A number of compounds were synthesized with the ultimate goal being the synthesis of C₂ symmetric molecules which displayed thermotropic liquid crystalline behaviour. The compounds prepared were 4-alkoxy benzophenones, 3,4-bis-alkoxy benzophenones, 4- alkoxy dibenzylidene acetones, 3,4-bis-alkoxy dibenzylidene acetones and 4-alkoxy- 1, 9-diphenyl-nona-l,3,6,8-tetraen-5-ones. The length of the linear alkoxy side chain was varied from C₆H₁₃ to C₁₂H₂₅. All compounds were characterized by FTIR, ¹H, and ¹³C NMR spectroscopy. Mesophase behaviour of the synthesized compounds was investigated using differential scanning calorimetry and polarizing optical microscopy. It was determined that both the alkoxy side chain length, as well as the number of alkoxy side chains have an effect on the ability of this class of C₂ symmetric compounds to selfassemble into liquid crystalline phases. In addition, the overall core size and extent of conjugation also affected mesophase formation. The mono-alkoxy benzophenones and dibenzylidene acetones were non-mesogenic, while all four of the mono-alkoxy 1,9- diphenyl-nona-l,3,6,8-tetraen-5-ones (alkoxy side chain of lengths C₆H₁₃, C₈H₁₇, C₁₀H₂₁ and C₁₂H₂₅)self-assembled into nematic liquid crystalline phases. Increasing the number of alkoxy side chains from one to two per aromatic moiety helped induce liquid crystalline formation: the corresponding bis-C₆H₁₃ benzophenone and bis-C ₆H₁₃, bis C₈H₁₇, and bis-C₁₀H₂₁ dibenzylidene acetones were mesogenic, displaying smectic A (benzophenone) and nematic (dibenzylidene acetone) mesophases respectively. / Forestry, Faculty of / Graduate

No description available.

No description available.

No description available.

No description available.

Flexoelectricity in liquid crystals is thought to be due to a coupling between dielectric properties and shape anisotropy of the molecules and described by the fiexoelectric coefficients el and e3. Two experiments are needed to measure el and e3 and it is usual to measure the difference (el - e3) and the sum (el + e3) and then calculate el and e3· The first experiment to measure the difference (el - e3) uses a TN structure with an in-plane applied electric field. Due to the dielectric coupling, the director aligns with the electric field and due to the fiexoelectric effect, the director tilts out of plane. This tilt is measured optically using two laser beams at oblique incidence, e.g. 45°. Using a theoretical model the experimental data is fitted and the difference (el - e3) extracted. The second experiment to measure the sum (el + e3) uses a Pi cell. Applying an ac voltage the transmission through the device is a repeating oscillating signal which contains 1st and 2nd harmonics. The 1st harmonic corresponds to the fiexoelectric effect and the 2nd harmonic to the dielectric effect. Using a lock-in amplifier, the harmonics were measured and the sum (el + e3) extracted using a theoretical model to fit the experimental data. Unfortunately, the data proved the experiment to be unreliable and another method was developed, which uses a BAN cell. The third experiment uses simple pulses in a BAN cell and also measures the sum (el + e3). The big disadvantage of the BAN cell is an internal voltage, which is created by the homeotropic alignment layer and the fiexoelectric polarisation. The internal voltage has the same effect on the director profile as the fiexoelectric effect, which is a big problem in measuring fiexoelectricity. Using a material, which is non ionic and has no fiexoelectricity, the internal bias could be measured and taken into account. Applying short de pulses of opposite sign, the fiexoelectric effect can be observed by the optical response and can be measured. Using these experiments, a number of investigation are being carried out such as the correlation between fiexoelectricity and the molecular structure, ions, elastic properties, molecular orientation, dielectric anisotropy 6E, and order parameter S. The results showed that fiexoelectricity only depends on ions and dielectric properties which was very interesting and surprising at the same time.

Liquid crystals (LCs) are anisotropic fluids that exhibit numerous thermodynamically stable phases in between an isotropic liquid and a three-dimensionally ordered solid. In their simplest ordered phase, the nematic, LCs show orientational order due to molecular self assembly and at the same time maintaining fluid flow properties. In the smectic phase, they show both orientational and partial translational order characterized by a 1-d density wave. Liquid crystalline substances have been extensively studied due to their applications and as important physical models of self-assembly. The effect of the disorder and impurities on LC systems is an important and challenging problem to the fundamental understanding of phases ordering or self-assembly and continually attracts the attention of researchers. The disordered systems often display complex and rich phenomena, being the generalization of the pure (ideal) systems. Disorder can dramatically alter the physical properties of multi-component, composite systems. In particular, the effect of disorder on phase transitions is important as the disorder typically couples to the order parameter, which can be usefully described as a random local field that is conjugate to the order parameter. This is usually realized in systems with random inclusions in a phase ordering media, e.g., a colloidal dispersion of solids in a complex fluid. Another form of disorder is presented by dilution effects, which imposes instead the random breaking or weakening of intermolecular bonds or interactions responsible for the phase ordering. Exploring a good physical system representing random dilution effects in a controlled manner offers a physical probe to unresolved problems in the understanding of mesophasic order. This Dissertation presents a series of studies of dilution and different form of disorder effect on liquid crystal phase transitions. We have used high-resolution AC-calorimetry, dielectric spectroscopy as well as polarizing microscopy to characterize the effects of solvent such as hexane, acetone, decane, and nanomaterials such as multiwall carbon nanotubes and ferroelectric nanoparticles on the phase transitions of several liquid crystals. The liquid crystals of interest are: pentylcyanobiphenyl (5CB), octylcyanobiphenyl (8CB), and decylcyanobiphenyl (10CB). Studies have been carried out as a function of solvent, nanotube, and nanoparticles concentration and temperature spanning the isotropic to nematic (I-N), nematic to smectic-A (N-SmA), and isotropic to smectic-A (I-SmA) phase transitions.

Apresentamos um novo cristal liquido liotrópico, onde o álcool (utilizado nas misturas liotrópicas com fases nemáticas biaxial e uniaxiais), é substituído por um detergente. Essa nova mistura é composta por laurato de potássio, cloreto de decilamônia e água e apresenta fases nemáticas uniaxiais, calamitica e discótica, e biaxial. É apresentada uma superfície do diagrama de fases dessa mistura e são determinados alguns parâmetros microscópicos. Nesse estudo são utilizadas as técnicas de microscopia óptica de luz polarizada, conoscopia e difração de raios x. É feito um estudo comparativo da estabilidade química desse sistema, em relação à mistura com álcool. É feito um estudo sistemático das propriedades de ancoramento de cristais líquidos liotrópicos em superfícies de vidro (lisas e com ranhuras), utilizando a técnica de microscopia óptica de luz polarizada. Um novo fenômeno é observado, o deslizamento do diretor na superfície de contorno. Um modelo qualitativo, é proposto, baseado na existência de urna bicamada anfifílica, com defeitos, na interface. São determinados os tempo de orientação e tempo de relaxação para as amostras e os resultados são comparados às previsões do modelo proposto. Também é apresentado um método de tratamento de superfícies de vidro para orientação de cristais líquidos liotrópicos. / We present a new lyotropic liquid crystal, where the alcohol (used in lyotropic mixtures that present biaxial and uniaxial nematic phases) is substituted by a detergent. This new mixture is composed by potassium laurate, ammonium decylchloride and water and presents uniaxial and biaxial nematic phases. A surface of the phase diagram os this mixture is presented and some microscopical parameters are determined. In this investigation the techniques of polarised optical microscopy, conoscopy and X ray- diffraction are used. The chemical stability of this mixture is compared to the mixture with alcohol. The anchoring properties of lyotropic liquid crystals on glass surfaces (smooth and with grooves) are also investigated, using optical polarising microscopy. A new phenomen is observed, the gliding of the director at the boundary surfaces. A qualitative model is proposed, based on the existence of an anphiphilic bilayer with defects, at the interface. The orientation and relaxation times are determined and compared to the ones predicted by the model. It is also presented, a method for treating glass surfaces with ferrofluid that is applied to orient lyotropic liquid crystal samples.

Apresentamos um novo cristal liquido liotrópico, onde o álcool (utilizado nas misturas liotrópicas com fases nemáticas biaxial e uniaxiais), é substituído por um detergente. Essa nova mistura é composta por laurato de potássio, cloreto de decilamônia e água e apresenta fases nemáticas uniaxiais, calamitica e discótica, e biaxial. É apresentada uma superfície do diagrama de fases dessa mistura e são determinados alguns parâmetros microscópicos. Nesse estudo são utilizadas as técnicas de microscopia óptica de luz polarizada, conoscopia e difração de raios x. É feito um estudo comparativo da estabilidade química desse sistema, em relação à mistura com álcool. É feito um estudo sistemático das propriedades de ancoramento de cristais líquidos liotrópicos em superfícies de vidro (lisas e com ranhuras), utilizando a técnica de microscopia óptica de luz polarizada. Um novo fenômeno é observado, o deslizamento do diretor na superfície de contorno. Um modelo qualitativo, é proposto, baseado na existência de urna bicamada anfifílica, com defeitos, na interface. São determinados os tempo de orientação e tempo de relaxação para as amostras e os resultados são comparados às previsões do modelo proposto. Também é apresentado um método de tratamento de superfícies de vidro para orientação de cristais líquidos liotrópicos. / We present a new lyotropic liquid crystal, where the alcohol (used in lyotropic mixtures that present biaxial and uniaxial nematic phases) is substituted by a detergent. This new mixture is composed by potassium laurate, ammonium decylchloride and water and presents uniaxial and biaxial nematic phases. A surface of the phase diagram os this mixture is presented and some microscopical parameters are determined. In this investigation the techniques of polarised optical microscopy, conoscopy and X ray- diffraction are used. The chemical stability of this mixture is compared to the mixture with alcohol. The anchoring properties of lyotropic liquid crystals on glass surfaces (smooth and with grooves) are also investigated, using optical polarising microscopy. A new phenomen is observed, the gliding of the director at the boundary surfaces. A qualitative model is proposed, based on the existence of an anphiphilic bilayer with defects, at the interface. The orientation and relaxation times are determined and compared to the ones predicted by the model. It is also presented, a method for treating glass surfaces with ferrofluid that is applied to orient lyotropic liquid crystal samples.

Il est actuellement bien connu que les molécules non sphériques peuvent former des mésophases (ou cristaux liquides), c'est-à-dire des phases dont les propriétés sont intermédiaires entre celles des liquides et celles des cristaux. La mésophase la plus connue est la phase nématique. Il s'agit d'une phase caractérisée par une distribution aléatoire des centres de masse des molécules, mais dans laquelle l'orientation des molécules présente une direction préférentielle, désignée par un vecteur unité appelé le directeur du nématique. Une telle phase possède donc la fluidité d'un liquide tout en présentant, tel un cristal, une biréfringence. C'est cette dernière propriété qui est exploitée dans les applications technologiques, principalement dans les dispositifs d'affichage. Dans un tel dispositif, le liquide nématique est contenu dans une cellule (il y a une cellule par pixel), et son directeur est manipulé à l'aide d'un champ extérieur, électrique ou magnétique. Pour une bonne compréhension du fonctionnement de ce dispositif, il est essentiel de connaître le profil du directeur à travers la cellule en l'absence de champ extérieur. Dans le cadre de ce travail, nous avons étudié un nématique torsadé, c'est-à-dire dont le directeur décrit une hélice à travers la cellule. Ce profil est déterminé principalement par les propriétés d'ancrage du liquide nématique sur les parois solides de la cellule. En effet, celles-ci peuvent posséder une direction d'ancrage privilégiée, qui favorise l'alignement du directeur dans une direction particulière. Nous avons considéré ici le cas de directions d'ancrage planaires, c'est-à-dire que le directeur est dans le plan des parois. Alors que l'ajout de parois identiques dans le système induit toujours une non-uniformité spatiale dans la densité du nématique (en comparaison avec un nématique en coeur de phase), l'utilisation de directions d'ancrage différentes induit une non-uniformité orientationnelle dans le directeur du nématique; dans notre cas une torsion. C'est principalement ce profil de directeur torsadé qui nous intéresse ici. L'objectif général de ce travail consiste donc à étudier les propriétés d'ancrage d'une phase nématique confinée et torsadée, d'une part par une théorie microscopique (théorie de la fonctionnelle de la densité), et d'autre part sur le plan de simulations de Monte Carlo, en particulier dans le cas où les molécules ont la forme de disques (discotiques).