Zirconia needles synthesized inside hexagonal swollen liquid crystals

We report the synthesis of zirconia microneedles by the direct nucleation of particles inside a hexagonal swollen liquid crystal (SLC) (cell parameter a = 27 nm) prepared by mixing with the proper ratio, an aqueous solution of sulfated zirconium colloids, a cationic surfactant (cetylpyridinium chloride), cychlohexane as swelling agent with an oil over water ratio of 2.5 (vol.), and 1-pentanol as cosurfactant. After a slow crystallogenesis that can be enhanced by an initial induction step under moderate temperature, particles in the centimeter range can be obtained, with a very high shape ratio (over 100). These particles are made of crystalline octahydrate zirconium oxychloride containing pores of 20 nm diameter, aligned along the main axis of the liquid crystal, as the fingerprint of the oil cylinders present in the hexagonal phase. The morphology of these particles confirms that the shaping mechanism is based on true liquid crystal templating (TLCT). Further thermal treatment of these particles, after extraction from the SLC, leads to the crystallization of zirconia with the same needlelike morphology as the zirconium oxychloride.

Highly swollen liquid crystals as new reactors for the synthesis of nanomaterials

Synthesis and self-assembly of nanomaterials can be controlled by the properties of soft matter. on one hand, dedicated nanoreactors such as reverse microemulsions or miniemulsions can be designed. on the other hand, direct shape control can be provided by the topology of liquid crystals that confine the reacting medium within a specific geometry. In the first case, the preparation of micro- or miniemulsions generally requires energetic mechanical stirring. The second approach uses thermodynamically stable systems, but it remains usually limited to binary (water + surfactant) systems. We report the preparation of different families of materials in highly ordered quaternary mediums that exhibit a liquid crystal structure with a high cell parameter. They were prepared with the proper ratios of salted water, nonpolar solvent, surfactant. and cosurfactants that form spontaneously swollen hexagonal phases. These swollen liquid crystals can be prepared from all classes of surfactants (cationic, anionic, and nonionic). They contain a regular network of parallel cylinders, whose diameters can be swollen with a nonpolar solvent, that are regularly spaced in a continuous aqueous salt solution. We demonstrate in the present report that both aqueous and organic phases can be used as nanoreactors for the preparation of materials. This property is illustrated by various examples such as the synthesis of platinum nanorods prepared in the aqueous phase or zirconia needles or the photo- or gamma-ray-induced polymerization of polydiacetylene in the organic phase. In all cases, materials can be easily extracted and their final shapes are directed by the structure-directing effect imposed by the liquid crystal.

Nonlinear dynamics of short traveling capillary-gravity waves

We establish a Green-Nagdhi model equation for capillary-gravity waves in (2+1) dimensions. Through the derivation of an asymptotic equation governing short-wave dynamics, we show that this system possesses (1 + 1) traveling-wave solutions for almost all the values of the Bond number θ (the special case θ=1/3 is not studied). These waves become singular when their amplitude is larger than a threshold value, related to the velocity of the wave. The limit angle at the crest is then calculated. The stability of a wave train is also studied via a Benjamin-Feir modulational analysis. ©2005 The American Physical Society.

Dispersed lipid liquid crystalline phases stabilized by a hydrophobically modified cellulose

Aqueous dispersions of monoolein (MO) with a commercial hydrophobically modified ethyl hydroxyethyl cellulose ether (HMEHEC) have been investigated with respect to the morphologies of the liquid crystalline nanoparticles. Only very low proportions of HMEHEC are accepted in the cubic and lamellar phases of the monoolein-water system. Due to the broad variation of composition and size of the commercial polymer, no other single-phase regions were found in the quasi-ternary system. Interactions of MO with different fractions of the HMEHEC sample induced the formation of lamellar and reversed hexagonal phases, identified from SAXD, polarization microscopy, and cryogenic TEM examinations. In excess water (more than 90 wt %) coarse dispersions are formed more or less spontaneously, containing particles of cubic phase from a size visible by the naked eye to small particles observed by cryoTEM. At high polymer/MO ratios, vesicles were frequently observed, often oligo-lamellar with inter-lamellar connections. After homogenization of the coarse dispersions in a microfluidizer, the large particles disappeared, apparently replaced by smaller cubic particles, often with vesicular attachments on the surfaces, and by vesicles or vesicular particles with a disordered interior. At the largest polymer contents no proper cubic particles were found directly after homogenization but mainly single-walled defected vesicles with a peculiar edgy appearance. During storage for 2 weeks, the dispersed particles changed toward more well-shaped cubic particles, even in dispersions with the highest polymer contents. In some of the samples with low polymer/MO ratio, dispersed particles of the reversed hexagonal type were found. A few of the homogenized samples were freeze-dried and rehydrated. Particles of essentially the same types, but with a less well-developed cubic character, were found after this treatment. © 2007 American Chemical Society.

C 3-symmetric discotic liquid crystalline materials for molecular electronics: versatile synthesis and self-organization

This thesis presents the versatile synthesis and self-organization of C3-symmetric discotic nanographene molecules as well as their potential applications as materials in molecular electronics. The details can be described as follows: 1) A novel synthetic strategy towards properly designed C3 symmetric 1,3,5-tris-2’arylbenzene precursors has been developed. After the final planarization by treatment with FeCl3 under mild conditions, for the first time, it became possible to access a variety of new C3-symmetric hexa-peri-hexabenzocoronenes (HBCs) and a series of triangle-shaped nanographenes. D3 symmetric HBC with three alkyl substituents and C2 symmetric HBC with two alkyl substituents were synthesized and found to show the surprising decrease of isotropic points., the self-assembly at the liquid-solid interface displayed a unique zigzag and flower patterns. 2) Triangle-shaped discotics revealed a unique self-assembly behavior in solution, solid state as well as at the solution-substrate interface. A mesophase stability over the broad temperature range with helical supramoelcular arrangement were observed in the bulk state. The honeycomb pattern as the result of novel self-assembly was presented. Triangle-shaped discotics with swallow alkyl tails were fabricated into photovoltaic devices, the supramolecular arrangement upon thermal treatment was found to play a key role in the improvement of solar efficiency. 3) A novel class of C3 symmetric HBCs with alternating polar/apolar substituents was synthesized. Their peculiar self-assembly in solution, in the bulk and on the surface were investigated by NMR techniques, X-ray diffraction as well as different electron microscope techniques. 4) A novel concept for manipulating the intracolumnar stacking of discotics and thus for controlling the helical pitch was presented. A unique staggered stacking in the column was achieved for the first time. Theoretical simulations confirmed this self-organization and predicted that this packing should show the highest charge carrier mobility for all discotics.

Photovernetzbare flüssigkristalline Polymere unterschiedlicher Kettentopologien

„Photovernetzbare flüssigkristalline Polymere unterschiedlicher Kettentopologien“, Patrick Beyer, Mainz 2007 Zusammenfassung In der vorliegenden Arbeit wurde die Synthese und Charakterisierung flüssigkristalliner Elastomere unterschiedlicher Polymertopologien vorgestellt. Dabei wurden Systeme synthetisiert, bei denen die mesogenen Einheiten entweder als Seitengruppen an ein Polymerrückgrat angebunden (Seitenkettenelastomere) oder direkt in die Polymerkette integriert (Hauptkettenelastomere) sind (siehe Abbildung). Bezüglich der Seitenkettensysteme konnten erstmals photovernetzbare smektische Seitenkettenpolymere, in denen aufgrund der Anknüpfung eines photoisomerisierbaren Azobenzols eine Photo- modulation der ferroelektrischen Eigenschaften möglich ist, dargestellt werden. Homöotrop orientierte freistehende Filme dieser Materialien konnten durch Spincoaten dargestellt und unter Ausnutzung des Dichroismus der Azobenzole durch geeignete Wahl der Bestrahlungsgeometrie photovernetzt werden. Aufbauend auf diesen Untersuchungen wurde anhand eines nicht vernetzbaren Modellsystems im Detail der Einfluss der trans-cis Isomerisierung des Azobenzols auf die ferroelektrischen Parameter untersucht. Durch zeitaufgelöste Messungen der Absorption der Azobenzole, der spontanen Polarisation und des Direktorneigungswinkels und Auswertung der kinetischen Prozesse konnte eine lineare Abhängigkeit der ferroelektrischen Eigenschaften vom Grad der Isomerisierungsreaktion festgestellt werden. Durch Vergleich dieser in der flüssigkristallinen Phase erhaltenen Ergebnisse mit der Kinetik der thermischen Reisomerisierung in Lösung (Toluol) konnte ferner eine deutliche Reduzierung der Relaxationszeiten in der anisotropen flüssigkristallinen Umgebung festgestellt und auf eine Absenkung der Aktivierungsenergie zurückgeführt werden. Makroskopische Formänderungen der Seitenkettenelastomere am Phasenübergang von der flüssigkristallinen in die isotrope Phase konnten jedoch nicht festgestellt werden. Aus diesem Grund wurden neue Synthesestrategien für die Darstellung von Hauptkettenelastomeren entwickelt, die sich aufgrund der direkten Kopplung von flüssigkristallinem Ordnungsgrad und Polymerkettenkonformation besser für die Herstellung thermischer Aktuatoren eignen. Auf Basis flüssigkristalliner Polymalonate konnten dabei lateral funktionalisierte smektische Hauptkettenpolymere synthetisiert werden, welche erstmals die Darstellung von LC-Hauptkettenelastomeren durch Photovernetzung in der flüssigkristallinen Phase erlauben. Durch laterale Bromierung konnte in diesen Systemen die Kristallisationstendenz der verwendeten Biphenyleinheiten unterdrückt werden. Bezüglich der Photovernetzung konnten zwei neue Synthesemethoden entwickelt werden, bei denen der Vernetzungsschritt entweder durch radikalische Polymerisation lateral angebundener Acrylatgruppen oder durch photoaktive Benzophenongruppen erfolgte. Basierend auf den Benzophenon funktionalisierten Systemen konnte ein neuartiges Verfahren zur Darstellung makroskopisch orientierter Hauptkettenelastomere durch Photovernetzung entwickelt werden. Die Elastomerproben, deren Ordnungsgrad durch Röntgenuntersuchungen ermittelt werden konnte, zeigen am Phasenübergang von der flüssigkristallinen in die isotrope Phase eine reversible Formänderung von 40%. Im Gegensatz zu anderen bekannten smektischen Systemen konnten die in dieser Arbeit vorgestellten Elastomere ohne Zerstörung der Phase bis zu 60% entlang der smektischen Schichtnormalen gestreckt werden, was im Kontext einer geringen Korrelation der smektischen Schichten in Hauptkettenelastomeren diskutiert wurde.

Molecular organization of n-cyanobiphenyl liquid crystals on a molybdenite surface

The alignement and anchoring of liquid crystals on solid surfaces is a key problem for modern device technology that until now has been treated empirically, but that can now be tackled by atomistic computer simulations. Molecular dynamics (MD) simulations were used in this thesis work to study two films of 7 and 8 n-alkyl-4’cyanobiphenyl (7CB and 8CB) liquid crystals , with a thickness of 15 nm, confined between two (001) surfaces of MoS2 (molybdenite). The isotropic and nematic phases of both liquid crystals were simulated, and the resulting structures characterized structurally. A new force field was designed to model the interactions between the liquid crystal (LC) molecules and the surface of molybdenite, while an accurate force field developed previously was used to model the 7CB and 8CB molecules. The results show that the (001) molybdenite surface induces a planar orientation in both the liquid crystals. For the nematic phase of 8CB, one of the two solid/LC interfaces is composed of a first layer of molecules aligned parallel to the surface, followed by a second layer of molecules aligned perpendicular to the surface (also called, homeotropic). The effect of the surface appears to be local in nature as it is confined to the first 15 Angström of the LC film. Conversely, for the nematic phase of 7CB, a planar ordering is established into the LC film. The LC molecules at the interface with the molybdenite appear to align preferentially their alkyl chains toward the solid substrate. The resulting tilt angle of molecules was found to be in good agreement with experimental measurements available in literature. Despite the fact that the MD simulations spanned a time range of more than 100 ns, the nematic phases of both 7CB and 8CB were found not to be completely formed. In order to confirm the findings presented in this thesis, we propose to extend the current study.

Modeling and simulations of nanoparticles in liquid crystalline systems

The aim of this work is to investigate, using extensive Monte Carlo computer simulations, composite materials consisting of liquid crystals doped with nanoparticles. These systems are currently of great interest as they offer the possibility of tuning the properties of liquid crystals used in displays and other devices as well as providing a way of obtaining regularly organized systems of nanoparticles exploiting the molecular organization of the liquid crystal medium. Surprisingly enough, there is however a lack of fundamental knowledge on the properties and phase behavior of these hybrid materials, making the route to their application an essentially empirical one. Here we wish to contribute to the much needed rationalization of these systems studying some basic effects induced by different nanoparticles on a liquid crystal host. We investigate in particular the effects of nanoparticle shape, size and polarity as well as of their affinity to the liquid crystal solvent on the stability of the system, monitoring phase transitions, order and molecular organizations. To do this we have proposed a coarse grained approach where nanoparticles are modelled as a suitably shaped (spherical, rod and disk like) collection of spherical Lennard-Jones beads, while the mesogens are represented with Gay-Berne particles. We find that the addition of apolar nanoparticles of different shape typically lowers the nematic–isotropic transition of a non-polar nematic, with the destabilization being greater for spherical nanoparticles. For polar mesogens we have studied the effect of solvent affinity of the nanoparticles showing that aggregation takes places for low solvation values. Interestingly, if the nanoparticles are polar the aggregates contribute to stabilizing the system, compensating the shape effect. We thus find the overall effects on stability to be a delicate balance of often contrasting contributions pointing to the relevance of simulations studies for understanding these complex systems.

Liquid crystalline cellulose derivatives for mirrorless lasing

In this thesis cholesteric films made of liquid crystalline cellulose derivatives with improved optical properties were prepared. The choice of the solvent, hydrogen bond influencing additives, the synthetic realization of a very high degree of substitution on the cellulosic polymer and the use of mechanical stirring at the upper concentration limit of the liquid crystalline range were the basis for an improved alignment of the applied cellulose tricarbamates. In combination with a tuned substrate treatment and film preparation method, cholesteric films were obtained, with optical properties that were theoretically predicted and only known from low molecular weight liquid crystals so far. Subsequent polymerization allowed a permanent fixing of the alignment and the fabrication of free standing and insensitive films.rnThe incorporation of inorganic nanorods into the cholesteric host material was mediated with tailored block copolymers, available via controlled radical polymerization methods. In addition to the shape match between the rodlike mesogens of the host and the nanorods it was possible to increase the miscibility of both materials. Nevertheless, the size of the nanorods, in comparison to the mesogens, in these densely packed liquid crystalline phases as well as their long equilibration times were the reasons for phase separation. Nanorods are, in principle, valuable substitutes for organics, but their utilization in cellulosic CLC was not to be combined with a high quality alignment of the cholesteric structure.rnA swelling process of polymerized films in a dye solution or dissolving dyes in non-polymerized CLC was used for incorporation of the organic chromophores. With the first method the CLC could be aligned and polymerized without any disturbance due to dye molecules. The optical properties of dye and CLC were matched, with regard to mirrorless lasing devices. The dye was optically excited and laser emission supported by the cholesteric cavity was obtained. The polarization and wavelength of the emitted radiation as well as its bandwidth, the obtained interference pattern and threshold behavior of the emission proofed the feedback mechanism that was not believed to be realizable in liquid crystalline polymers. rnUtilization of a microfluidic co-flow injection device enabled us to transfer the properties of cellulosic CLC from the planar film shape to spherical micrometer sized particles. The pure material yielded particles with distorted mesogen alignment similar to films prepared by capillary flow. Dilution of the CLC with a solvent that migrated into the carrier phase during particle preparation provided the basis for particles with well ordered areas. rnAlthough cellulose derivatives were known for their liquid crystalline behavior for decades and synthesized in mass production, their application as feedback material was affected by bad optical properties. In comparison to low molar mass compounds, the low degree of order in the CLC phase was the cause. With the improved material, defined lasing emission was shown and characterized. Derivatives of cellulose are desirable materials, because, as a renewable resource, they are available in large amounts for a low price and need only simple derivatization reactions. The fabrication of CLC films with tunable lasing emission, for which this thesis can provide a starting point, is in good agreement with today's requirements of modern technology and its miniaturization.rn

Security devices based on liquid crystals doped with a colour dye

Liquid crystal properties make them useful for the development of security devices in applications of authentication and detection of fakes. Induced orientation of liquid crystal molecules and birefringence are the two main properties used in security devices. Employing liquid crystal and dichroic colorants, we have developed devices that show, with the aid of a polarizer, multiple images on each side of the device. Rubbed polyimide is used as alignment layer on each substrate of the LC cell. By rubbing the polyimide in different directions in each substrate it is possible to create any kind of symbols, drawings or motifs with a greyscale; the more complex the created device is, the more difficult is to fake it. To identify the motifs it is necessary to use polarized light. Depending on whether the polarizer is located in front of the LC cell or behind it, different motifs from one or the other substrate are shown. The effect arises from the dopant colour dye added to the liquid crystal, the induced orientation and the twist structure. In practice, a grazing reflection on a dielectric surface is polarized enough to see the effect. Any LC flat panel display can obviously be used as backlight as well.

2D tunable graded index prism beam steering device based on nematic liquid crystals

Liquid crystal devices are being used in many non-display applications in order to construct small devices controlled by low voltage electronics without mechanical components. In this work, we present a novel liquid crystal device for laser beam steering. In this device the orientation of the liquid crystal molecules can be controlled. A change in the liquid crystal orientation results in a change of the refractive index. When a laser beam passes through the device, the beam will be deviated (Fig.1) and the device works a prism. The main difference between this device and a prism is that in the device the orientation profile of the liquid crystal molecules can be modified so that the laser beam can be deviated a required angle: the device is tuneable.

Magneto-Optical deflector with nematic liquid crystals

In this paper we report a new metod for optical switching based on the magneto-optical properties of liquid crystal materials. In order to improve previous response times, we used a wedge structure.

Digital light beam deflector with liquid cristal (1980)

In this letter , we report a new method for óptical switching based on the electro-optical properties of liquid crystal materials and, in particular, of the nematic type. The basis of this new method is the use of twisted wedge structure that has not been reported before elsewhere. In the past several years , great efforts in integrated optics have been made to develop optical switching devices with fast speed by using electro-optic, acousto-optic or magneto -optic materials. A mechanically operated óptical switch made of grade-index rod 1enses and e1ectromagnets has been proposed too . Switches of this kind include one input and two output waveguides and, depending on the app1ied voltage, one incident light on the switch exits either in one or another of the two output waveguides.

Wedged analog tunable beam steering device based on cholesteric liquid crystals

In this work we propose a novel cholesteric liquid crystal beam steering device based on the Kerr effect. The first version of the device consists of two ITO coated glass plates, with intentionally prepared electrodes, assembled together with a thickness gradient between both sides of the device. One side of the cell has two substrates at direct contact; the other side has separated substrates to form the wedge. The cell was filled with a cholesteric liquid crystal. The liquid crystal material is an innovative mixture called 1892E with extremely low viscosity doped with a ZLI chiral nematogen. The proposed beam steering device based on cholesteric liquid crystals has great potential for many photonic applications. Results describing the performance of the device and the properties of the selected liquid crystals are presented.

Dielectric and electro-optical properties of some cyanobiphenyl liquid-crystals

A variety of methods have been reviewed for obtaining parallel or perpendicular alignment in liquid-crystal cells. Some of these methods have been selected and developed and were used in polarised spectroscopy, dielectric and electro-optic studies. Also, novel dielectric and electro-optic cells were constructed for use over a range of temperature. Dielectric response of thin layers of E7 and E8 (eutectic mixture liquid-crystals) have been measured in the frequency range (12 Hz-100 kHz) and over a range of temperature (183-337K). Dielectric spectra were also obtained for supercooled E7 and E8 in the Hz and kHz range. When the measuring electric field was parallel to the nematic director, one loss peak (low-frequency relaxation process) was observed for E7 and for E8, that exhibits a Debye-type behaviour in the supercooled systems. When the measuring electric field was perpendicular to the nematic director, two resolved dielectric processes have been observed. The phase transitions, effective molecular polarisabilities, anisotropy of polarisabilities and order parameters of three liquid crystal homologs (5CB, 6CB, and 7CB), 60CB and three eutectic nematic mixtures E7, E8, and E607 were calculated using optical and density data measured at several temperatures. The order parameters calculated using the different methods of Vuks, Neugebauer, Saupe-Maier, and Palffy-Muhoray are nearly the same for the liquid crystals considered in the present study. Also, the interrelationship between density and refractive index and the molecular structure of these liquid crystals were established. Accurate dielectric and dipole results of a range of liquid-crystal forming molecules at several temperatures have reported. The role of the cyano-end group, biphenyl core, and flexible tail in molecular association, were investigated using the dielectric method for some molecules which have a structural relationship to the nematogens. Analysis of the dielectric data for solution of the liquid-crystals indicated a high molecular association, comparable to that observed in the nematic or isotropic phases. Electro-optic Kerr effect were investigated for some alkyl cyanobiphenyls, their nematic mixtures and the eutectic mixture liquid-crystals E7 and E8 in the isotropic phase and solution. The Kerr constant of these liquid crystals found to be very high at the nematic-isotropic transition temperatures as the molecules are expected to be highly ordered close to phase transition temperatures. Dynamic Kerr effect behaviour and transient molecular reorientation were also observed in thin layers of some alkyl cyanobiphenyls. Dichroic ratio R and order parameters of solutions containing some azo and anthraquinone dyes in the nematic solvent (E7 and E8), were investigated by the measurement of the intensity of the absorption bands in the visible region of parallel aligned samples. The effective factors on the dichroic ratio of the dyes dissolved in the nematic solvents were determined and discussed.