and threshold voltages for Williams domains are discussed. If the undisturbed optical axis is parallel to the applied electric field and the dielectric anisotropy is positive, no Williams domains appear. This result is in agreement with the Carr-Helfrich model.
84 citations
TL;DR: In this paper, the analogous series of p,p'-di-nalkyl-substituted azoxy- and azobenzenes are synthesized and dielectric constants parallel and perpendicular to the molecular axis are measured for some typical members of the series.
Abstract: The analogous series of p,p'-di-n-alkyl-substituted azoxy- and azobenzenes are synthesized. These compounds show low melting points and nematic or smectic behaviour. The melting points are generally lower for the azoxy compounds than for the analogous azo compounds. A probable explanation is the non-planar conformation of the azoxy compounds as compared with the planar conformation of the azo compounds. The influence of a permanent dipole moment in the azoxy series and the absence of a permanent dipole moment in the azo series is investigated. The clearing point is increased by the presence of a permanent dipole moment. The dielectric constants parallel and perpendicular to the molecular axis are measured for some typical members of the series. For both series we find e‖ > e⊥, the difference being greater for the azo series than for the azoxy series.
83 citations
TL;DR: In this paper, X-ray diffraction patterns of 14 substances have been studied in non-oriented samples in the smectic modification sA, sC, and sB, and the results of the investigation of the equatorial intensity distribution are described.
Abstract: X-ray diffraction patterns of 14 substances have been studied in non-oriented samples in the smectic modification sA, sC, and sB. In all three modifications a sharp reflexion was found at small angles. At large angles a diffuse reflexion was found in the modifications sA and sC, and a sharp reflexion in the modification sB. In all cases the calculated layer separation d is smaller than the molecular length L in the most stretched molecular structure. The X-ray patterns obtained from oriented samples of two substances are described and compared with the results obtained by other investigators. Further the results of the investigation of the equatorial intensity distribution are described.
79 citations
TL;DR: In this paper, a new type of smectic phase (smectic H) was described, using microscopic and X-ray data, and the structure of this phase can be assigned a three dimensional monoclinic lattice, with a = 10.32 A, b = 4.986 A, c = 20.73 A, and β = 123.27 A.
Abstract: A new type of smectic phase (smectic H) is described, using microscopic and X-ray data. The structure of this phase can be assigned a three dimensional monoclinic lattice, with a = 10.32 A, b = 4.986 A, c = 20.73 A, and β = 123.8°, probably space group P21, or Pa. The ab plane is parallel to the smectic layers; the thickness of the layers is 17.27 A. The molecules are most probably approximately parallel to the c axis, thus making an angle of 56.2° with the smectic layers and forming continuous columns of molecules parallel to the c axis. In planes perpendicular to the c axis the arrangement of the molecular axes is approximately hexagonal, with a spacing of 4.97 A. Successive layers of molecules parallel to the bc plane are shifted 2.87 A with respect to each other in the direction of the c axis. The most probable arrangement of the molecular planes is a herringbone-type packing. It appears that within the regular three-dimensional structure there are a considerable number of columns of molecule...
77 citations
TL;DR: In this paper, the authors studied the nuclear quadrupolar spin-lattice relaxation rate 1/T 1 of the deuterons of p-hexyloxybenzoic acid in the liquid state.
Abstract: In the liquid state, the molecules of the carboxylic acids are associated in pairs by hydrogen bonds connecting their acid groups. If these dimers are long and rigid, they can form a nematic phase. The dimer molecules of the liquid crystal p-hexyloxybenzoic acid have been deuterated on the hydrogen bonds, and the nuclear quadrupolar spin-lattice relaxation rate 1/T 1 of these deuterons has been studied. In the nematic phase, 1/T 1 is frequency dependent between 2.7 and 13.8 MHz; its temperature behavior is identical with that of the parameter S describing the orientational order of the nematic phase. In the isotropic phase, 1/T 1 behaves in the same way as in conventional nematics: at high nuclear frequencies, it is still frequency dependent, and rather unsensitive to temperature in the vicinity of the nematic-isotropic transition; at low frequencies, it has no frequency dependence, but diverges when the transition temperature T c is approached from above: The “critical frequency” which separates...
TL;DR: In this article, the dielectric constant and loss of crystalline cyclohexanol (CHOL) and 2,3-dimethylbutane (DMB) were measured from 4°K to the melting point, 299°K for CHOL and 145°k for DMB.
Abstract: The dielectric constant and loss of crystalline cyclohexanol (CHOL) and 2,3-dimethylbutane (DMB) were measured from 4°K to the melting point, 299°K for CHOL and 145°K for DMB. The supercooled plastic crystal of CHOL above the glass temperature Tg exhibits a prominent relaxation (α) due to molecular rotation. The temperature dependence of therelaxation time was found to obey the Fogel-Tamman equation. A secondary relaxation (β) was observed below Tg , 149°K for CHOL and 76°K for DMB. The mechanism proposed for β is internal rotation; rotationof OH bond for CHOL and trans-gauche transformation for DMB. The β relaxation of DMB is well correlated with acoustic relaxation in the liquid state in the relaxation time versus timperature plot. Relaxations in the intermediate state, crystal III, of these two substances and the most stable state, crystal II of DMB were studied.
TL;DR: In this article, the activation energy of the excition hopping process has been determined to be Ea = 0.055 eV and the lifetime of pyrene excimer fluorescence is decreased by quenchers in accordance with the steady-state measurements of relative quantum efficiencies.
Abstract: Sensitized fluorescence and fluorescence quenching has been studied in pyrene layers, doped with small amounts of coronene, anthraquinone and acenaphthylene. Coronene exhibits strong sensitized fluorescence at room temperature, but not at 77°K. This behaviour is due to decreased exciton mobility. The activation energy of the excition hopping process has been determined to be Ea = 0.055 eV. The lifetime of pyrene excimer fluorescence is decreased by quenchers in accordance with the steady-state measurements of relative quantum efficiencies. Three possible mechanisms of excimer state hopping in jpyrene are discussed
TL;DR: For example, liquid crystals comprise the states of intermediate order that may have properties in common with normal isotropic liquids as well as with solid crystals and, for this and other reasons, provide a...
Abstract: Liquid crystals comprise the states of intermediate order that may have properties in common with normal isotropic liquids as well as with solid crystals and, for this and other reasons, provide a ...
TL;DR: In this article, a variety of spectroscopic techniques have been used to demonstrate that genuine monomer emission can be observed from pyrene crystals and that emitting monomers appear to be located in disordered regions of the crystal situated within 5 × 102 nm of the surface.
Abstract: The phenomenon of monomer emission from excimer forming crystals has been investigated for pyrene and perylene. A variety of spectroscopic techniques have been used to demonstrate that genuine monomer emission can be observed from pyrene crystals. The emitting monomers appear to be located in disordered regions of the crystal situated within 5 × 102 nm of the surface. Both delayed monomer and excimer emission have been observed at 4.2°K and information on the range of the monomer exciton migration has been obtained. The low temperature emission observed from the dimeric α-form of perylene crystals appears to be either from highly perturbed perylene monomers located in disordered regions of the crystal or from impurities. The unusual temperature dependence previously observed for the emission spectrum of the monomeric β-form of perylene crystals is shown to be due largely to reabsorption effects.
TL;DR: One hundred and seven compunds belonging to ten homologous series of aromoatic anils were synthesized, purified and analyzed by differential thermal analysis and pahse contrast microscopy.
Abstract: One hundred and seven compunds belonging to ten homologous seriesof aromoatic anils were synthesized, purified and analyzed by differential thermalanalysis and pahse contrast microscopy. The freedo...
TL;DR: In this article, the exceptionally strong Kerr effect of some nematogenic compounds with strong electric dipole moments parallel to their long molecular axes was investigated, and the relaxation times were 4.10−7 sec or less.
Abstract: The exceptionally strong Kerr effect of some nematogenic compounds with strong electric dipole moments parallel to their long molecular axes was investigated. The Kerr constants just above the clearing point T cl were about 100 times larger than with nitrobenzene. The temperature dependence of the Kerr effect obeyed (T-T cl)−y, with 0.5<γ<0.7. The relaxation times were 4.10−7 sec or less.
TL;DR: In this article, a survey has been made of reported namatic-isotropic points of NLI's having the following structure: where X and Z are: R-, Ro-, RCO2, ROCO2, Ro-, Ro, RCO 2, RCO O 2, Ro, Ro O, ROC O, R CO, R OCO O, OCO-, R CO-, OCO-ROCO-O, Ro, N-alkyl varying form C1 to C6; andy is: the variation of the nam
Abstract: A survey has been made of reported nematic-isotropic points of nematic liqid crytals having the following structure: where X and Z are: R-, Ro-, RCO2-, ROCO2-, RCO-,ROCO-; R is n-alkyl varying form C1 to C6; andy is: The variation of the nematic-isotropic points with structure is sufficiently regular to permit the generation of sets of parameters for X, Y and Z that can be used to make resonable predictons of the namatic-isotropic points for all combinationf of X, Y and Z.
TL;DR: In this article, a calculation of the Frank elastic contants based on the latice model of liquid crystals is presented, and the calculated temperature dependence of K11, K22 and K33 is is agreement with experment.
Abstract: A calculation of the Frank elastic contants based on the latice model of liquid crystals is presented. The calculated temperature dependence of K11, K22 and K33 is is agreement with experment. The X-ray structue data for p-azoxyanisole is employed to caculated themanitude of 1/2 (K11 + K22) and the result is found to agree with experiment.
TL;DR: In this article, the thermodynamic conditions of nematic stability are discussed on the basis of the molecular statistical theory of orientational order developed in previous papers, and theoretical calculations of the order parameter in the nematic phase and the volume change at the nematic-isotropic transition point are presented for 2-4 non-adienic acid, 2 -4undecadienistic acid, p-azoxyanisole, p -azoxyphenetole and anisaldazine.
Abstract: The thermodynamic conditions of nematic stability are discussed on the basis of the molecular statistical theory of orientational order developed in previous papers. Theoretical calculations of the order parameter in the nematic phase and the volume change at the nematic-isotropic transition point are presented for 2-4-nonadienic acid, 2-4-undecadienic acid, p-azoxyanisole, p-azoxyphenetole and anisaldazine. The significance of the results are discussed briefly.
TL;DR: In this paper, the phase diagram of the anthracene-acridine system by thermal analysis revealed the compositions of terminal solid solutions amounting to 2 and 88 mole % of Anthracene respectively.
Abstract: The revision of the phase diagram of the anthracene-acridine system by thermal analysis revealed the compositions of terminal solid solutions amounting to 2 and 88 mole % of anthracene respectively, and a eutectic at 8% of anthracene and 384.4[ddot]K. Relatively stable binary single crystals of composition ranging from 0 to 45 mole % of acridine in anthracene and from 2 to 10 mole % of anthracene in acridine have been obtained by the vapour growth technique. The lattice constants of the mixed crystals have been determined.
TL;DR: A number of new Schiff's bases showing nematic mesomrophism of low thermal stability (to below O°C) were found to also deviate somewhat from known compounds in the relation of transition temperatur as discussed by the authors.
Abstract: A number of new Schiff's bases showing nematic mesomrophism of low thermal stability (to below O°C) were found to also deviate somewhat from known compounds in the relation of transition temperatur...
TL;DR: In this article, it was shown that thermal dissociation of impurities is responsible for bulk residual conductivity while protonic injection plays a leading part in high field conduction in the liquid phase of MBBA.
Abstract: Different mechanisms of ions generation that take place, either in the liquid volume or at a chemically inert electrode, have been distinguishedby polarographic study of hydrolysis and electrochemical oxidation which characterizes the p-methoxybenzylidene-p-n-butylaniline (MBBA) molecule. In both cases, the protonated form of MBBA has been identified. The analysis of low field electrical conductivity of highly purified samples gives information on mobility values and kinetic constants of dissociation and recombination of a presumed protonic weak electrolyte in solution. The main conclusion is that thermal dissociation of impurities is responsible for bulk residual conductivity while protonic injection plays a leading part in high field conduction in the liquid phase of MBBA.
TL;DR: In a continued effort to synthesize compounds which exhibit nematic mesomorphism at room temperature, homologous p-alkoxy-p'-acyloxyazobenzenes and p -alkoxy p'-acy loxyazoxybenzenes were prepared as discussed by the authors.
Abstract: In a continued effort to synthesize compounds which exhibit nematic mesomorphism at room temperature, homologous p-alkoxy-p'-acyloxyazobenzenes and p-alkoxy-p'-acyloxyazoxybenzenes were prepared. The p-substituted groups were normal alkyl chains containing from one to seven carbon atoms. Of the 98 compounds which were prepared in both the azo and azoxy series, 87 exhibited enantiotropic and 3 monotropic nematic properties while only 8 showed smectic behavior. The azoxy “compounds” were generally lower melting than the azo compounds. This is presumably due to the fact that the former are mixtures of isomers (oxygen may be on nitrogen attached to the alkoxyphenyl group or on nitrogen attached to the acyloxyphenyl group). The higher nematic thermal stability of the azoxy materials compared to the azo compounds and to analogous Schiff base compounds may be attributed to broadening of the molecule as a result of the presence of a lateral oxygen atom. This would have the effect of increasing the termin...
TL;DR: In this paper, it was shown that silver paste behaves as an injecting contact for electrons in naphthalene single crystals, and the trap density of this material was found to be between 1016-1017 cm−2.
Abstract: Measurements of the current in naphthalene single crystals, as a function of the voltage, showed space charge limited current characteristics. Using this theory we found for the trap density of this material, values between 1016-1017 cm−2. The mean depth was about 0.65 eV, the capture cross section, 10−17 cm2 and the frequency of escape, 109 sec−1. For the current range encountered in this work, it is shown that silver paste behaves as an injecting contact for electrons.
TL;DR: In this paper, the importance of charge carrier mobility measurements for the study of the charge carrier transport mechanism in solids has been widely accepted, and the authors proposed a method to measure the mobility of a charge carrier.
Abstract: The importance of charge carrier mobility measurements for the study of charge carrier transport mechanism in solids has been widely accepted.
TL;DR: In this paper, a series of Schiff bases derived from 4-alkoxybenzylidene-2-hexane-2−alkoxy-5−aminopyridines were prepared in order to investigate the effect of heterocyclic nitrogen on the mesomorphic behavior.
Abstract: A series of Schiff bases derived from 4-alkoxybenzylidene-2′-alkoxy-5′-aminopyridines were prepared in order to investigate the effect of heterocyclic nitrogen on the mesomorphic behavior. Nematic mesomorphism was obtained when the alkoxy group in the benzaldehyde portion of the molecule was methoxy or ethoxy while the higher homologs exhibited two smectic phases. The higher and lower temperature smectic phases were identified as smectic A and B respectively on the basis of their optical properties and transition enthalpies. Plots of mesomorphic-isotropic transition temperature as a function of the number of carbon atoms in the benzaldehyde portion of these Schiff bases exhibited some unusual odd-even effects which are discussed in terms of current theory. No particularly meaningful trends were observed, however, when the transition entropies were plotted against the number of carbon atoms in the alkoxy chains. The smectogenic tendencies of this series of compounds, compared to the benzene analog...
TL;DR: The theory of melting of molecular crystals developed in Parts I and II is extended to include short range orientational order, and the thermodynamic properties are evaluated by the quasi-chemical method, which is mathematically equivalent to Bethe's treatment.
Abstract: The theory of melting of molecular crystals developed in Parts I and II is extended to include short range orientational order. The thermodynamic properties are evaluated by the quasi-chemical method, which is mathematically equivalent to Bethe's treatment, and the theory is applied to liquid crystalline transitions. The interesting result is obtained that the nematic-isotropic transition is always first order, and that although the long range orientational order disappears suddenly at this transition point, a certain degree of short range orientational order persists in the isotropic phase. The specific heat exhibits both pre- and post-transition anomalies, in conformity with observations. The thermal expansion and isothermal compressibility show pre-transition anomalies but, contrary to experimental facts, no post-transition anomalies, indicating that even the quasi-chemical approach is not quite adequate for explaining all the properties.
TL;DR: Fryburg, Gelerinter and Fishel as mentioned in this paper studied the structure of two smectic A phases using electron resonance spectroscopy with vanadyl acetylacetonate as the spin probe.
Abstract: Fryburg, Gelerinter and Fishel have studied the structure of two smectic A phases using electron resonance spectroscopy with vanadyl acetylacetonate as the spin probe. (1) Their analysis of the electron resonance spectra shows that in 4-n-butyloxybenzylidene-4′-aceto-aniline the orientational order of the probe does not change at the nematic-smectic transition and that the temperature dependence of the order is the same in both phases. In addition the angular dependence of the vanadium coupling constant, in the smectic A phase, was found to be in poor agreement with theory. These findings are unexpected for the orientational order of spin probes dissolved in smectic A mesophases has been found(2,3) to be essentially temperature independent in contrast to the behaviour in a nematic mesophase. Further the angular dependence of the hyperfine coupling was found to be in excellent agreement with theory. (2,3) Finally earlier results indicate that there should be a discontinuity in the orientational or...
TL;DR: In this article, it was shown that the cores of non-basal dislocations function as preferred regions for the appearance of the product of 9-cyanoanthracene.
Abstract: Optical microscopic studies of UV-irradiated single crystals of anthracene, and of some of its derivatives, reveal(1,2) that nucleation of the dimer within the host matrix occurs heterogeneously; and that the cores of certain non-basal dislocations function as preferred regions for the appearance of the product. That other types of dislocations present in aromatic solids may also be involved in chemical transformations within the solid-state is indicated both by the facile photochemical decoration (with dimer nuclei) of several families of dislocations in acenaphthylene,(3) and by the unexpected stereochemical product of dimerization of 9-cyanoanthracene.(4.5) We have recently demonstrated(6) that, for 9-cyanoanthracene, stacking faults produced by partial dislocations gliding on (221) planes could account for the invariable production of trans dimer of 9-cyanoanthracene upon UV irradiation. In this communication we report the preliminary results of in situ investigations aimed at revealing direc...