A basic introduction to liquid crystals and surfactants has been given, along with a description of the main techniques employed in the study of liquid crystals (in particular optical polarising microscopy, differential scanning calorimetry, nuclear magnetic resonance spectroscopy and x-ray diffraction studies). Conventional surfactants comprise a polar head group and a hydrophobic, hydrocarbon chain ie. they are amphiphilic. Because long chains have high melting points the length of the alkyl chain in these compounds is limited to < ca C[18], as long chain surfactants are usually insoluble. Therefore, in this project the alkyl group has been replaced by a long, hydrophobic polydimethylsiloxane chain. Polydimethylsiloxanes are low melting materials (glass transition at ca -120&deg;C) with very flexible chains, hence surfactants based on them might be readily soluble in water. This project involves chemical attachment of amphiphilic mesogens to alpha-SiH terminated siloxanes of varying lengths and the examination of their surfactant properties. The following type of structure was successfully synthesised: CH[3]CH[2]CH[2]CH[2](Si (CH[3]) [2]O)n Si(CH[3])[2] - m where n = integer; m = amphiphilic mesogen. The amphiphilic head groups of these novel surfactants contained the salts of either a mono-, or a dicarboxylic acid. After the synthesis of these surfactants, the liquid crystal and micelle properties of the sodium and calcium salts, were investigated utilising a number of physical techniques eg. optical microscopy and differential scanning calorimetry. Finally, some work on synergism has been described. When different types of surfactants are purposely mixed, what is sought is synergism, the condition when the properties of the mixture are better than those attainable with the individual components by themselves.

Results are presented from a series of simulations undertaken to investigate the effect of adding small spherical particles to a fluid of rods which would otherwise represent a liquid crystalline (LC) substance. Firstly, a bulk mixture of Hard Gaussian Overlap particles with an aspect ratio of 3:1 and hard spheres with diameters equal to the breadth of the rods is simulated at various sphere concentrations. Both mixing-demixing and isotropic-nematic transition are studied using Monte Carlo techniques. Secondly, the effect of adding Lennard-Jones particles to an LC system modelled using the well established Gay-Berne potential is investigated. These rod-sphere mixtures are simulated using both the original set of interaction parameters and a modified version of the rod-sphere potential proposed in this work. The subject of interest is the internal structure of the binary mixture and its dependence on density, temperature, concentration and various parameters characterising the in-termolecular interactions. Both the mixing-demixing behaviour and the transitions between the isotropic and any LC phases have been studied for four systems which differ in the interaction potential between unlike particles. A range of contrasting microphase separated structures including bicontinuous, cubic, and micelle-like arrangement have been observed in bulk. Thirdly, the four types of mixtures previously studied in bulk are subjected to a static magnetic field. A variety of novel phases are observed for the cases of positive and negative anisotropy in the magnetic susceptibility. These include a lamellar structure, in which layers of rods are separated by layers of spheres, and a configuration with a self-assembling hexagonal array of spheres. Finally, two new models are presented to study liquid crystal mixtures in the presence of curved substrates. These are implemented for the cases of convex and concave spherical surfaces. The simulation results obtained in these geometries indicate segregation of spheres at the topological defect of the director field induced by the surface.

No description available.

We have analyzed the NMR spectra of a series of 19 di-halobenzenes having Cs or C₂v symmetry dissolved in a special mixture of nematic liquid crystals, i.e., 55 wt% 1132/EBBA-d₂. This mixture has the unique feature that dideuterium dissolved in it experiences a zero average electric field gradient. Therefore one can assume that for this mixture the contribution to the molecular order due to the interaction between the average electric field gradient and the quadrupole moment of the molecule can be neglected. It has been suggested that the orientational order also depends on the size and shape of the solutes and a model has been used to calculate the order parameters. Another model relates the ordering to the molecular polarizability anisotropy. A description of the ordering of molecules having Cs symmetry requires 3 independent order parameters and thus provides a strong test for the different models of orientational order. Comparisons are made between our experimentally determined order parameters and the theoretical values obtained using the size and shape, and the polarizability models. Very good agreement is obtained for the size and shape model. However, it is found that any mechanism, involving a molecular property which is approximately bond additive, can not predict the differences found between the ortho- and meta-dihalobenzenes. Consequently, a poor agreement results from the polarizability model when a bond-additive scheme is used to calculate the molecular polarizability. Further investigation of the correlation between order parameters and solute molecules suggests that the molecular shape dominates the orientational behavior. / Science, Faculty of / Chemistry, Department of / Graduate

The spectrum of 1,3-dichloro-2-ethenylbenzene partially oriented in nematic liquid crystalline solvents has been analysed using a number of complimentary nuclear magnetic resonance (NMR) techniques. The analysis of this spectrum poses a considerable problem due to its complexity and the uncertain geometry and orientation of the molecule. The technique of multiple quantum NMR (MQNMR) has the capability of tremendously simplifying complex spectra. The multiple quantum spectrum was measured and proved very useful in the analysis but unavoidable resolution difficulties in the MQNMR experiment prevented a complete solution of the problem using this spectrum alone. While the spectrum of 1,3-dichloro-2-ethenylbenzene is dominated by only a few large interactions, the lines are split by relatively small dipolar couplings and the limited resolution available in the multiple quantum spectrum makes the determination of the smaller couplings difficult. To overcome this difficulty a frequency selective excitation of the multiple quantum spectrum was adapted and developed. After testing the experiment on the relatively simple spectrum of 1,1,2-trichloroethane dissolved in a nematic solvent, this selective experiment was applied to the much more complex spectrum of 1,3-dichloro-2-ethenylbenzene where it proved capable of directly measuring the small couplings in the spectrum without interference from any of the larger interactions. This information contributed greatly to the eventual analysis of the spectrum. MQNMR experiments can be very time consuming and as a result the spectroscopist must frequently make do with very limited time domain signals from which the spectrum must be extracted. This creates a number of difficulties when the signals are analysed with the fast Fourier transform (FFT), the standard method of spectral analysis used in NMR. With these problems in mind, the suitability of MQNMR time domain signals for analysis by a method of spectral estimation due to Burg, commonly called the maximum entropy method (MEM), was examined. By testing Burg's MEM with the MQNMR spectra of a number of different solutes partially oriented in nematic phases, it was found to be a useful adjunct to the FFT when dealing with MQNMR interferograms. While some care is required in its application, this method of spectral analysis should find important uses in the estimation of MQNMR spectra. Solution of the spectrum yielded information on the molecular geometry and the orientation of the 1,3-dichloro-2-ethenylbenzene in the nematic solvents used. While an extensive analysis of molecular geometry proved impossible, the information on molecular orientation was examined in terms of two different models. The orientation data shows excellent agreement with a recently developed model for orientation based upon the shape of the solute. / Science, Faculty of / Chemistry, Department of / Graduate

No description available.

A "liquid crystal" is a substance whose properties are intermediate between those of a crystalline solid and an isotropic liquid. The molecules within a liquid crystal are partially ordered, but they are able to translate easily in at least one direction.

Several hydroxy-terminated, low molecular weight polymers were reacted to form esters bearing mesogenic endgroups. The effect of the different repeating unit structures with a common mesogenic unit based on an oxybenzoylterephthalate dyad ester on the formation and properties of liquid crystals was studied by microscopic and thermal analysis techniques. The polymeric glycols used were polyethylene glycols (PEG), polytetramethyl ene glycols (PTMG), polybutadiene glycol (PB), and hydrogenated polybutadiene glycol (HPB) with molecular weight of 650 ^ 6000, in addition to 1 ,12-dodecanediol for the model compound. Conversion of the mesogenic dyad ester to a dyad containing a carboxylic acid group was shown to permit mesophase formation over a wide temperature range. The HPB product of this series was found to be a liquid crystalline elastomer with a low viscosity above the melting point, and with film forming ability. On examining the effect of endgroups on physical properties, it was found that both the length v of mesogen and hydrogen bonding interaction by carboxyl groups proved to be the most important factors to form liquid crystals with the PB and HPB polymers. Texture observation of macroglycol products on a polarizing microscope gave neither sharp T.. (clearing temperature) nor typical pattern of mesomorphic state. This result may be explained on the basis of the low stability of liquid crystal phase. These results were consistent with the differential scanning calorimetry (DSC) measurement that showed isotropic transition endotherms on heating but no peak on cooling. Low molecular weight precursors with liquid crystalline end groups associate to form high molecular weight polymers which resulted in films. Until now physical properties have been poor, but there is a possibility that further research in this area might provide a route to useful polymers which could be adaptable to the RIM process.

No description available.

No description available.