Studies of liquid crystal (LC) /Au nanoparticle (NP) composites have been pursued in columnar and in nematic phases of thermotropic LCs. Using LCs forming a columnar phase, we found that different functionalities on the corona of the Au NPs (hydrophobic vs. hydrophilic) display unique effects on the stability and ordering of the columnar LC phase. Doping nematic LCs with non-chiral or chiral Au NPs causes the formation of textures commonly observed for chiral nematic LCs, i.e., the formation of somewhat uniform stripe textures or patterns separated by areas of homeotropic alignment of LC molecules. Two scenarios are proposed. In the first scenario, the Au NPs form topological chain-like defects and the remaining Au NPs reside at the interface inducing vertical alignment of the LC molecules. In the second scenario, chiral Au NPs transfer chirality to the nematic LC host. Further, induced circular dichroism studies proved the second scenario. Using the same chiral Au NP systems, the origin of chirality of Au NPs has also been studied, and a powerful methodology has been proposed to unravel the puzzle of chirality of chiral ligand-protected Au NPs. Further investigations of these texture phenomena led to the discovery of using metal NPs to control the orientation and alignment of LCs. In due course, a dual alignment and electro-optical switching behaviour was found using alkylthiol-capped Au NPs doped into a nematic LC with positive dielectric anisotropy in planar namatic LC cells. This study was also expanded to Ag and CdTe NPs, which showed the same phenomenon, and all investigated NPs significantly reduced the voltage needed to re-orient the LCs in an electric field (threshold voltage). Starting from basic and moving on to more application-oriented research, we finally also initiated structure-property relationship studies of LC/NP composites.

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Concentrated solutions of hydroxypropylcellulose in aqueous and in polar organic solvents form lyotropic liquid crystalline phases. The critical concentration of hydroxypropylcellulose needed for anisotropic phase separation is dependent upon the solvent and is much greater than that expected for rod-like species in solution. The flexibility of the cellulosic chains in a particular solvent is believed to be the main factor governing the critical hydroxypropylcellulose volume fraction at phase separation. The anisotropic phase exhibits birefringence, iridescence, and very high optical activity, properties characteristic of cholesteric liquid crystals. The helicoidal pitch for most of these cholesteric samples in water and organic solvents varies from 280 to 700 nm, but in certain organic solvents the helicoidal pitch ranges up to 6000 nm. The latter samples exhibit fingerprint-like periodicity lines in the light microscope and distinctive "shimmering" colors. For both types of samples the helicoidal pitch is found to vary inversely with the third power of the hydroxypropylcellulose volume fraction. A model is proposed for the cholesteric structure in which the average separation, d, between the chain molecules varies with (phi)(,2)('- 1/2), where (phi)(,2) is the polymer volume fraction. The experimental x-ray diffraction data are in accord with this relationship. The angular twist between the molecules in adjacent cholesteric layers is calculated to vary from 0.30 to 1.8(DEGREES) over the mesophase concentration range investigated. / Light scattering measurements give twice the weight average molar mass for hydroxypropylcellulose as do sedimentation equilibrium measurements. Viscosity measurements in organic solvents show that the hydroxypropylcellulose conformation in dilute solution is neither that of a random coil nor that of a rigid rod. All aqueous hydroxypropylcellulose solutions show a lower consolute temperature that varies more with the sample molar substitution than with its molar mass. A qualitative "phase" diagram for aqueous hydroxypropylcellulose solutions is also included.

This thesis investigates the applications of liquid crystals in three areas of aerodynamic testing, namely temperature, shear stress and pressure measurement over surfaces. The use of the selective reflection colours from encapsulated chiral nematic materials to map surface temperatures is assessed. This method is very successful in a wide range of applications for determining heat transfer rates, but has limitations where high heat transfer rates are present, due to the thermal response time of the material structure, and the effects of temperature gradients. The thermal time constant is determined as a function of material viscosity. It is typically 10ms for a chiral nematic material at room temperature. The effect of a temperature gradient on the selective reflection is studied in terms of the pitch gradient produced in the material structure. Two improved methods are then proposed. The first makes use of the cholesteric to isotropic transition to indicate an isotherm on a heated surface, while the second uses changes in the birefringence colours produced by an aligned nematic layer applied to a surface and viewed between crossed poiarisers. Changes in the selective reflection colours from cholesteric liquid crystals can be used to assess surface shear stress levels. This method is assessed and determined to be too problematical for accurate measurements. A new method is proposed, based on the shear induced texture change from the focal-conic to the Grandjean texture in the cholesteric phase. This method may be used to quantify surface shear stress levels as well as to provide excellent flow visualisation once flow alignment has occurred. Tests have been conducted on the pressure sensitivity of chiral nematic liquid crystal materials. The results indicate that the change in transition temperature with pressure is of the order of 40°/kbar. which may not be sensitive enough for wind tunnel purposes.

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The copolymerisation of N-ethyl maleimide (NEM) with ethyl vinyl ether (EVE) in benzene (Bz) was investigated. The copolymerisation displays a tendency towards alternation but is not alternating and propagation involves the formation of a 1:1 NEM/EVE charge-transfer complex. This complex and the monomer/solvent complexes were investigated by ¹H NMR spectroscopic methods. The equilibria were combined mathematically to model the NEM/EVE/Bz system. The copolymerisations of functionalised N-substituted maleimides, N-4-[6-(4'-methoxy-4-azobenzoxy) hexoxycarbonyl] phenylmaleimide (NMeAz6PM) and N-4-[6-(4'-nitro-4-azobenzoxy)hexoxycarbonyl] phenylmaleimide (NNitAz6PM), with styrene (St), 4-vinyl pyridine (4VP), or EVE as the comonomer were studied and the reactivity ratios calculated. The monomer pairs NMeAz6PM/St, NMeAz6PM/4VP and NNitAz6PM/St produce alternating copolymers but NMeAz6PM/EVE does not. The absence of liquid crystallinity in these materials was attributed to the rigidity of the polymer backbone coupled with the low mesogen content arising from the alternating backbone. Blends of amorphous maleimide based copolymers with 6-(4-methoxy-4'-azobenzoxy)hexanoic acid, 4.2, were investigated. The copolymers chosen were believed to be alternating and are the product of the copolymerisation of N-phenyl, N-(4-methoxy) phenyl, or N-(4-hydroxy) phenyl maleimide with St or 4VP. 4.2 is essentially immiscible with the St based copolymers. However 4.2 is miscible with the 4VP based copolymers and liquid crystallinity is induced. This behaviour is attributed to the formation of hydrogen bonds between 4.2 and the pyridyl units of the copolymer backbone. Blends of 4.2 or 6-(4-nitro-4'azobenzoxy)hexanoic acid, 5.2, with copoly(NMeAz6PM-alt-St) or copoly(NMeAz6PM-alt-4VP) were investigated. The limited miscibility and liquid crystalline behaviour of the acid/styrene copolymer blends was attributed to interactions between the mesogens on the copolymer and the acid. However, the enhanced miscibility and liquid crystalline behaviour seen in the pyridine blends can be attributed to the formation of hydrogen bonds between the acids and the pyridyl units of the backbone, in addition to the acid/copolymer mesogen interactions.

Compounds which exhibit liquid crystal phases have been widely used in display technology. The majority of display applications utilize the nematic liquid crystal phase, which is a liquid-like phase which has partial orientational order at the molecular level. The nematic phase exhibits birifringence which can be manipulated through the application of an external field. Subsequently, all liquid crystal-based display technology utilizes the application of an external field to ???switch??? or tune the optical properties of a nematic domain into a desired optical state. In addition to an external field, the geometry and surface interactions of the liquid crystal domain must be precisely controlled in order for the display to operate properly. Liquid crystal displays (LCDs) utilize a rectangular domain, or pixel, within which the nematic domain is exposed to surface anchoring conditions that result in a twist of the nematic alignment through the thickness of the domain. In this work, a different type of liquid crystal domain that is elliptic is studied which is formed through ???bottom-up??? techniques, such as phase separation of a liquid crystal/polymer mixture to form a polymer-dispersed liquid crystal (PDLC) composite. Nematic domains within PDLCs are spheroidal, as opposed to rectangular for a pixel, and thus exhibit substantially different behaviour in the presence of an external field. The fundamental difference between spheroidal and rectangular nematic domains is that the former requires the presence of defects in nematic order while the latter does not. The overall objective of this work is to study, for a simplified elliptic cylinder domain, the formation of the nematic domain, the resulting domain texture in the presence of an external field, and the domain texture following release of the external field. These three states are directly related to applications of PDLC films as optical functional materials, where an external (electric) field is used to manipulate the optical properties of the film. The effects of geometry (aspect ratio), surface anchoring, and external field strength are studied through a simulation-based approach using the Landau-de Gennes theory of the nematic phase.