Showing papers in "Liquid Crystals in 1999"
TL;DR: In this paper, six members of a new homologous series of achiral banana-shaped molecules have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy and X-ray diffraction.
Abstract: Six members of a new homologous series of achiral banana-shaped molecules have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy and X-ray diffraction. According to X-ray diffraction measurements on oriented samples, four homologues form an XB2 phase behaviour. From electro-optical studies the spontaneous polarization and the tilt angle could be measured. An orientational order parameter of 0.8 was determined by 13C NMR and this is nearly independent of the temperature. NMR investigations also give information about the real conformation of the molecules in the XB2 phase. Dielectric measurements indicate that the rotation around the molecular long axis is clearly hindered because of the packing of the bent molecules within the smectic layers. which exhibits antiferroelectric switching
112 citations
TL;DR: In this article, the same authors found that 9,9-dioctylfluorene copolymers alternating with benzothiadiazole and dithiophene moieties in the main chain were found to be "hairy rod" type thermotropic liquid crystalline polymers as well as yellow to red photoluminescence emitters.
Abstract: Conjugated copolymers of 9,9-dioctylfluorene alternating with benzothiadiazole and dithiophene moieties in the main chain were found to be 'hairy rod' type thermotropic liquid crystalline polymers as well as yellow to red photoluminescence emitters. Thin films of both copolymers were found to align into monodomains on standard liquid crystal alignment layers as well as on rubbed precursor route poly(para-phenylenevinylene) (precPPV), which is transparent to red light. This observation is of importance for the use of precPPV as a hole transporting alignment layer for fluorene copolymers in polarized electroluminescence devices.
89 citations
TL;DR: The structure of a liquid crystalline (LC) carbosilane dendrimer of the fifth generation bearing 128 terminal cyanobiphenyl mesogenic groups has been studied as mentioned in this paper.
Abstract: The structure of a liquid crystalline (LC) carbosilane dendrimer of the fifth generation bearing 128 terminal cyanobiphenyl mesogenic groups has been studied. This dendrimer was synthesized by a hydrosilylation reaction and then the cyanobiphenyl mesogenic groups were chemically linked to the dendritic matrix via a-OOC-(CH2)10-Si(CH3)2OSi(CH3)2-spacer. Structural studies carried out by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction methods revealed unusual phase behaviour. At room temperature the dendrimer forms a lamellar (smectic A) phase which develops in-plane ordering above 40C.This is due toa tendency to form columns ofmolecules which areprobably perpendicular to the layers. Above 121C the material transforms into another more disordered mesophase which is probably a disordered hexagonal columnar phase. The proposed structures and molecular packing in these different types of mesophase are discussed.
81 citations
TL;DR: The self-assembling process involved in the formation of the mesomorphic state apparently deforms the spherical disposition of the supermolecular dendrimers to give rod-like entities that pack together to yield disordered Smectic A and smectic C mesophases as discussed by the authors.
Abstract: Liquid crystalline dendrimers are proving to be a fascinating class of materials that possess unusual physical properties. The self-assembling process involved in the formation of the mesomorphic state apparently deforms the spherical disposition of the supermolecular dendrimers to give rod-like entities that pack together to yield disordered Smectic A and smectic C mesophases.
75 citations
Journal Article•
75 citations
TL;DR: In this paper, the geometrical shape (aspect ratio) of the monodomain acrylate-based networks is found to affect drastically the dynamics and mechanics of the reorientation transition.
Abstract: Monodomain acrylate-based networks have been synthesized by a two-step cross-linking procedure using gamma-irradiation. Strain-induced reorientation of the nematic director is studied by X-ray diffraction measurements. The geometrical shape (aspect ratio) of the monodomain films is found to affect drastically the dynamics and mechanics of the reorientation transition. Uniform continuous rotation of the director occurs in narrow samples with an aspect ratio AR=12 when the external mechanical field is applied perpendicular to the initial orientation. Under the same conditions, films having a lower aspect ratio (AR=2.5) demonstrate the formation of stripe domains with an alternating sense of director rotation (clockwise and counter-clockwise towards the stress axis). Deformation of a square film (AR=1) generates stripe domains in the geometrical centre of the sample, whereas a uniform continuous rotation is observed in other regions of the film. Finally, a comparison of experimental data and theoretical pred...
71 citations
TL;DR: In this paper, two series of calamitic liquid crystals containing a benzothiazole ring within the central core and two different linkage groups (amide and azo) have been prepared and their liquid crystalline properties studied.
Abstract: Two series of calamitic liquid crystals containing a benzothiazole ring within the central core and two different linkage groups (amide and azo) have been prepared and their liquid crystalline properties studied and compared with those of the analogous series of imines. The influence of the linkage group within the central core has been proven to determine the variety of mesomorphism displayed by the compounds. The compounds with imine and azo linkages behave in a similar way and exhibit typical nematic and smectic C mesophases. Compounds incorporating an amide linkage show a poorer mesomorphism and mainly present a smectic C mesophase.
65 citations
TL;DR: In this paper, the authors report a chiral liquid crystal in which a tilt of the molecular orientation away from the layer normal in the Smectic C phase of up to 24 occurs with minimal layer contraction.
Abstract: We report a smectic liquid crystal in which a tilt of the molecular orientation away from the layer normal in the Smectic C phase of up to 24 occurs with minimal layer contraction. This characteristic, accompanied by an exceptionally large underlying negative thermal layer expansion coefficient, enables the formation of chiral smectic electro-optical cells having nearly the ideal 'bookshelf' layer geometry, free from the complexities of the chevron layer structure produced by contraction.
64 citations
TL;DR: In this article, the influence of charged impurities and flexoelectric polarization on the optical transmission of a hybrid aligned nematic liquid crystal cell was analyzed under the Poisson Boltzmann equation and Frank elastic theory.
Abstract: W e analyse the influence of charged impurities and flexoelectric polarization on the optical transmission of a hybrid aligned nematic liquid crystal cell. The theoretical results obtained within the framework of the Poisson-Boltzmann equation and Frank elastic theory are compared with the observed optical response [N. V. Madhusudana and G. Durand,J. Phys.Lett. 46, L-195 (1985)]. We show that impurities can be very important for the behaviour of the system in the low field regime where the flexoelectric effect is relevant, and we determine the flexoelectric coefficient, the anchoring strength, and the concentration of impurities in the sample previously studied by Madhusudana and Durand.
64 citations
TL;DR: In this paper, the mesomorphic and transitional properties of dimesogens have been investigated using optical microscopy, differential scanning calorimetry, helical pitch measurements, and electric field and miscibility studies.
Abstract: New non-symmetric dimesogens consisting of a cholesteryl moiety and an aromatic mesogenic unit interconnected through a pentamethylene spacer have been prepared. The aromatic units consist of two phenyl rings linked by carboxy, oxycarbonyl, ethylene, ethynylene, azo or Schiff's base groups. The mesomorphic and transitional properties have been investigated by using optical microscopy, differential scanning calorimetry, helical pitch measurements, and electric field and miscibility studies. The structures of the mesophases have been studied using X-ray diffraction. Several kinds of smectic packings are observed depending on the nature of the linking group and an attempt to understand the origin of the different phase structures at a molecular level is made.
61 citations
TL;DR: In this paper, the authors propose a liquid crystal templating of porous materials, which is similar to the one we use in this paper, but with a slightly different approach.
Abstract: (1999). Invited article Liquid crystal templating of porous materials. Liquid Crystals: Vol. 26, No. 3, pp. 305-339.
TL;DR: In this article, a new smectic type mesophase without in-plane order is presented which is formed by nitro substituted banana-shaped compounds and the structure of this phase is not yet known in detail.
Abstract: A new smectic type mesophase without in-plane order is presented which is formed by nitro substituted banana-shaped compounds. The structure of this phase is not yet known in detail. The growth of germs and also the textures point to a helicoidal structure. The helical structure is obviously the result of the chirality of the smectic layers which is caused by the tilt of the molecules together with the polar arrangement of the angled molecules.
TL;DR: The double-twisted helical structure was confirmed for the smectic phase of a banana-shaped molecular system from observation of the microscopic pattern Optically polarized microscopic textures and phase transitions of 1,3-phenylene bis\[4-(3-chloro-4 n-octyloxyphenyliminomethyl) benzoate (PBCOB) and related compounds are described.
Abstract: The double-twisted helical structure was confirmed for the first time for the smectic phase of a banana-shaped molecular system from observation of the microscopic pattern Optically polarized microscopic textures and phase transitions of 1,3-phenylene bis\[4-(3-chloro-4- n-octyloxyphenyliminomethyl) benzoate (PBCOB) and related compounds are described The chloro-substituent introduced into the 3-position of the Schiff's base moiety of the banana-shaped molecule destabilized the smectic phase and enhanced new mesomorphic properties The mesophases exhibited by the octyloxy homologue of the chloro-substituted compound were of the smectic phases, SmX1 and SmX2 The mixing behaviour between chloro-substituted and non-chloro-substituted compounds resulted in the appearance of a wide miscibility gap and enlargement of the SmX1 phase which can be seen in a lateral chloro-substituted compound
TL;DR: In this article, the influence of the alignment layer on V-shaped switching in thin homogeneous cells of a chiral smectic liquid crystal was studied by means of electro-optic and switching current measurements.
Abstract: The influence of the alignment layer on V-shaped switching in thin homogeneous cells of a chiral smectic liquid crystal were studied by means of electro-optic and switching current measurements.Severalpolyimides withdifferent chemicalstructures wereusedas thealignment layer in thin homogeneous cells; V-shaped switching was observed with some of them and W-shaped switching with others. It was also shown that the switching current constantly flows during the V-shaped transmittance change. In order to clarify this effect of the alignment layer on V-shaped switching, polyimide films with various polarity and thickness were examined. It is concluded that thick alignment layers assist liquid crystal materials in realizing V-shaped switching, even if the materials are not ideal for this process.
TL;DR: In this paper, a lattice model with pairwise additive interactions is presented, where the parameters defining the pair potential are expressed as linear combinations of elastic constants. But the interaction energies or potentials used in these studies are short ranged but, in general, not pairwise-additive, unless the three elastic constants are set to a common value, thus reducing the potential to that in the well-known Lebwohl-Lasher lattice models.
Abstract: Director configurations in a nematic liquid crystal can be determined by minimizing its total elastic free energy, for given elastic constants and specific boundary conditions. In some cases, these configurations have been obtained by numerical procedures where the elastic free energy density plays the same role as the overall potential energy in a standard Metropolis Monte Carlo simulation. The interaction energies or potentials used in these studies are short ranged but, in general, not pairwise additive, unless the three elastic constants are set to a common value, thus reducing the potential to that in the well-known Lebwohl-Lasher lattice model. On the other hand, we can construct, in different ways, a lattice model with pairwise additive interactions, which approximately reproduces the elastic free energy density, where the parameters defining the pair potential are expressed as linear combinations of elastic constants. An anisotropic nematogenic pair interaction of this kind, originally proposed by...
TL;DR: Polarizing microscope textures of the twist grain boundary A* (TGBA*) phase are reviewed for two different compounds in different geometries with different surface treatments giving monostable planar and homeotropic boundary conditions as discussed by the authors.
Abstract: Polarizing microscope textures of the twist grain boundary A* (TGBA*) phase are reviewed for two different compounds in different geometries with different surface treatments giving monostable planar and homeotropic boundary conditions. The textures are discussed in the light of the helical structure of the TGBA* phase. Depending on the compound, the underlying phase is either SmA* or SmC*, whereas the adjacent phase at higher temperature is cholesteric (N*). Sample preparations in wedge-shaped cells subjected to a slight temperature gradient exhibit TGBA* textures much more typical for the cholesteric than for the ordinary SmA* phase. For instance, Grandjean steps and fingerprint textures are observed for planar and homeotropic boundary conditions, respectively. Preparation of smectic droplets clearly reveals the helical axis of the TGBA* phase to be perpendicular to the helical axis of the helielectric SmC* phase. For thin samples, a suppression of the TGBA* helix leading to a surface-induced structure ...
TL;DR: In this article, it was shown that the arrangement of molecular cores has a higher degree of order in the isotropic (D and cubic) phases than in the SmC phase.
Abstract: Heat capacity measurements have been made on ANBC(18) at temperatures from 8 to 490 K by adiabatic calorimetry. All known phases were detected. The temperatures, enthalpies and entropies of transition were determined for the phase transitions observed. On the basis of the entropy of transition to the SmC phase from the D or cubic phases, it is pointed out that the D phase of ANBC and the cubic phase of BABH might be identical in nature. It is shown that the arrangement of 'molecular' cores has a higher degree of order in the isotropic (D and cubic) phases than in the SmC phase, whereas the terminal alkoxy chains are more disordered in the isotropic phases than in the SmC phase. The degrees of disorder in the D and cubic phases relative to the SmC phase are very similar in terms of the entropy of transition per methylene group. The inverted phase sequence in ANBC (SmC D on heating) and BABH (cubic SmC) can be accounted for in terms of the competing roles in the entropy between the molecular core and the ch...
TL;DR: In this article, the origin of V-shaped switching observed in thin cells of antiferroelectric/ ferrielectric liquid crystals was investigated by means of helical pitch and conoscope measurements using thick free-standing films of binary mixtures with various mixing ratios.
Abstract: In order to clarify the origin of the V-shaped switching observed in thin cells ofantiferroelectric/ ferrielectric liquid crystals, bulk properties have been studied by means of helical pitch and conoscope measurements using thick free-standing films of binary mixtures with various mixing ratios. In the temperature range showing V-shaped switching in thin cells, helical structure clearly exists, indicating the existence of ordered phases. Some indistinct phase changes with temperature, coexistence of phases and quasi-continuous phase changes with an applied electric field were observed, suggesting a system with weak inter-layer correlation. By comparing the phase diagrams made using thin homogeneous cells and thick free-standing films, it was found that V-shaped switching occurs in the region where various subphases exist in the bulk. The appearance of many indistinct phases is consistent with the weak interlayer correlation. In this way, it was concluded that the V-shaped switching occurs in tilted smect...
TL;DR: In this article, a series of mesomorphic complexes formed through intermolecular hydrogen bonding between p-n-alkoxybenzoic acids (where alkoxy denotes chains from propoxy- to decyloxy- and dodecyloxy-) and non-mesogenic p-hydroxybutyl benzoate, have been synthesized and characterized by thermal microscopy, differential scanning calorimetry, infrared spectroscopy (IR) and 1H NMR studies.
Abstract: A new series of mesomorphic complexes formed through intermolecular hydrogen bonding between p-n-alkoxybenzoic acids (where alkoxy denotes chains from propoxy- to decyloxy- and dodecyloxy-) and non-mesogenic p-hydroxybutyl benzoate, have been synthesized and characterized by thermal microscopy, differential scanning calorimetry, infrared spectroscopy (IR) and 1H NMR studies. A detailed IR spectral investigation in the solid state and in solution suggests that the acid and phenol groups are complementary to each other, each acting as both proton donor and proton acceptor. The results of comparative thermal analyses of both free p-n-alkoxybenzoic acids and H-bonded complexes exhibited an induced crystal smectic G phase in the complexes throughout the series, its thermal range increasing with alkoxy carbon number.
TL;DR: In this article, four different types of dimesogenic compounds involving the cholesteryl moiety as one of the mesogenic constituents were synthesized and their liquid crystalline properties were investigated.
Abstract: We have synthesized four different types of dimesogenic compounds involving the cholesteryl moiety as one of the mesogenic constituents, and have investigated their liquid crystalline properties. The molecular structures of these dimesogens have been confirmed by spectral analyses; they exhibit a rich polymorphism, as revealed by optical microscopic and differential scanning calorimetric observations. The studies show that the mesomorphic behaviour is sensitive to the nature of the terminal alkyl chains, and to the structure of the 'second mesogen' that is attached to the cholesteryl unit through a polymethylene spacer.
TL;DR: In this article, the aggregation structures in organic gels and xerogels formed from L-glutamic acid-derived lipids were investigated by scanning and transmission electron microscopies, X-ray analyses, and 1H NMR and IR spectroscopic methods.
Abstract: Aggregation structures in organic gels and xerogels formed from L-glutamic acid-derived lipids were investigated by scanning and transmission electron microscopies, X-ray analyses, and 1H NMR and IR spectroscopic methods. These analyses showed that the gels were produced through the formation of highly oriented aggregates based on a single layer and a remarkable development of their fibrous morphology. We also describe how the critical aggregation concentration can be observed at a concentration below the critical gel concentration by using a dye-complexation method with a cyanine dye, NK-77.
TL;DR: In this paper, the same cholesteryl unit and the same spacer length were used to select molecules for the analysis of incommensurate low ordered smectic phases (Sic) in non-symmetric dimesogens with molecular parameters varying from those of the standard compound KI-5(4).
Abstract: Systematic physical chemistry studies are in progress concerning the occurrence of incommensurate low ordered smectic phases (Sic) in non-symmetric dimesogens with molecular parameters varying from those of the standard compound KI-5(4). In the present study, the selected molecules possess the same cholesteryl unit and the same spacer length, but the terminal chain of the aromatic mesogenic part is varied. Up to a critical molecular length, a competition between two incommensuratelengths is observed and the dimesogeniccompounds respond to the frustration connected to this competition with the formation either of an incommensurate phase Sic or of a two dimensional modulated phase. For longer dimesogens, a commensurate lock-in of the two characteristic lengths systematically occurs.
TL;DR: In this article, a new class of chiral gels consisting of a lightly cross-linked network containing non-reactive molecules were produced. But the gels showed sharp switching characteristics with a hysteresis.
Abstract: Chiral nematic liquid crystal compositions containing mono- and di-acrylates in mixtures with non-reactive liquid crystal molecules were produced. Defect-free planar macroscopic alignment of the molecules was induced between uniaxially rubbed substrates. Polymerization of the acrylate groups was initiated photochemically leading to the formation of a new class of chiral gels consisting of a lightly cross-linked network containing non-reactive molecules. In this way, the chiral pitch and, hence, the reflection colour became permanently fixed. Under the influence of an electric field the gels could be switched to the homeotropic state and reverted to the planar defect-free state upon removal of the electric field. Depending on the composition of the gel, either the position of the reflection band remained the same while its magnitude and/or width decreased, or its position shifted to smaller wavelengths with increasing electric field. Some of the gels showed sharp switching characteristics with a hysteresis...
TL;DR: In this paper, the structure of 4-cyano-4-hexylbiphenyl (6CB) and 7CB with low melting points has been determined at 73 and 33 C, respectively, and compared with previously reported structures of other homologues (n = 2-5, 8-11).
Abstract: Crystal structures of 4-cyano-4-hexylbiphenyl (6CB) and 4-cyano-4-heptylbiphenyl (7CB) with low melting points (13.5 and 30 C, respectively) have been determined at 73 and 33 C, respectively, and compared with previously reported structures of other homologues (n = 2-5, 8-11). In the crystal of 7CB, chains and cores stack alternately, where core regions are largely overlapped with distances of 3.575(2) and 3.597(2)A for N (in CN)... C (in phenyl), showing almost the same packing mode as those of 9CB and 11CB. In the crystal of 6CB, CN groups are antiparallel between neighbouring molecules though the interatomic distances are much longer N... C, 3.673(3) and 3.563(3)A than in 8CB and 10CB (3.28-3.43A), resulting in a quite different packing mode from those of 8CB and 10CB, which had been found to be similar, with CN-CN interactions. Systematic comparison of the homologous series (n = 2-11) reveals odd-even effects, CN-phenyl and CN-CN interactions for odd and even members, respectively, in the crystal stru...
TL;DR: Lee-Goldburg decoupling has been applied to enhance dipolar oscillations during the t(1) period of the two-dimensional experiment used for obtaining order parameters of the liquid crystal N-(4-ethoxybenzylidene)-4-n-butylaniline (EBBA) at different temperatures.
Abstract: Lee-Goldburg decoupling has been applied to enhance dipolar oscillations during the t(1) period of the two-dimensional experiment used for obtaining order parameters of the liquid crystal N-(4-ethoxybenzylidene)-4-n-butylaniline (EBBA) at different temperatures. The order parameters of the aromatic core obtained by this method are in agreement with values obtained from the H-2 NMR spectra of a selectively labelled compound and are observed to follow the empirical Haller equation.
TL;DR: In this paper, the mesomorphic properties of a 4-biphenyl unit linked to a fluorinated tail via different spacers were investigated using polarized light microscopy and differential thermal analysis.
Abstract: In order to determine the influence of modification of the shape of the connector, within the fluorinated series, on the formation of mesophases in low molecular mass compounds, we decided to examine the mesomorphic behaviour of molecules which consist of a 4-biphenyl unit linked to a fluorinated tail via different spacers. The synthesis and the physical properties of these new materials are reported. The mesomorphic properties were characterized by polarized light microscopy and by differential thermal analysis, thereby showing the peculiar contribution of each of the spacers. The data collected from the 4-biphenyl derivatives synthesized allow us not only to confirm the enhancement of the smectic properties owing to the fluorinated alkyl chains, but also to display the impact of the spacer on these properties. Actually, some spacers like the amide or thioether type, as well as that of the hemithioacetal derivatives give suppression of the smectic character. Some others like that in the esters with an OC...
TL;DR: In this article, a family of new hydrogen bonded complexes based on comb-shaped LC copolymers containing alkyloxy-4-oxybenzoic acid mesogenic fragments and chiral dopant molecules, derivatives of pyridine-4 carboxylic acid has been prepared.
Abstract: A family of new hydrogen bonded complexes based on comb-shaped LC copolymers containing alkyloxy-4-oxybenzoic acid mesogenic fragments and chiral dopant molecules, derivatives of pyridine-4-carboxylic acid has beenprepared. At concentrations of chiral groups in the range 1± 25 mol%, induction of the cholesteric phase is observed. The temperature .
TL;DR: In this paper, the authors presented the results of studies of the dielectric relaxation of 6CHBT obtained for different values of the angle between the directions of the macroscopic orientation of the sample (director n) and the probing electric field E. A model of the molecular dynamics in the oriented nematics is proposed.
Abstract: This paper presents the results of studies of the dielectric relaxation of nematic 6CHBT obtained for different values of the angle between the directions of the macroscopic orientation of the sample (director n) and the probing electric field E. Analysis of the evolution of the relaxation spectrum from e*‖(ω) (E‖n) to e*⊥(ω) (E⊥n)allows one to explain the hitherto existing inconsistency in the molecular interpretation of the spectra. A model of the molecular dynamics in the oriented nematics is proposed.
TL;DR: In this article, a new type of functionalized discotic oligomer system has been realized whose molecular architecture consists of a trisubstituted benzene as central core and three nitro-functionalized triphenylenes as regional cores.
Abstract: A new type of functionalized discotic oligomer system has been realized whose molecular architecture consists of a trisubstituted benzene as central core and three nitro-functionalized triphenylenes as regional cores. These regional cores are linked to the central core via ether linkages through an alkyl chain spacer. Polarizing microscopy and calorimetric measurements reveal the existence of a monotropic columnar mesophase in this novel functionalized oligomer. The product does not crystallize at room temperature over a long period (c. five months) or on cooling to 50 C. The corresponding unfunctionalized trimer is not liquid crystalline. The dipolar interaction of nitro groups is probably responsible for the induction of mesogenicity. The introduction of functionality into liquid crystal oligomers permits the variation of their properties on a wide scale and opens the route to new synthetic supramolecular systems for various device applications.
TL;DR: A series of chiral bimesogenic compounds have been synthesized and phase characterization of these materials are described in this article, with particular emphasis on their flexoelectric properties.
Abstract: A series of chiral bimesogenic compounds has been synthesized. The molecules comprise beta-estradiol as the central chiral unit, which is attached to a terminal 4'-cyanobiphenyl-4-yl group at C-17 by an alkanoate spacer containing 5, 6 or 11 carbon atoms. At C-3 the estradiol moiety is substituted with an omega-alkenyloxy chain, also containing 5, 6 or 11 carbon atoms. We will describe the synthesis and phase characterization of these materials. The compounds with 5 or 11 carbon atoms in the spacer separating the mesogens display chiral nematic mesophases with temperature ranges up to 40 ° C, entering the isotropic state at about 100 ° C. The pitch lengths of these pure materials are remarkably invariant with temperature. Their electro-optic behaviour has been characterized, with particular emphasis on their flexoelectric properties. The materials have the highest flexoelectric coefficients reported so far in any chiral nematic liquid crystal [with e (kappa-1) < + 0.5 C N -1 m -1], with flexoelectro-optic...