Showing papers in "Liquid Crystals in 1987"
TL;DR: In this article, a generalized Landau-de-Gennes elastic free energy of liquid crystals, in powers of a symmetric and traceless tensor order parameter, polarization field, of external fields and all respective derivatives, is constructed.
Abstract: Using angular momentum representation a method is proposed that allows the systematic construction of a generalized Landau-de Gennes elastic free energy of liquid crystals, in powers of a symmetric and traceless tensor order parameter, polarization field, of external fields and all respective derivatives. By this method all linearly independent elastic invariants and surface terms are constructed for nematics and cholesterics up to fourth order terms. In particular it is shown that up to fourth order in the tensor order parameter there are nineteen bulk elastic constants and four surface terms in the free energy of a general, biaxial nematic. In addition, the stability of this expansion is studied in detail. Some special cases of the elastic free energy of liquid crystals, already discussed in the literature, are reexamined and discrepancies with our results are emphasized. Finally, a thermo-dynamically correct way of establishing contact between the generalized de Gennes elastic free energy and ...
146 citations
TL;DR: In this article, the Landaude Gennes formalism is used to describe the phase transition temperature of a system placed between two parallel plates, subject to identical homeotropic or homogeneous boundary conditions at each plate.
Abstract: We discuss, using a Landaude Gennes formalism, the nematic-isotropic transition temperature for a system placed between two parallel plates, subject to identical homeotropic or homogeneous boundary conditions at each plate. The temperature at the phase transition may increase or decrease as the inverse sample thickness, D-1, increases, depending on the nature of the boundary conditions. In all cases the transition terminates at a critical point for sufficiently large D-1, beyond which the nematic and isotropic phases are no longer distinct. The phase transition temperature is well described by a liquid crystal analogy of the Kelvin equation which can be generalized to give an exact Clausius-Clapeyron relation. Under many circumstances the system behaves from a thermodynamic point of view as though it were in a bulk ordering field. The finite geometry restricts the growth of nematic or isotropic wetting films. We discuss the disjoining pressure experiment of Horn, Israelachvili and Perez [15]. Fin...
103 citations
TL;DR: In this article, the electroclinic effect in the smectic A phase of 4-(3-methyl-2-chlorobutanoyloxy)-4'-heptyloxybiphenyl has been investigated.
Abstract: The electroclinic effect in the smectic A phase of 4-(3-methyl-2-chlorobutanoyloxy)-4'-heptyloxybiphenyl has been investigated. Values up to 10° are obtained for the induced tilt angle, which is proportional to the applied d.c. field. The amplitude of the tilt angle modulation shows exactly the same dependence on temperature and frequency as the dielectric constant which indicates a linear coupling between the tilt angle and the polarization. The relaxation frequency increases linearly with temperature from 0.4 MHz at the SA—S∗C transition to 3.5 MHz near the SA-I clearing point. Optical switching times below 0.5 μs were established.
95 citations
TL;DR: The columnar mesophase has been investigated by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction as discussed by the authors, and it has been shown to transition to a thermotropic columnar phase at about 110-120°C for each compound of the series n = 12 to n = 22, n being even and equal to the number of carbon atoms in the corresponding fatty acid.
Abstract: Binuclear copper(11) complexes of fatty acids crystallize at room temperature in a lamellar lattice that has been characterized by X-ray diffraction. A transition to a thermotropic columnar mesophase is observed at about 110-120°C for each compound of the series n = 12 to n = 22, n being even and equal to the number of carbon atoms in the corresponding fatty acid. This columnar mesophase has been investigated by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. Columns of polar copper carboxylate groups are surrounded by disordered aliphatic chains, and form a two-dimensional hexagonal lattice. The repeating unit in a column is a binuclear dicopper tetracarboxylate complex.
90 citations
TL;DR: In this article, the attenuated total reflection technique is used to examine such guided modes allowing simultaneous observation of the surface plasmonpolariton (S.P.) at a metal/liquid crystal interface.
Abstract: An aligned layer of nematic liquid crystal with suitable optical anisotropy and under the application of appropriate applied voltages, may support various guided modes of light. Here the attenuated total reflection technique is used to examine such guided modes allowing simultaneous observation of the surface plasmonpolariton (S.P.P.) at a metal/liquid crystal interface. We report observations of the interaction between the bulk guided modes and the S.P.P., an interaction made possible by the strong refractive index anisotropy of the chosen liquid crystal. The influence of applied voltage upon the guided modes, the S.P.P. and their interaction is studied. Detailed reflectivity results are compared with theory for layered uniaxial media.
82 citations
TL;DR: In this paper, a mixture of liquid-crystalline polyesters with cholesteric and probably chiral smectic C∆ phases were used to obtain cross-linked polymers with elastic properties.
Abstract: Liquid-crystalline polyesters with cholesteric and probably chiral smectic C∗ phases were prepared using combined liquid-crystalline polymers (that is polymers with the mesogenic groups in the main chain as well as in the side groups). Copolyesters of these polymers and polymers with olefinic double bonds could be cross-linked retaining the liquid-crystalline phases. This resulted in cross-linked polymers with elastic properties.
74 citations
TL;DR: In this paper, the synthesis, characterization and thermal behavior of 12 dinuclear, orthopalladated complexes derived from Schiff's bases are reported and the connection between their molecular structures and their mesogenic properties is discussed.
Abstract: The synthesis, characterization and thermal behaviour of 12 dinuclear, orthopalladated complexes derived from Schiff's bases are reported. The complexes are of the type [Pd2(H-X)2C6H3(OR)–C(Z) = N-C6H4 R' 2] (X = OAc, Cl, Br, SCN; Z = H, CH3; R' = OR, R; R = C10H21) and ten of them exhibit ordered mesophases (SC and SA). The molecules were characterized structurally by I.R. and 1H N.M.R. spectroscopy in order to elucidate the possible existence of isomers. The connection between their molecular structures and their mesogenic properties is discussed.
67 citations
TL;DR: In this article, the X-ray reflectivity from docosanoic acid spread on 2.5 × 10−4 molar solutions of caesium chloride, cadmium chloride and lanthanum chloride was compared.
Abstract: The technique of X-ray reflection is used to study the structure of an insoluble monolayer spread on an aqueous subphase. The feasibility of the technique as a non-invasive structural probe for liquid surfaces is demonstrated. A comparison of the X-ray reflectivity from docosanoic acid spread on 2.5 × 10−4 molar solutions of caesium chloride, cadmium chloride and lanthanum chloride shows that practically all of the cadmium and lanthanium ions are within 15 A of the acid head groups whereas there is no evidence for such binding with caesium. The pH dependence of the amount of bound cadmium is used to show that a pK value of 5.35 ± 0.05 describes the conversion from acid to cadmium soap.
58 citations
TL;DR: In this article, a rod-like aromatic core with four to six long aliphatic terminal groups, which all possess a hexagonal mesophase, has been studied by X-ray diffraction.
Abstract: Eighteen compounds having the same molecular architecture, a rod-like aromatic core with four to six long aliphatic terminal groups, which all possess a hexagonal mesophase, have been studied by X-ray diffraction. From the lattice constants of these mesophases, we have deduced some information about the molecular organization. We discuss the curvature and the density of the interface between the aromatic and the paraffinic moieties, the orientational disorder and the packing of the rod-like central parts of the molecules. The occurrence of smectic, oblique or cubic mesophases in the same series underlines the similarities with lyotropic polymorphism and justifies a description of the molecular organization in terms of interfaces.
55 citations
TL;DR: In this article, the mesogenic properties of poly-α-chloroacrylate side-chain polymers are characterized by differential scanning calorimetry, optical microscopy and X-ray diffraction.
Abstract: Novel methacrylate and α-chloroacrylate side-chain polymers in which the mesogenic moieties are based on the 4-n-alkoxyphenyl-4'-(4'-methylhexyloxy) benzoates have been characterized by differential scanning calorimetry, optical microscopy and X-ray diffraction. Polymethacrylates give smectic A phases although the homologue where n = 11 exhibits a smectic C∗-smectic A transition. There is no real evidence for liquid-crystalline properties in the poly-α-chloroacrylates where n = 2 or 6. The poly-α-chloroacrylate for which n = 11 exhibits a layer structure with the side chains orthogonal to the layer planes as in the smectic A phase. However, this smectic phase has a higher order of the molecules within the layers; the chlorine atoms do impose a local ordering of the mesogenic side groups.
52 citations
TL;DR: X-ray measurements on liquid-crystalline main chain polymers and on combined polymers (polymers with the mesogenic groups in the main chain as well as in the side groups) before and after cross-linking allow the identification of the different liquid crystalline phases as mentioned in this paper.
Abstract: X-ray measurements on liquid-crystalline main chain polymers and on combined polymers (polymers with the mesogenic groups in the main chain as well as in the side groups) before and after cross-linking allow the identification of the different liquid-crystalline phases. Smectic A, smectic B and probably smectic C phases are found at higher temperatures. The fibre pattern of the oriented combined polymers are analogous to the fibre pattern of the liquid-crystalline main chain polymers. The comparison implies that the mesogenic groups in main chains and side groups are oriented parallel to each other to form the liquid-crystalline phase. No hint of a microscopic bidirectional orientation is observed. Two different types of orientation were observed in the polymers on drawing, depending on the molecular weight (non-cross-linked polymers) or the elongation (cross-linked polymers). In lower molecular weight polymers the smectic layers are oriented parallel to the fibre axis on drawing. For higher mole...
TL;DR: A comparative study of three related families of ferroelectric liquid crystals and some distinctly different analogues has been made with the aim of clarifying systematic trends in smectic C phase stability and the appearance of macroscopic polarization.
Abstract: A comparative study of three related families of ferroelectric liquid crystals and some distinctly different analogues has been made with the aim of clarifying systematic trends in smectic C phase stability and the appearance of macroscopic polarization. The substances are interesting not only because the C phases are broad and occur around ambient temperature, but also because the more highly ordered tilted phases (smectic I, J, …) are suppressed. Their collective behaviour also sheds light on the molecular origin of the spontaneous polarization.
TL;DR: In this paper, a novel alignment technique which utilizes mixtures with extended cholesteric pitches and has been found to give excellent alignment of ferroelectric smectics using the standard surface treatments employed in the nematic display industry.
Abstract: Until now a major problem in achieving practical ferroelectric displays has been the lack of a suitable technique for producing uniform alignment over large areas. This preliminary communication reports a novel alignment technique which utilizes mixtures with extended cholesteric pitches and has been found to give excellent alignment of ferroelectric smectics using the standard surface treatments employed in the nematic display industry.
Journal Article•
TL;DR: In this paper, the authors describe a new class of light control films consisting of submicron liquid crystal droplets dispersed in ultraviolct-cured polymer matrices, which can respond optically to both applied electric fields and temperature changes.
Abstract: Abstract This paper describes a new class of light control films consisting of submicron liquid crystal droplets dispersed in ultraviolct-cured polymer matrices. These films, which can respond optically to both applied electric fields and temperature changes, are potentially useful for displays and light shutters. The optical performance of these films depends on a variety of structural, electro-optical and thermal properties. This report describes scanning electron microscope studies of film structure, measurements of voltage dependent film transmittance and light scattering, and calorimetric studies which indicate that microdroplet formation in the films occurs as a result of phase separation which takes place during the cure process.
TL;DR: In this paper, a new range of transition metal-containing liquid crystals is formed by complexing classical organic mesogens directly to a metal, which exhibits a monotropic nematic phase.
Abstract: A new range of transition metal-containing liquid crystals is formed by complexing classical organic mesogens directly to a metal. Thus, reaction of [PdCl2(PhCN)2] with 4-n-pentyl-4′-cyanobiphenyl (5CB) leads to [PdCl2(5CB)2] which exhibits a monotropic nematic phase. Complexes of long chain 4-alkyloxy-4′-cyanobiphenyls show enantiotropic mesophases. Analogously, reaction of [PtCl2(PhCN)2] or [Rh2Cl2(CO)4] with 4-alkyl-4′-cyanobiphenyls (L) yields mesogenic [PtCl2L2] or cis-[RhCl(CO)2 L] respectively. The crystal and molecular structure of [PdCl2(5CB)2] and [PtCl2(8CB)2] are also reported and show different packing in the solid state.
TL;DR: In this article, it was shown that the director configuration in smectic C phases is bistable in strong electric fields for suitable field directions with respect to the smectric layer planes.
Abstract: The director configuration in smectic C phases is bistable in strong electric fields for suitable field directions with respect to the smectic layer planes. Therefore different domains separated by walls are possible. Bistability can be accompanied by director reorientation which causes domain wall motion and pronounced hysteresis effects. Transitions of this type have been investigated both experimentally and theoretically.
TL;DR: In this article, a review is given of several experimental methods for the measurement of the performance parameters of ferroelectric liquid crystals, including spontaneous polarization, elastic modulus and dielectric susceptibility for the tilt of the director.
Abstract: A review is given of several experimental methods for the measurement of the performance parameters of ferroelectric liquid crystals. We discuss (a) the technique for measuring the spontaneous polarization, the elastic modulus and dielectric susceptibility for the tilt of the director (the pyro-electric method), (b) the measurement of the tilt angle and the optical anisotropy (the electro-optical method) and (c) the determination of the non-linear susceptibility of the chiral smectic C∗ phase (the optical second harmonic generation technique).
TL;DR: In this paper, two related ester systems having fluoro, chloro, bromo, methyl or cyano lateral substituents have been synthesized and the effect of lateral substitution on the thermal stability of the liquid crystal phases, and particularly the smectic C phase, was examined.
Abstract: Two related ester systems having fluoro, chloro, bromo, methyl or cyano lateral substituents have been synthesised. The effect of lateral substitution on the thermal stability of the liquid crystal phases, and particularly the smectic C phase, was examined. In some instances, a small polar group was shown to increase the stability of the smectic C phase relative to the unsubstituted analogue. The position of the lateral substituent is important. Some of the compounds exhibit a very wide smectic C phase.
TL;DR: In this article, the authors present a neutron scattering study of oriented samples for the lamellar phase of the ternary mixture sodium decyl sulphate/1-decanol/water.
Abstract: We present a neutron scattering study of oriented samples for the lamellar phase of the ternary mixture sodium decyl sulphate/1-decanol/water. Diffuse scatterings are observed, around the Bragg reflections and away from them, which show that the structure of this lamellar phase deviates from the periodic stacking of infinite homogeneous lamellae of water and amphiphilic molecules usually proposed for the structure of lamellar phases. The nature of this deviation evolves with the soap/decanol ratio, according to the location of the sample in the lamellar domain of the phase diagram. In the middle of the domain the deviation relates to the organization of the lamellar stacking, without apparent modification of the structure of the lamellae of amphiphiles. Moving away from the middle, for higher soap/decanol ratios, the structure of the lamellae appears to be randomly perturbed, eventually by the presence of a few water regions piercing them. When the boundary of the lamellar domain is approached, f...
TL;DR: In this article, a series of copper alkanoate dimers were obtained for the Raman spectra and band assignments were made using isotopic substitution, temperature studies and in part by Raman Spectra-based analysis.
Abstract: Infrared spectra have been obtained for a series of copper alkanoate dimers which include the caproate (dicopper tetrahexanoate), pelargonate (nonanoate), undecanoate, laurate (dodecanoate), palmitate (hexadecanoate) and docosanoate Band assignments have been made wherever possible through the use of isotopic substitution, temperature studies and, in part, by Raman spectra The results indicate that it should be possible to follow phase changes to the respective columnar mesophases of these complexes by monitoring the position of the methylene stretching frequency
TL;DR: In this paper, a new class of discotic liquid crystals is presented, which consists of a trisubstituted central core (either a benzene or a cyclohexane ring) and three regional cores, with three decyloxy-chains in the 3,4,5 positions of a particular benzene ring.
Abstract: Two examples of a new class of discotic liquid crystals are presented. Their molecular architecture consists of a trisubstituted central core (either a benzene or a cyclohexane ring) and three regional cores, with three decyloxy-chains in the 3,4,5-positions of a particular benzene ring. These regional cores are linked to the central core via ester groups. Polarizing microscopy and calorimetric measurements reveal the existence of monotropic and enantiotropic discotic mesophases respectively. The products exhibit a pronounced hindering of crystallization at room temperature.
TL;DR: In this article, a liquid-crystalline polysiloxane for thermo-optical recording is presented, and an unoptimized sensitivity of 12nJ/μm2 for 632°8 nm light was found at 24°C.
Abstract: Initial experimental evaluation of a novel liquid-crystalline polysiloxane for thermo-optical recording is presented. A versatile system using a single laser source has been used to gather information on the dynamics of laser addressing. To ensure the most stable, highest-contrast pixel the polymer must be heated through its biphasic region and some way into the isotropic phase. It was found that at 60°C using laser pulses of 300μs or less, write-in and selective erasure times were on the submillisecond timescale, and that bulk erasure required 1–2 min. The use of the polymer in an analogue optical store has been demonstrated and an unoptimized sensitivity of 12nJ/μm2 for 632°8 nm light was found at 24°C.
TL;DR: In this article, the authors present calculations of the light intensity transmitted through a ferroelectric liquid crystal light valve with uniformly tilted layers during electrooptical switching and show that the rise and fall times of light intensity are in general different.
Abstract: We present calculations of the light intensity transmitted through a ferroelectric liquid crystal light valve with uniformly tilted layers during electrooptical switching. We discuss the switching characteristics of the light valve for different layer tilts and at different applied voltage steps. We also show that the rise and fall times of the light intensity are in general different.
TL;DR: In this article, different molecular structures of the mesogenic pendant group connected to the main chain via a variable intermediate group have been investigated, and the influence of the intermediate group (spacer length n, of the central group (O or COO), of the interphenyl group (COO) as well as that of the terminal group (CN or OCH3), upon the nature of the relaxations (α, β, γ or ω) and the critical times of the molecular dynamics in side-chain liquid crystal polymers are discussed separately.
Abstract: Dielectric studies have been performed on a series of oriented polyacrylates in broad frequency and temperature ranges, 5 Hz to 1 GHz and – 170°C, to + 200°C respectively. In order to assist in the identification of the observed relaxation mechanisms, different molecular structures of the mesogenic pendant group connected to the main chain via a variable intermediate group have been investigated. The influence of the intermediate group (spacer length n), of the central group (O or COO), of the interphenyl group (COO) as well as that of the terminal group (CN or OCH3), upon the nature of the relaxations (α, β, γ or ω) and the critical times of the molecular dynamics in side-chain liquid crystal polymers are discussed separately.
TL;DR: In this paper, the Freedericksz transition of planar oriented smectic C phases has been investigated experimentally and the optical picture of the effect observed under a polarizing microscope is described in detail.
Abstract: The Freedericksz transition of planar oriented smectic C phases has been investigated experimentally. The optical picture of the effect observed under a polarizing microscope is described in detail. For a number of selected materials the threshold voltage and the switching times were measured. The experimental data were used to estimate the elastic constant and the rotational viscosity of smectic C phases. The experimental results are discussed and interpreted theoretically.
TL;DR: In this paper, the effect of altering the point of alkyl chain branching on smectic C thermal stability was investigated in six types of ester system. But the effect was minimal after the third carbon.
Abstract: Fifty-five esters were prepared to show the effect of altering the point of alkyl chain branching on smectic C thermal stability. Six types of ester system were studied. Moving the position of alkyl chain branching away from the main molecular core enhanced smectic C thermal stability; after about the third carbon this effect was minimal. Extending the alkyl chain after the point of chain branching gradually lowered the smectic C thermal stability.
TL;DR: In this article, two principal mechanisms are shown to be important for the orientational order of small solute molecules in the nematic liquid crystal 4-(α,β-d 4-n-pentyl)-4'-cyanobiphenyl (5CB-α, β-d4 4).
Abstract: Two principal mechanisms are shown to be important for the orientational order of small solute molecules in the nematic liquid crystal 4-(α,β-d 4-n-pentyl)-4'-cyanobiphenyl (5CB-α,β-d 4). The interaction between the mean electric field gradient due to the liquid crystal and the molecular quadrupole moment of the solute is particularly important for small molecules such as hydrogen. For larger solutes, like 1,3,5-trichlorobenzene, the short-range interactions which are proportional to the size and shape of the solute are important. A model which takes both mechanisms into account is used to calculate solute molecular order parameters. Excellent agreement between the calculated parameters and those obtained from N.M.R. measurements is found.
TL;DR: In this paper, the X-ray diffraction patterns of columnar columnar mesophases of rufigallol have been studied and their discotic mesomorphism studied.
Abstract: Aliphatic ethers of rufigallol have been prepared and their discotic mesomorphism studied. All the compounds give enantiotropic, columnar mesophases with a regular stacking of the discs along the columns, as revealed by the X-ray diffraction patterns of the mesophases. Calorimetric data indicate a wide range of stability of the mesophase. Solid polymorphism has been demonstrated that involves both solution crystallized and melt crystallized samples.
TL;DR: In this paper, the synthesis of new liquid-crystalline comb-like polymers with oxygen-containing spacers and mesogenic cyanbiphenyl groups is described.
Abstract: Synthesis of new liquid-crystalline comb-like polymers with oxygen-containing spacers and mesogenic cyanbiphenyl groups is described. Liquid-crystalline homo-polymers of acrylates and methacrylates, their copolymers and liquid-crystalline polysiloxane are obtained. It is shown that the incorporation of oxygen atoms in spacer leads to decrease of the clearing temperature. The mesophase types of the homopolymers and copolymers are determined.
TL;DR: In this article, single crystals and polycrystalline samples of the compounds bis(1,3-di(p-n-octylphenyl)propane-l, 3-dionato)copper(II) have been studied by ESR.
Abstract: Single crystals and polycrystalline samples of the compounds bis(1,3-di(p-n-octylphenyl)propane-l,3-dionato)copper(II) [I], and bis(l,3-di(p-n-octyloxy-phenyl)propane-l,3-dionato)copper(II) [II] have been studied by ESR The ESR spectra of single crystals of [I] at 298 K have the features associated with a spin 1/2 one-dimensional Heisenberg antiferromagnet while the spectra of single crystals of [II] do not ESR of single crystals of [I] and [II] provides a reliable means of detecting previously reported crystalline to discotic phase transitions Single crystals of [I] heated beyond the temperature of the first discotic phase transition yield single line ESR spectra which have an angular dependence and an asymmetric lineshape; these observations suggest that, in the discotic phase, exchange interactions are still significant and a degree of long range order is maintained