Dynamics of transient domain structures in nematic liquid crystals: The splay Fréedericksz transition

We discuss the dynamics of the transient pattern formation process corresponding to the splay Fréedericksz transition. The emergence and subsequent evolution of the spatial periodicity is here described in terms of the temporal dependence of the wave numbers corresponding to the maxima of the structure factor. Situations of perpendicular as well as oblique field-induced stripes relative to the initial orientation of the director are both examined with explicit indications of the time scales needed for their appearance and posterior development.

Studies of Liquid-liquid Separation in Fibre Coalescer

This thesis focuses on fibre coalescers whose efficiency is based on the surface properties/characteristics. They have the ability to preferentially wet or interact with one or more of the fluids to be separated. Thus, the interfacial phenomena governing the separation efficiency of the coalescers is investigated depending on physical factors such as flowrates, phase ratios and coalescer packing density. Design of process equipment to produce and separate of the emulsions was carried out.The experimentation was carried out to test the separation efficiency of the coalescing medias, namely fibreglass, polyester I and polyester II. The performances of the coalescing medias were assessed via droplet size information. In conclusion, the objectives (design of process equipment and experimentation) were achieved. Fibre glass was the best coalescing media, next was polyester I and then finally polyester II. Droplets sizes increased with decreased flowrates and increased packing density of the coalescer. Phase ratio had effect on the droplet sizes of the feed but had no effect on the coalescence of droplets of the feed.

Desenvolvimento de sistemas líquido-cristalinos empregando silicone fluido de co-polímero glicol e poliéter funcional siloxano

For the construction of the phase diagrams, the method of the aqueous titration was used. There were prepared 5 ternary diagrams, varying the surfactant and the oil phase. The liquid-crystalline phases were identified by polarized light microscopy. The formulations prepared with silicon glycol copolymer, polyether functional siloxane (PFS) and water (S1) and with diisopropyl adipate, PFS and water (S4) presented liquid-crystalline phases with lamellar arrangement. Moreover, after 15 days in hot oven (37 ºC), the formulations presented hexagonal arrangement, evidencing the influence of the temperature in the organization of the system.

Propriétés supramoléculaires des cations diimidazolium disubstitués : des complexes d’inclusion en solution aux interactions à l’état cristallin et cristal liquide

Les sels d’imidazolium ont un rôle important dans certaines protéines et acides nucléiques et ont été utilisés à de nombreuses reprises dans des assemblages supramoléculaires en raison de leurs propriétés uniques. Les sels de diimidazolium dérivés sont toutefois moins connus. Ils ont pour l’instant uniquement été utilisés comme des précurseurs de carbènes N-hétérocycliques. Ils sont donc à la base de plusieurs catalyseurs utilisés pour des réactions de couplage croisés mais leurs propriétés sont toutefois méconnues dans le cadre de la chimie supramoléculaire. Cette classe de composés a nottament attiré notre attention en raison de la facilité de modification de leurs propriétés physico-chimiques par modification de leur structure chimique. L’objectif général des travaux présentés dans cette thèse est l’étude des propriétés supramoléculaires des sels de diimidazolium disubstitués en solution (aqueuse ou organique), ainsi qu’en phase solide ou cristal-liquide. L’influence de l’espaceur entre les deux noyaux imidazolium ainsi que l’influence des substituants latéraux et des contre-ions a été étudiée. Dans un premier temps, les propriétés de complexation des sels de diimidazolium à des macrocycles sont étudiées. Les sels bromure sont étudiés en solution aqueuse avec plusieurs cyclodextrines et le cucurbit[7]uril, et les sels hexafluorophosphate sont étudiés en solution organique pour leur complexation avec l’éther couronne DB24C8 et un calix[4]arène. Cette nouvelle classe de composés a montré de très bonnes propriétés de complexation à ces différents macrocycles en solution et a également permis de contrôler différents assemblages supramoléculaires à l’interface air-eau. Dans un deuxième temps, l’étude des sels de phénylènediimidazolium a permis de modifier les propriétés de complexation en solution pour obtenir la formation de complexes multiples avec le cucurbit[7]util en solution aqueuse. Cette même famille de composés a également permis la formation de cristaux liquides ioniques lorsque les substituants sont des chaînes alkyles plus longues. La résolution de plusieurs structures cristallines de différents sels de diimidazolium a finalement permis de comprendre la nature des interactions intermoléculaires à l’état cristallin. La recherche présentée dans cette thèse a donc permis une étude détaillée des propriétés supramoléculaires des sels de diimidazolium dans tous les états de la matière qui leur sont accessibles.

Nanopatterning and functionalization of phospholipid-based Langmuir-Blodgett and Langmuir-Schaefer films

Durant les dernières décennies, la technique Langmuir-Blodgett (LB) s’est beaucoup développée dans l’approche « bottom-up » pour la création de couches ultra minces nanostructurées. Des patrons constitués de stries parallèles d’environ 100 à 200 nm de largeur ont été générés avec la technique de déposition LB de monocouches mixtes de 1,2-dilauroyl-sn-glycéro-3-phosphatidylcholine (DLPC) et de 1,2-dipalmitoyl-sn-glycéro-3-phosphatidylcholine (DPPC) sur des substrats de silicium et de mica. Afin d’amplifier la fonctionnalité de ces patrons, la 1-palmitoyl-2-(16-(S-methyldithio)hexadécanoyl)-sn-glycéro-3-phosphatidylcholine (DSDPPC) et la 1-lauroyl-2-(12-(S-methyldithio)dodédecanoyl)-sn-glycéro-3-phosphatidylcholine (DSDLPC) ont été employées pour la préparation de monocouches chimiquement hétérogènes. Ces analogues de phospholipide possèdent un groupement fonctionnel méthyldisulfide qui est attaché à la fin de l’une des chaînes alkyles. Une étude exhaustive sur la structure de la phase des monocouches Langmuir, Langmuir-Schaefer (LS) et LB de la DSDPPC et de la DSDLPC et leurs différents mélanges avec la DPPC ou la DLPC est présentée dans cette thèse. Tout d’abord, un contrôle limité de la périodicité et de la taille des motifs des stries parallèles de DPPC/DLPC a été obtenu en variant la composition lipidique, la pression de surface et la vitesse de déposition. Dans un mélange binaire de fraction molaire plus grande de lipide condensé que de lipide étendu, une vitesse de déposition plus lente et une plus basse pression de surface ont généré des stries plus continues et larges. L’addition d’un tensioactif, le cholestérol, au mélange binaire équimolaire de la DPPC/DLPC a permis la formation de stries parallèles à de plus hautes pressions de surface. La caractérisation des propriétés physiques des analogues de phospholipides a été nécessaire. La température de transition de phase de la DSDPPC de 44.5 ± 1.5 °C comparativement à 41.5 ± 0.3 °C pour la DPPC. L’isotherme de la DSDPPC est semblable à celui de la DPPC. La monocouche subit une transition de phase liquide-étendue-à-condensée (LE-C) à une pression de surface légèrement supérieure à celle de la DPPC (6 mN m-1 vs. 4 mN m-1) Tout comme la DLPC, la DSDLPC demeure dans la phase LE jusqu’à la rupture de la monocouche. Ces analogues de phospholipide existent dans un état plus étendu tout au long de la compression de la monocouche et montrent des pressions de surface de rupture plus basses que les phospholipides non-modifiés. La morphologie des domaines de monocouches Langmuir de la DPPC et de la DSDPPC à l’interface eau/air a été comparée par la microscopie à angle de Brewster (BAM). La DPPC forme une monocouche homogène à une pression de surface (π) > 10 mN/m, alors que des domaines en forme de fleurs sont formés dans la monocouche de DSDPPC jusqu’à une π ~ 30 mN m-1. La caractérisation de monocouches sur substrat solide a permis de démontrer que le patron de stries parallèles préalablement obtenu avec la DPPC/DLPC était reproduit en utilisant des mélanges de la DSDPPC/DLPC ou de la DPPC/DSDLPC donnant ainsi lieu à des patrons chimiquement hétérogènes. En général, pour obtenir le même état de phase que la DPPC, la monocouche de DSDPPC doit être comprimée à de plus hautes pressions de surface. Le groupement disulfide de ces analogues de phospholipide a été exploité, afin de (i) former des monocouches auto-assemblées sur l’or et de (ii) démontrer la métallisation sélective des terminaisons fonctionnalisées des stries. La spectrométrie de photoélectrons induits par rayons X (XPS) a confirmé que la monocouche modifiée réagit avec la vapeur d’or pour former des thiolates d’or. L’adsorption de l’Au, de l’Ag et du Cu thermiquement évaporé démontre une adsorption préférentielle de la vapeur de métal sur la phase fonctionnalisée de disulfide seulement à des recouvrements sub-monocouche.

The effect of the molecular weight on the electro-optic properties of methacrylate-based side-chain liquid crystal polymers

A series of methacrylate-based side-chain liquid crystal polymers has been prepared with a range of molecular weights. For the high molecular weight polymers a smectic phase is observed with a very narrow nematic range; however, for low molecular weight polymers only the nematic phase is observed. A marked reduction in the glass transition temperature, TSN and TNI is observed with a reduction in the molecular weight. The orientational order parameters for these polymers in the liquid crystal phase have been determined using infra-red dichroism. It is found that the higher the molecular weight of the polymer, the greater is the threshold voltage of the electro-optic response and the lower the order parameter. The increase in the threshold voltage with increasing molecular weight may be related to the intrinsic curvature elasticity and hence to the coupling between the mesogenic units and the polymer backbone.

A novel study of the polymerisation process involved in the formation of the network component of polymer-stabilised liquid crystals within electro-optic cells using high performance liquid chromatography

Polymer-stabilised liquid crystals are systems in which a small amount of monomer is dissolved within a liquid crystalline host, and then polymerised in situ to produce a network. The progress of the polymerisation, performed within electro-optic cells, was studied by establishing an analytical method novel to these systems. Samples were prepared by photopolymerisation of the monomer under well-defined reaction conditions; subsequent immersion in acetone caused the host and any unreacted monomer to dissolve. High performance liquid chromatography was used to separate and detect the various solutes in the resulting solutions, enabling the amount of unreacted monomer for a given set of conditions to be quantified. Longer irradiations cause a decrease in the proportion of unreacted monomer since more network is formed, while a more uniform LC director alignment (achieved by decreasing the sample thickness) or a higher level of order (achieved by decreasing the polymerisation temperature) promotes faster reactions.

Crystallization of random aromatic copolyesters containing flexible spacer chains and side-groups

The crystallization behaviour of a series of random copolymers of varying chemical composition is reported. For polymers containing a high proportion of alternating rigid aromatic units and flexible spacers, conventional liquid crystalline and crystalline phase behaviour is observed. The introduction of a substantial fraction of a second shorter rigid unit containing side-chains leads to a broad endotherm in the d.s.c. scan covering some 150°C. Subsequent isothermal crystallization at any point within the broad endotherm leads to the generation of sharp endotherms at temperatures just above the recrystallization temperature. We attribute this behaviour to the crystallization of clusters of molecules containing similar random sequences. Such crystals are non-periodic along the chain direction.

Memory effects in liquid crystal elastomers

Free-standing monodomain liquid crystal elastomer samples are shown to have a complete memory of the orientational configuration at the time of cross-linking. This memory is demonstrated through samples in which the parent polymer system is first aligned in a magnetic field prior to cross-linking. These films show reversible nematic-isotropic phase transitions and x-ray scattering patterns characteristic of nematic phases. The liquid crystal elastomer films exhibit a remarkable memory effect, in that the sample may be held at temperatures well above the nematic-isotropic transition for extended periods ( > 2 weeks), but on cooling into the liquid crystal phase region, both the original director alignment and the degree of preferred orientation are recovered. It is demonstrated that these novel memory effects are equilibrium in nature. The origins of this phenomena in terms of coupling between the mesogenic side-chains and the polymer network are discussed.

Conformational analysis of oriented non-crystalline polymers using wide angle X-ray scattering

A procedure is presented for obtaining conformational parameters from oriented but non-crystalline polymers. This is achieved by comparison of the experimental wide angle X-ray scattering with that calculated from models but in such a way that foreknowledge of the orientation distribution function is not required. X-ray scattering intensity values for glassy isotactic poly(methylmethacrylate) are analysed by these techniques. The method could be usefully applied to other oriented molecular systems such as liquid crystalline materials.

2D hexagonal mesoporous platinum films exhibiting biaxial, in-plane pore alignment

The synthesis of 2D hexagonal mesoporous platinum films with biaxial, in-plane pore alignment is demonstrated by electrodeposition through an aligned lyotropic liquid crystal templating phase. Shear force is used to align a hexagonal lyotropic liquid crystalline templating phase of an inexpensive and a commercially available surfactant, C16EO10, at the surface of an electrode. Electrodeposition and subsequent characterisation of the films produced shows that the orientation and alignment of the phase is transferred to the deposited material. Transmission electron microscopy confirms the expected nanostructure of the films, whilst transmission and grazing incidence small angle X-ray scattering analysis confirms biaxial, in plane alignment of the pore structure. In addition further electrochemical studies in dilute sulfuric acid and methanol show that the pores are accessible to electrolyte solution as indicated by a large current flow; the modified electrode therefore has a high surface area, that catalyses methanol oxidation, and the pores have a very large aspect ratio (of theoretical maximum 2 × 105). Films with such aligned mesoporosity will advance the field of nanotechnology where the control of pore structure is paramount. The method reported is sufficiently generic to be used to control the structure and order of many materials, thus increasing the potential for the development of a wide range of novel electronic and optical devices.

Chapter 17: X-Ray rheology of liquid crystal polymers

X-ray Rheology is an experimental technique which uses time-ressolved x-ray scattering as probe of the molecular level structural reorganisation which accompanies flow. It provides quantitative information on the direction alignment and on the level of global orientation. This information is very helpful in interpreting the classic rheological data on liquid crystal polymers. In this research we use data obtained from a cellulose derivate which exhibits a thermotropic liquid crystal phase. We show how increased shear rates lead to a rapid rise in the global orientation and we related this to therories of flow in liquid crystal polymers from the literature. We show that the relaxation time is independent of the prior shear rate.

Phospholipid-based microemulsions suitable for use in foods

The preparation of nonaqueous microemulsions using food-acceptable components is reported. The effect of oil on the formation of microemulsions stabilized by lecithin (Epikuron 200) and containing propylene glycol as immiscible solvent was investigated. When the triglycerides were used as oil, three types of phase behavior were noted, namely, a two-phase cloudy region (occurring at low lecithin concentrations), a liquid crystalline (LC) phase (occurring at high surfactant and low oil concentrations), and a clear monophasic microemulsion region. The extent of this clear one-phase region was found to be dependent upon the molecular volume of the oil being solubilized. Large molecular volume oils, such as soybean and sunflower oils, produced a small microemulsion region, whereas the smallest molecular volume triglyceride, tributyrin, produced a large, clear monophasic region. Use of the ethyl ester, ethyl oleate, as oil produced a clear, monophasic region of a size comparable to that seen with tributyrin. Substitution of some of the propylene glycol with water greatly reduced the extent of the clear one-phase region and increased the extent of the liquid crystalline region. In contrast, ethanol enhanced the clear, monophasic region by decreasing the LC phase. Replacement of some of the lecithin with the micelle-forming nonionic surfactant Tween 80 to produce mixed lecithin/Tween 80 mixtures of weight ratios (Km) 1:2 and 1:3 did not significantly alter the phase behavior, although there was a marginal increase in the area of the two-phase, cloudy region of the phase diagram. The use of the lower phosphatidylcholine content lecithin, Epikuron 170, in place of Epikuron 200 resulted in a reduction in the LC region for all of the systems investigated. In conclusion, these studies show that it is possible to prepare one-phase, clear lecithin-based microemulsions over a wide range of compositions using components that are food-acceptable.

Morphology of polymer networks formed in the chiral and non chiral phases of an anti-ferroelectric liquid crystal

We introduced photo-polymer networks into the various liquid crystalline phases of the antiferroelectric liquid crystal AS612 and studied the effects of these networks by measuring the temperature dependence of the Bragg wavelengths selectively reflected. After polymerization, the decrease in Bragg wavelengths with respect to the original values is consistent with a shorter helical pitch due to polymer network shrinkage. Also, by removing the liquid crystalline material, we are able to image the residual polymer network using scanning electron microscopy and polarized light microscopy. The polymer strands are a few microns thick and the networks show both chiral and non-chiral features.