No description available.

The guest-host [GH) effect of a tetracyanoquinodimethane (TCNQ) dye in nematic liquid crystals (LC) and polymer dispersed liquid crystals (PDLC) has been studied and compared with commercial azo and anthraquinone dyes. The adduct (MORPIP) is a conjugated molecule with a high dipole moment of 15 Debyes. E7 LC doped with MORPIP was found to give a dichroic ratio of 2.34 and an order parameter of 0.31; these values are lower than for the other dichroic dyes, typically 0.6 - 0.78. However, MORPIP showed a useful property When doped in LC and PDLC samples. The rise time of E7 was decreased by 21% when doped with 0.3 wt% MORPIP. It was suggested that MORPIP increased the dielectric anisotropy of E7 and hence reduced the rise time. Nevertheless, no change in dielectric anisotropy was observed. This observation favours a model in which the dipolar dopant reorients rapidly in the applied field and drives the reorientation of the surrounding LC The effects of the dyes incorporated in PDLGs were also studied, in order to explore their suitability for use in a colour reflective display. Dichroic PDLQ were made using the photo-initiated phase separation method. 0.37 wt7o MORPIP doped PDLCs showed a 45% faster response to an applied electric field than that of the undoped PDLCs. However, the contrast ratio of the dichroic PDLCs was not sufficiently high for display use due to insufficient scattering and dye trapping. Two new fabrication methods were developed to overcome this problem One method involved the preparation of a network type PDLC into which a doped LC was dispersed. Another method involved the preparation of voids by removing the LC from the PDLC In both methods, subsequent re-filling of the doped LC into the pre-made polymer matrix improved the contrast ratio and reduced the decay time of the dichroic PDLC

No description available.

Molecular simulation of macromolecular liquid crystal (LC) systems has so far been limited by a number of factors: the large size of the molecules themselves and the fact that mesophase formation takes place on length and timescales that are not reasonable to simulate. The work in this thesis develops three methods that can be used to assist in the computer simulation of macromolecular LC systems. Coarse-graining is a technique where instead of representing every atom within a molecule as a single site, a number of atoms are grouped into interaction centres. This coarse-graining procedure has been applied to a liquid crystal dendrimer to enable the bulk phase simulation of the molecule to be studied. The analysis of the results shows that the behaviour for the coarse-grained model closely matches that of a more detailed atomistic model. Phase behaviour in the bulk matches results from X-ray data. The parallel-tempering method (replica exchange method) uses a series of replicas of the same system at different temperatures to improve the sampling of phase space. This technique was applied to two different systems, a bulk phase simulation of an alkane chain and the gas phase simulation of a silsesquioxane liquid crystal dendrimer. The method was then extended to work with a set of replicas which used different potentials. The Tsallis potential was used to soften potentials and allow replicas to sample a greater area of phase space. A third simulation method was applied which used soft-core potentials. This attempted to address the problem of the long timescales needed to see the formation of mesophases in macromolecular systems. Three different anisotropic single site soft-core models were developed and tested for liquid crystals. The results show that the time needed for mesophase formation is reduced for soft-core models and that these models are able to form multiple liquid crystal phases. In addition, the most promising of these soft-core models has been applied to the simulation of more complex liquid crystal systems, represented by multi-site models.

The main subject of this thesis is structure determination of self-assembled wedge-shaped liquid crystal-forming molecules. The main part of the work involves 3,4,5-trialkoxygallate alkali metal salts. These salts, with two short and one long alkyl chains, are studied in search for close packed micellar phases, which have never been observed before in thermotropic liquid crystals. The molecular structures of these compounds were designed on the basis of the theoretical prediction of volume filling of Voronoi polyhedra of close-packed structures. The idea is that the longer alkyl chain in the molecule would help fill the octahedral interstices, furthest points from the centers of surrounding micelles, in a structure of close packed spheres. To fill the octahedral interstices even better, various proportions of n-paraffin CnH2n+2, where n= 15, 17, 19, are also added to these dendrons. For comparison, 3,4,5-tridodecyloxygallates are also studied with varying proportions of added C19H40. Small-angle X-ray diffraction (SAXD) experiments have been carried out to identify the phase structures at different temperatures. It is found that neither of the pure compounds yields a close-packed structure, and it is only in the mixtures of Na or Rb salts with added C19H40 which the hexagonal close-packed (HCP) phase is obtained. The first order phase transition from the hexagonal columnar phase to another hexagonal columnar phase is also observed in pure compounds of Na salts and their mixtures with added C19H40 whereas the hexagonal columnar superlattice is also obtained from the mixtures of Li salt and n-alkane CnH2n+2 (n=15, 17). The phase diagrams of the binary systems are constructed, to understand the principles of close-packed structure formation in self-assembled tapered mesogens and dendrimers. The second subject of this thesis is structure determination of wedge-shaped liquid crystal molecules of hybrid dendrons of methyl 3,4- -alkoxy biphenylmethyl-4-yloxy) benzoate, with different lengths of alkyl chains. The molecular structure of these dendrons is a combination of molecular structures of previously studied benzyl ether based dendrons and biphenyl methyl ether based dendrons. Using SAXD, it is found that these dendrons yield wiggly lamellar phases, as well as hexagonal columnar phases consisting of core-shell supramolecular columns. The core-shell structure is confirmed by the good results of the intensity fitting between experimental intensities and intensities calculated from geometric models, as well as by reconstructing electron density maps and performing molecular dynamics simulations.

Confined liquid crystals (LCs) have attracted much interest because their richness in physical phenomena and potential applications. In this thesis the configuration of novel liquid crystal phases confined in cylindrical cavities from tens of nanometres to hundred of microns primarily were investigated by means of small angle X-ray scattering (SAXS), grazing incident X-ray scattering (GSAXS) and atomic force microscopy (AFM). The arrangement of honeycomb phases and 3-d channeled-layer phases (ChL) formed by polyphilic side-branched compounds in nanoporous anodic aluminium oxide (AAO) templates as well as in glass capillaries with micrometer diameters were investigated. 3-d diffraction patterns were reconstructed from the SAXS data. A fracture method associated with AFM observation was performed on the AAO samples to complement the X-ray study. Details of assembly of individual columns inside the nanochannel were directly observed by AFM for the first time. Surprisingly, even the planar-anchored columns were found reluctant to orient axially (parallel to the 1-d channel), which was explained by the strong deformation energy of the 2-d lattice. Besides, the in-plane orientation of the 2-d lattice was observed in both planar and homeotropic anchoring conditions and different mechanisms were proposed. For the ChL phase, axial orientation was observed due to the high rigidity of the columns. The 3-d layered structure could be suppressed by the nanopores and an induced smectic structure was observed. Configuration of several discotic columnar LCs in cylindrical confinement was studied. The investigation of pure triphenylene derivatives in nanopores refutes the only detailed experimental work on discotics published as far. Again the discotic columns are mainly perpendicular to the channel axis, even with planar anchoring condition, to avoid the distortion of the 2-d lattice. The axial configuration was observed only when the rigidity of the columns was increased by dopant. The structure of LC-directed mesoporous silica nanofibers fabricated via dual structure-directing agents in 200 nm AAO membranes was characterized by SAXS and TEM. Different mesoporous structures were observed when changing the alkyl chain length of the cationic surfactant.

This thesis is concerned with the microemulsion phase behaviour of polymerizable surfactants with polymerizable oils and water, and the effect of various properties and characteristics of the equilibrium microemulsion phases and their components on the structure of solid polymers prepared by polymerisation of the liquid microemulsion phases. The polymerizable surfactants each contain a polymerizable methacrylate group in one of two possible locations. In two of the surfactants, the methacrylate group is located on the head group of the surfactant and on one the methacrylate group is located on the surfactant tail group. Three oils were used, each possessing an acrylate polymerizable group at either end of the oil molecule. The characterisation of polymerizable surfactants in aqueous/ polymerizable oil systems at low [surfactant] was observed initially. It was found that the solubility of the surfactants is similarly low in each of the three oils and each surfactant was found to be soluble in water. Stable o/w emulsions were prepared using each of the surfactants with no evidence of phase inversion with temperature observed in any system but phase inversion with electrolyte was observed in some systems. Salt was found to affect the partitioning of each surfactant between water and oil and C3/11-M surfactant was found to exhibit an electrolytic effect of its own. Winsor I microemulsion systems were prepared at low [salt] and Winsor II microemulsions were prepared at high [salt] for systems containing HDDA or DPGDA oils for each of the surfactants. The effects of each of the components present in each system on surfactant partitioning and cmc in water and oil and were observed. High surfactant concentration microemulsion phases were then prepared using each of the three surfactants with each of the three oils (nine systems) so that the effect of surfactant and oil molecular structure on phase equilibration (location of aggregates, equilibrium phase composition etc.) could be determined. A broad range of phase types were observed, dependent on the structures of the surfactant and oil molecules. Phase inversion due to increasing initial [surfactant] alone (i.e. no added salt) was observed in several systems. The possibility of control of equilibrium water content of the equilibrium microemulsion phases with surfactant concentration was studied. Systems that did not demonstrate phase inversion with initial [surfactant] displayed control of equilibrium water content of the equilibrium microemulsion phases with initial [surfactant]. It was thought that control of porosity of the solid polymers may be possible by controlling the equilibrium water content of the liquid microemulsion. Solid polymer films were prepared by polymerisation of equilibrium microemulsion phases containing high initial [surfactant] using a photopolymerisation method. 0.1 min using 1 wt. % Darocur 1173 initiator was sufficient UV irradiation time to prepare sold films. The opacity of the polymer films was seen to increase with increasing water content of the parent microemulsion. The strength of the films was seen to decrease with increasing water content of the parent microemulsion. Three different microstructures of polymer films were observed by SEM and were dependent on the structures of the surfactants and oils in the microemulsion. The pore volume fraction of the polymers was found to be dependent on the microemulsion phase type, including surfactant and oil types and was controllable in several cases by equilibrium water content of the parent microemulsion, therefore by initial [ surfactant] in the microemulsion.

Discotic liquid crystal research is now more than 30 years old, however it is still early days compared with the well-established area of calamitic liquid crystal research which dates back for more than a century. Since the 1990’s there has been great deal of interest in discotic liquid crystal research, due to applications such as discotic compensation films for enhancing liquid crystal display (LCD) viewing angles and exciting new potential applications such as organic semi-conductors and photovoltaics.The work contained in this thesis can be broken down into three major areas: investigation of synthetic methodologies of hexasubstituted triphenylene-based discotic liquid crystals, examination of structure-property relationships within novel triphenylene-based discotic liquid crystalline materials, and the design and mesomorphic properties of novel disc-shaped molecular architectures.The investigation of the synthetic methodologies of triphenylene-based liquid crystals examined two main areas of the synthesis of triphenylene-based discotic materials: iodination reactions to generate valuable intermediates, and the oxidative strategies and methods for the generation of hexasubstituted triphenylenes. These investigations have revealed that careful control over reaction conditions can offer great selectivity in the synthesis of halogenated intermediates for the synthesis of triphenylene-based liquid crystals and that electron rich intermediate species offer the greatest scope for the synthesis of hexasubstituted triphenylenes in good yield and purity.The determination of structure-property relationships within triphenylene-based discotic liquid crystals has examined various peripheral substituents and various near-core modifications to the conventional discotic liquid crystal.A small number of novel spiral-shaped materials have been synthesised, which have revealed interesting mesomorphic properties. These materials have shown the ability to self-organise in such a manner as to fill free space around central disc units so as to generate columnar mesophases in materials which would not otherwise be expected to exhibit mesomorphism.

The evaluation of novel polyimides for nematic liquid crystal alignment by UV irradiation was the aim of this thesis. Homeotropic liquid crystal (LC) alignment was observed for the majority of the in-house synthesised hairy-rod polyimides (PIs). Oblique unpolarised UV irradiation of branched PIs results in pretilted LCs of 2 deg to 88 deg depending on chemical composition, processing history and fluence. The initial pretilted alignment collapsed to homogeneous alignment at higher fluences, which was dependent on the main chain, side chain combination. Minimal changes in the UV/VIS and FTIR absorption spectra were found for fluences at which pretilted LC alignment occurred. For higher fluences photodegradation could be observed in the FTIR spectra. AFM measurements revealed an increased surface roughness after UV irradiation, but no surface anisotropy could be observed. X-ray reflectivity measurements showed film shrinkage after in-situ UV irradiation with identical trends as was observed for the collapse of the LC pretilt. Modeling revealed an increased electron density at the surface and a less diffuse interface following UV exposure. Bragg features in the reflectivity curve suggested a periodical internal film morphology, which was confirmed by grazing incidence X-ray diffraction (GIXRD). Models of chain orientations are proposed for three PIs and the UV induced morphology change analysed. GIXRD probes the surface or bulk of the thin film and an anisotropic increase in order was found for the in-plane surface scan at low fluences, followed by a general order decrease for higher fluences. The LC tilts in the direction of UV irradiation and not toward higher order. An entropy-related photodegradation model is suggested whereby the LC tilts toward maximum surface disorder to minimise free energy. Polarised UV irradiation of main chain only PIs results in homogeneous LC alignment. This was used to photoalign a LC semiconductor material, resulting in a polarised UV absorption ratio of 11 to 1.

A range of novel liquid crystals and amorphous organic conjugated semiconductors were synthesised by the chemistry branch of the Organophotonics group at Hull University. These compounds are studied electrochemically and optically to investigate the suitability of these materials as good donor/acceptor with suitable pairs energy levels for use in organic photovoltaic devices. Liquid crystal compounds with a fluorene-thiophene structure were identified as potential electron donors in combination with perylene based compounds as electron acceptors. Time-of-flight was used to study the charge transport of organic semiconductors in this thesis. The nature of the functional groups of the molecules was found to have a significant influence on the charge carrier mobility. The incorporation of a reactive end group with spacer affected the charge carrier mobility of electron donors negatively, suggesting that the mobility depends on the intermolecular separation. Based on the need to correlate the charge transport of the donor/acceptor blends with photovoltaic devices, the electron and hole mobility were studied for blends. For all blends the hole mobility is lower than that of the pure electron donor. The electron mobility of the blends is much higher than that of the pure electron acceptor. The thermal activation of charge transport in the liquid crystals is investigated by applying the Gill model. The result shows that charge transport in the liquid crystals is thermally activated and the activation energy is field dependent. The Gaussian disorder model and correlated disorder model were used to analyze the mobility data of four liquid crystals compounds with the same conjugated core and different end groups. We show that the thin film nanoscale morphology and the phase separation of the donors/acceptors blends depend on the chemical structures of donors and acceptors, the casting solvents and the annealing temperature of the film. The functional groups of the perylene bisimide are found to influence the roughness. The surface roughness of the blended thin film is minimum and its phase separation finest when the electron donors component has short terminal aliphatic groups rather than long polymerisable chains. Chlorobenzene shows the best performance as coasting solvent. The annealing temperature is significant in controlling the nanoscale morphology and the phase separation of an intermixed network of the blends. We successfully demonstrate photovoltaic performance using blends of our novel donors and acceptors. The annealing temperature is very important to optimise the solar cell performance by optimisation of the phase separation. The perylene based liquid crystals have disappointing performance as electron acceptors. The donor with the shortest terminal end group gives the best result. The device performance fully correlates with the blend nanoscale morphology of the blends; the blend with the smallest domains gives best power conversion efficiency; the best device has a value of 1.1%.