Showing papers in "Liquid Crystals in 1998"
TL;DR: In this paper, the structure of dispersed liquid crystal droplets elasticity, surface tension, and surface anchoring is analyzed. And the equilibrium states of large droplets contain topologically stable defects, e.g., monopoles, boojums, etc.
Abstract: The structure of dispersed liquid crystal droplets elasticity, surface tension, and surface anchoring. For sufficiently large droplets with radius R K / W a, where K is the bulk elastic constant and W a is the anchoring coefficient, the surface terms prevail. As a result, the equilibrium states of large droplets contain topologically stable defects. Application of topological theorems to and hedgehogs is reviewed. is controlled by a balance of the bulk defect structures, e.g. monopoles, boojums
259 citations
TL;DR: In this article, the authors performed time resolved diffraction experiments in order to obtain a better insight upon the metastable phases surrounding some thermotropic mesophases of cubic Ia 3 d and Im 3 m symmetries.
Abstract: We have performed time resolved diffraction experiments in order to obtain a better insight upon the metastable phases surrounding some thermotropic mesophases of cubic Ia 3 d and Im 3 m symmetries. These metastable phases are columnar hexagonal, smectic or tetragonal, depending on the nature of the mesogenic molecule. Moreover, it appears that the structure of the cubic phase of the 4-alkyloxybiphenyl-4-carboxylic acids previously labelled smectic D varies with the lateral group substituted on the biphenyl core.
120 citations
TL;DR: In this article, wide band dielectric and electro-optic spectroscopy of an antiferroelectric liquid crystal (AFLC) has been carried out over a range of frequencies from 1 kHz to 1 GHz.
Abstract: Wide band dielectric and electro-optic spectroscopy of an antiferroelectric liquid crystal (AFLC) has been carried out over a range of frequencies from 1 kHz to 1 GHz. The AFLC sample under investigation possesses a variety of different ferri-, ferro-, and antiferro-electric phases. Dielectric spectroscopy of the LC cells in the antiferroelectric phases reveals both collective and individual dynamics of molecules. In the antiferroelectric SmCA phase, three dielectric relaxation processes are found in the absence of the bias and an additional relaxation process appears under the bias field. A solution of the dynamical equation of the director subject to a weak alternating field for the anitferroelectric helix has been found. A comparison of the results of electro-optic and dielectric spectroscopy with a theoretical study enables a determination of the origin of the relaxation processes in antiferroelectric phases. The mechanism for the distortion of the antiferroelectric helix has been determined using die...
107 citations
TL;DR: In this article, a study of the ferroelectric and antiferroelectric behavior in low molar mass organosiloxane liquid crystal materials classed as mono-mesogens (AB type) and bi-mesogen (ABA type) is presented.
Abstract: The present paper is a study of the ferroelectric and antiferroelectric behaviour in low molar mass organosiloxane liquid crystal materials classed as mono-mesogens (AB type) and bi-mesogens (ABA type). A systematic study of series of materials all based on the same chiral mesogenic moiety is presented. The mesogen is a biphenylyl benzoate with a halogen X attached laterally to the benzoate ring which is closest to the chiral centre. Three homologous series, where the halogen is either fluorine, chlorine or bromine, are investigated. The number of silicon atoms in the linkage or end group is varied between two and nine. It is found that the materials have a broad (40 C) ferroelectric or antiferroelectric phase with a high P s (90nC cm2). The siloxane moiety forces the tilt angle to a temperature independent value close to 45. The antiferroelectric order is observed only in bi-mesogens with an odd number of silicon atoms in the siloxane link. The antiferroelectric order is attributed to the conformation of...
89 citations
TL;DR: In this article, a symmetric low-molecular-mass liquid crystal based on the oxadiazole ring was synthesized and characterized, and the symmetric position of the heterocyclic group in the centre of the liquid crystal, bis(4-hexyloxyphenyl)4,4-(1,3,4-oxadiazoles-2,5-diyl)dicarboxylate, yields a very rigid and non-linear mesogenic core.
Abstract: A symmetric, low-molecular-mass liquid crystal based on the oxadiazole ring was synthesized and characterized. The symmetric position of the heterocyclic group in the centre of the liquid crystal, bis(4-hexyloxyphenyl)4,4-(1,3,4-oxadiazole-2,5-diyl)dicarboxylate, yields a very rigid and non-linear mesogenic core. Despite this non-linear structure, a broad liquid crystalline range with a smectic C and a smectic A phase was found. Conoscopic experiments on freely suspended films revealed the existence of two optical axes in the smectic A phase, indicating a phase symmetry anticipated for either a 'McMillan' biaxial smectic phase or a biaxial (achiral) ferroelectric smectic phase.
87 citations
TL;DR: In this article, the mesomorphic properties of a variety of bio-materials are discussed, indicating that such structures have properties that are dependent on liquid crystallinity for their behavior.
Abstract: The molecules of living systems invariably exhibit both thermotropic and/or lyotropic liquid crystalline properties. In some cases the mesophases formed by bio-materials have lamellar structures, whereas in other situations they form columnar phases. Many liquid crystal biomaterials are found in cell membranes, indicating that such structures have properties that are dependent on liquid crystallinity for their behaviour of a variety of bio-materials is discussed. function. In this article the mesomorphic
80 citations
TL;DR: In this paper, photoisomerizable chiral dopants have been studied, including menthone derivatives, a chiral stilbene derivative and a nematic copolymer.
Abstract: New photoisomerizable chiral dopants have been studied. The dopants used were menthone derivatives, a chiral stilbene derivative and a nematic copolymer of a menthone derivative and a benzoyloxybenzonitrile derivative. NMR, HPLC and UV results showed that the E-Z-isomerization of all the compounds indicated proceeded rapidly upon UV exposure, without the formation of undesired by-products. Isomerization of the menthone derivatives, including the copolymer, induced a substantial decrease in the helical twisting power. The changes in helical twisting power induced by the isomerization of the chiral stilbene derivative were limited to a factor of 2. In mixtures of the photoisomerizable dopants with commercial dopants of opposite twisting senses and a nematic host mixture, the sign of the twisting sense could be reversed by illuminating the mixture with UV light. The viewing angle dependence of irradiated regions of a 90 -twisted nematic cell was rotated 90 with respect to the viewing angle dependence of the ...
77 citations
TL;DR: In this paper, a statistical approach to the image data observed by polarized light microscopy (orthoscopic mode) was developed in order to detect with high sensitivity any structural change in the image frame.
Abstract: A study of textural changes is presented concerning the nematic phase of 4- n-alkyloxybenzoic acids (in particular, 4-n-heptyl- and 4-n-octyl-oxybenzoic acids), by means of a statistical approach to the image data observed by polarized light microscopy (orthoscopic mode). A new image processing method is developed in order to detect with high sensitivity any structural change in the image frame. To do this, a set of parameters is introduced, charaterizing the observed textures. Such a set is a vector, working like a pathfinder strongly increasing the human eye or in general the sensor skilfulness to appreciate any change of the optical texture, both in space and in time. This is suitable for revealing smooth transitions, such as phase transitions between smectic and nematic phases (or between different smectic phases), or order transitions, like alignment transitions in poorly oriented nematic layers. In fact, by using this method for detecting the order transition between two nematic 'subphases' of 4-n-a...
75 citations
TL;DR: In this paper, the effect of curing temperature on the electro-optic behavior and network morphology of reverse mode polymer stabilized cholesteric textures (PSCTs) has been studied.
Abstract: The effect of curing temperature on the electro-optic behaviour and network morphology of reverse mode polymer stabilized cholesteric textures (PSCTs) has been studied. Scanning electron micrographs indicate that increasing the curing temperature generally results in a larger average void size within a polymer network, while the morphology of the individual strands (which varies significantly for different monomers) remains largely unchanged. PSCTs with sufficiently large network voids exhibit a two-stage switching behaviour consistent with a model in which the cholesteric liquid crystal is divided between two distinct environments one in which the liquid crystal is strongly dominated by the polymer network, the other in which a bulk-like behaviour, comparable to the unstabilized cholesteric material, is observed.
74 citations
TL;DR: In this article, the second harmonic (SHG) of an optical wave was studied and the temperature and electric field dependence of the SH activity of the apparent thermodynamic phases of several homologues were studied and first quantitative values of the effective nonlinear optical coefficients were given.
Abstract: New liquid crystals consisting of non-chiral, banana shaped molecules are investigated by the generation of the second harmonic (SHG) of an optical wave. The temperature and electric field dependence of the SH activity of the apparent thermodynamic phases of several homologues are studied and first quantitative values of the effective non-linear optical coefficients are given.
63 citations
TL;DR: In this paper, a polymer network was incorporated into the ULH structure by means of photopolymerization of a reactive monomer added to the cholesteric, which still exhibits the flexoelectro-optic effect, it is stable and it is also retained after heating to the isotropic phase and going back to the Cholesteric phase.
Abstract: Flexoelectric coupling gives rise to a linear electro-optic response in cholesterics (flexoelectrooptic effect) with a uniformly lying helix (ULH) structure and this electro-optic effect is strongly reliant on the homogeneity and quality of the texture. The ULH structure, unfortunately, is complicated in itself and may be perturbed by factors such as dielectric coupling, surface/liquid crystal interactions and phase transitions, and often there is a tendency for relaxation into the Grandjean texture (standing helix structure) with time. Hence, in order to exploit the flexoelectro-optic effect in cholesterics any instability of the ULH structure must be ruled out. We have overcome these problems by incorporating a polymer network by means of photopolymerization of a reactive monomer added to the cholesteric. The volume stabilized ULH structure still exhibits the flexoelectro-optic effect, it is stable and it is also retained after heating to the isotropic phase and going back to the cholesteric phase. In a...
TL;DR: The mesomorphic behavior of a liquid crystalline sheet-shaped 2,4,6-triarylaminothe 1,3,5-triazine bearing six peripheral decyloxy chains has been investigated in binary mixtures with two-fold alkoxy substituted non-mesomorphic benzoic acids as mentioned in this paper.
Abstract: The mesomorphic behaviour of a liquid crystalline sheet-shaped 2,4,6-triarylaminothe 1,3,5-triazine bearing six peripheral decyloxy chains has been investigated in binary mixtures with two-fold alkoxy substituted non-mesomorphic benzoic acids. Each investigated equimolar mixture exhibits an enantiotropic columnar phase with either a hexagonal or a rectangular lattice. The two-dimensional lattice parameters depend on length and positions of the flexible alkoxy chains of the aromatic carboxylic acids. The structure formation of the mixed systems giving rise to variations of the mesophase structure of the pure triarylaminotriazine results from intermolecular hydrogen bonding between the complementary melamine and acid components.
TL;DR: In thin films of C liquid crystals, localized regions containing additional smectic layers form circular inclusions that carry a topological charge, such inclusions nucleate a companion topology.
Abstract: In thin films of smectic C liquid crystals, localized regions containing additional smectic layers form circular inclusions that carry a topological charge. Such inclusions nucleate a companion top...
TL;DR: In this paper, a group of 4-n-octyloxybenzoyloxy (noxoxyloxycarbonyl)ethenyl]benzoic acid 4-perfluoro-alkyloxyphenyl esters with varied lengths of the perfluorinated chains were investigated by X-ray measurements.
Abstract: A group of 4-[4-(4-n-octyloxybenzoyloxy)benzoyloxy]benzylidenemalonic acid bis(perfluoralkyl) esters and 4-{4-[2,2-bis(n-octyloxycarbonyl)ethenyl]benzoyloxy}benzoic acid 4-perfluoro- alkyloxyphenyl esters with varied lengths of the perfluorinated chains were investigated by X-ray measurements. The liquid crystalline behaviour (SmA, SmC, Col) and the structures of the phases are described. The substances show the influence of the steric interaction caused by the wedge shape of the molecules and the repulsion caused by chemically incompatible segments (fluorophobic effect). In the first group of compounds investigated the steric interaction clearly dominates the structure, so that an antiparallel packing of the molecules in SmA and SmC phases results. In the second group, the influence of the incompatibility is more and more enhanced and a low temperature phase with a columnar structure arises in which the incompatible parts are separated on a microscopic scale.
TL;DR: In this article, the effect of UV curing intensity and curing time on the electro-optic behavior and network morphology of reverse mode polymer stabilized cholesteric textures (PSCTs) has been studied.
Abstract: The effect of UV curing intensity and curing time on the electro-optic behaviour and network morphology of reverse mode polymer stabilized cholesteric textures (PSCTs) has been studied. Scanning electron micrographs indicate that increasing the curing intensity generally results in a more open polymer network characterized by a larger average void size, while the morphology of the individual strands remains largely unchanged. In addition, as the polymerization process proceeds, voids within the network are observed to decrease in size. PSCTs with sufficiently large network voids exhibit a two-stage switching behaviour consistent with a model in which the cholesteric liquid crystal is divided between two distinct environments one in which the liquid crystal is strongly dominated by the polymer network, the other in which a bulk-like behaviour, comparable to the unstabilized cholesteric material, is observed.
TL;DR: The reaction of 3,6,7,10,11,pentahexyloxy-2-hydroxytriphenylene with 2-(2-chloroethoxy)ethanol followed by acryloyl chloride or methacryl-closest to ours is as discussed by the authors.
Abstract: The ferric chloride mediated oxidative coupling of 3,3,4,4-tetrahexyloxybiphenyl with 2-hexyloxyanisole followed by demethylation gave 3,6,7,10,11-pentahexyloxy-2-hydroxyor triphenylene. Reaction with tert -butyldimethylsilyl-protected 6-bromohexanol, removal of the protecting group, and condensation with acryloyl chloride methacryloyl chloride gave acrylate and methacrylate monomers with hexamethylene 'spacers'. Alternatively, reaction of 3,6,7,10,11-pentahexyloxy-2-hydroxytriphenylene with 2-(2-chloroethoxy)ethanol followed by acryloyl chloride or methacryloyl chloride gave acrylate and methacrylate monomers with diethylenoxy 'spacers'. Both of the poly(acrylate) homopolymers and the poly(methacrylate) homopolymer with the diethylenoxy 'spacer' gave columnar liquid crystal phases. This is contrary to the current perception that the methacrylate polymer backbone is too inflexible to be incorporated in columnar phases.
TL;DR: In this paper, the phase behavior of binary systems ANBC(nC)-n-tetradecane, for nC = 8, 16 and 18, was examined using DSC, paying special attention to the role of the alkoxy chain of the ANBC molecule in the D phase.
Abstract: The phase behaviour of the binary systems ANBC(nC)-n-tetradecane, for nC = 8, 16 and 18, was examined using DSC, paying special attention to the role of the alkoxy chain of the ANBC molecule in the D phase. The dependence of the SmC-D and D-isotropic liquid transition temperatures upon the apparent average number of paraffinic carbon atoms closely resembles the nC dependence in the series of neat ANBCs, demonstrating that the alkoxy chain behaves, at least in part, like the solvent in lyotropic liquid crystals. The D phase was not detected in ANBC(8)-n-tetradecane.
TL;DR: A brief review of theoretical predictions and experimental observations concerning polar (pyroelectric, ferroelectric and antiferroelectric) achiral mesomorphic phases is given in this article.
Abstract: A brief review is presented of theoretical predictions and experimental observations concerning polar (pyroelectric, ferroelectric and antiferroelectric) achiral mesomorphic phases.
TL;DR: In this paper, a liquid crystalline 1,3,5-triazines are presented that incorporate three mesogenic groups based on rod-like azobenzene moieties.
Abstract: New liquid crystalline 1,3,5-triazines are presented that incorporate three mesogenic groups based on rod-like azobenzene moieties. The synthesis was carried out by reaction of cyanuric chloride with 4-alkoxy-4-aminoazobenzene derivatives. The mesomorphic behaviour was investigated by polarizing microscopy, differential scanning calorimetry and X-ray scattering. Besides a nematic and a SmA phase, the trisazomelamines form a higher ordered mesophase within a broad temperature range below the smectic A phase. With respect to the rod-like azobenzene sub-units, the low temperature phases display a smectic 'bilayer' structure. The molecules are antiparallel aligned within one layer. Furthermore, 1,3,5-triazines incorporating just two azobenzene groups are presented, these were prepared to investigate the influence of the number of rod-like sub-units linked to the triazine core on the mesomorphic properties.
TL;DR: In this paper, the nematic phase of 4,4-(p-terphenyl)-bis and 2,3,4-tri(dodecyloxy)benzalimine is found to be biaxial, the biaaxial angle increasing with schlieren textures exhibited by the nematic phase have been between glass plates, as well as free standing consisting entirely of s or 2-brush disclinations.
Abstract: Conoscopic studies reveal that the nematic phase of 4,4-(p-terphenyl)-bis\[2,3,4-tri(dodecyloxy)benzal]imine is biaxial, the biaxial angle increasing with schlieren textures exhibited by the nematic phase have been between glass plates, as well as free-standing consisting entirely of s or 2-brush disclinations, confirming an earlier suggestion that the absence of 4-brush disclinations is evidence of biaxiality decreasing temperature The examined Films sandwiched films, exhibit a nematic schlieren texture
TL;DR: In this article, three series of ferroelectric liquid crystals have been synthesized, having three ester groups in the mesogenic core and one ester group in the chiral chain.
Abstract: Three series of ferroelectric liquid crystals have been synthesized, having three ester groups in the mesogenic core and one ester group in the chiral chain. The ester groups introduced into the core decrease the temperature of crystallization, which results in a broad temperature range ferroelectric smectic C phase in homologues with a long non-chiral chain. These phases exhibit a high spontaneous polarization and relatively short pitch of the helical structure.
TL;DR: In this article, the phase behavior and optical properties of a photo-sensitive chiral dopant with left-handed and righthanded cholesteric copolymers were studied.
Abstract: Mixtures of a photosensitive chiral dopant based on (—)-menthone with left-handed and righthanded cholesteric copolymers were prepared. The phase behaviour and optical properties of the mixtures prepared were studied. The action of UV radiation on planar oriented films of such systems was shown to induce dramatic changes in the maximum reflection wavelength as a result of E-Z isomerization of the dopant molecules. The kinetics of photoisomerization of such mixtures in solution and in the bulk were investigated at different temperatures. The above mixtures can be considered as promising and uprecedented materials for coloured data recording and storage.
TL;DR: In this paper, a chiral bifunctional H-bond acceptor and achiral trifunctionsal donor was synthesized for cholesteric networks, and the results suggest that supramolecular liquid crystalline networks with macroscopic helical structures have been formed by intermolecular hydrogen bonds.
Abstract: Supramolecular hydrogen-bonded networks have been prepared by self-assembly of a chiral bifunctional H-bond acceptor and achiral trifunctional H-bond donors. A stilbazole dimer, (R)-1,2-bis{4-\[2-(4-pyridyl)ethenyl]phenoxy}propane (1) has been synthesized for use as a chiral component for cholesteric networks. Compound 1 has been complexed with tri functional H-bond donors, p,q-bis{2-\[2-(4-carboxyphenoxy)ethoxy]ethoxy}benzoic(2-6) \[p, q =2,4 (2); 2,5 (3); 2,6 (4); 3,4 (5); 3,5 (6)], maintaining the 1 :1 donor/acceptor group stoichiometry. With the exception of the complex from 1 and 6, these H-bonded complexes exhibit cholesteric phases and glass transition behaviour, while all individual components are non-mesomorphic. For example, the H-bonded complex consisting of 1 and 2 shows a cholesteric phase from 75 to 184 C on heating. These results suggest that supramolecular liquid crystalline networks with macroscopic helical structures have been formed by intermolecular hydrogen bonds between non-mesomorph...
TL;DR: Anisotropic networks, elastomers and gels exhibit piezoelectric, pyroelectric, ferroelectric and NLO properties of potential interest for use communication and processing technologies as discussed by the authors.
Abstract: Anisotropic networks, elastomers and gels exhibit piezoelectric, pyroelectric, ferroelectric and NLO properties of potential interest for use communication and processing technologies. The formation, properties and applications of such anisotropic, mainly liquid crystalline, networks are described. If some of the molecules in a liquid mixture contain at least two reactive groups which can be either photochemically or thermally polymerized, then crosslinked, anisotropic networks, elastomers and gels can be produced. Solid macroscopically aligned elastomers or networks can be formed as required beforehand or simultaneously by orientation of the sample. Anisotropic gels consist of a solid anisotropic network and non-covalently bonded, but strongly oriented domains of low molar mass liquid crystals. Anisotropic networks, elastomers preformed amorphous or liquid crystalline polymers incorporating additional reactive groups, which can be macroscopically oriented in the additional crosslinking reactions. Reversi...
TL;DR: In this article, the twisting ability of bridged binaphthol derivatives with substituents of various lengths and chemical nature in the 6,6-positions, recently synthesized and used as dopants in nematic solvents, is investigated with the help of the model based upon surface- helicity tensors.
Abstract: The twisting ability of a novel series of bridged binaphthol derivatives with substituents of various lengths and chemical nature in the 6,6-positions, recently synthesized and used as dopants in nematic solvents, is investigated with the help of the model based upon surface- helicity tensors. Structures of the low energy conformers of these compounds have been generated by molecular mechanics calculations. Their orientational behaviour and the coupling between anisotropy in the alignment and molecular chirality, which are at the origin of the helical twisting power, are analysed on the basis of the anisometry and the chirality of the shape.
TL;DR: The history of liquid crystals for display applications and recent progress in new liquid crystal structures is presented.
Abstract: Active Matrix (AM) LCDs and STN LCDs are the leading display technologies for portable applications such as notebook computers. Recently, new LCD device configurations and new LCD operation modes have been introduced. The existing technologies and the new LCD operation modes require improved liquid crystal materials. The history of liquid crystals for display applications and recent progress in new liquid crystal structures is presented.
TL;DR: In this article, the mesophase behavior of fiber-ring compounds was investigated and the constitution of the fiber ring compounds was varied, e.g. with the type and direction of connecting groups and, especially, the influence of substituents situated at different positions of the angled middle part.
Abstract: Banana-shaped molecules have been prepared and their mesophase behavior was investigated. The constitution of the fiber-ring compounds was varied, e.g. with the type and direction of connecting groups and, especially, the influence of substituents situated at different positions of the angled middle part. According to preliminary investigations by x-ray measurements, polarizing microscopy and mixing behavior several mesophases could be distinguished. Ferroelectricity could be detected only for one of the phases by electro-optical switching and pyroelectrical investigation.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
TL;DR: In this article, the cubic mesophases of a chiral and an achiral hydrazine derivative have been investigated by means of X-ray diffraction experiments, and it was shown that both cubic phases are incompletely miscible which can be explained by different space groups of the cubic lattices.
Abstract: The structures of the cubic mesophases of a chiral and an achiral hydrazine derivative have been investigated by means of X-ray diffraction experiments. As seen from the phase diagram of the binary system, both cubic phases are incompletely miscible which can be explained by the different space groups of the cubic lattices. In contrast to the isotropic liquid, the cubic phase of the chiral compound exhibits a considerable optical rotation indicating phase chirality.
TL;DR: In this article, a series of cholesteric copolymers containing combined chiral photochromic side groups has been synthesized and the phase behaviour and optical properties of the copolymer have been characterized.
Abstract: For the first time, a series of cholesteric copolymers containing combined chiral photochromic side groups has been synthesized and the phase behaviour and optical properties of the copolymers have been characterized. Specific features of the photochemical and photo-optical behaviour of such systems were studied, and the quantum yields of the photo-induced process in solution and in the condensed state of the cholesteric copolymers were calculated. The selective light reflection wavelength was found to be controlled by the UV radiation. The synthesized polymers were shown to be promising candidates for colour data recording.
TL;DR: In this article, the authors investigated the phase ordering kinetics of various liquid crystalline materials from well known series (PCH, CCH, nO. m, BCH and CB) when turning from a disordered to an ordered phase induced by a rapid temperature quench.
Abstract: The phase ordering kinetics of various liquid crystalline materials from well known series (PCH, CCH, nO. m, BCH and CB) when turning from a disordered to an ordered phase induced by a rapid temperature quench have been investigated. The process of phase ordering can generally be described by the growth of a characteristic length L. The exponents n of the theoretically predicted universal growth law L (t) tn have been determined. In this paper we present in particular the results for the kinetics of phase formation which takes place under the mechanism of nucleus growth. As characteristic length we chose the diameter of the nuclei. We obtained values for n between 0.5 and 1 dependent on the depth of the temperature quench. These results for mainly spherical shaped nematic and smectic germs are discussed in terms of the Allen-Cahn theory for the movement of an antiphase boundary (APB). As a lower limiting value, 0.5 confirms the classical theory, while 1 as an upper limiting value can be explained by the i...