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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

Bibliography: pages 103-106. / Beta-alumina solid electrolytes studied were subjected to charge and discharge cycles in a secondary, high energy density Na/beta-alumina/NaAlCl₄/FeCl₂ cell (known as the Zebra cell) at 250⁰ C. These electrolytes were studied by means of optical microscopy, as well as SEM and EDS analyses to est ablish possible failure modes. After cycling little discolouration, or impurity pickup was found to have occurred in the electrolyte. The forms of degradation of the beta-alumina electrolyte identified appeared to be a result of inherent flaws in the beta-alumina electrolyte tube, problems due to protracted storage of the tubes, or an apparent interfacial film on the cathode/electrolyte interface. A lead wetting agent was used in the cells to enhance the wetting of the beta-alumina electrolyte by the liquid Na. A study of the coating after cycling of the cell showed that the coating was adherent irrespective of the number of cycles completed. The coating did not appear to influence cracking of the electrolyte during cycling. Cracking of the beta-alumina electrolyte was found to have initiated from the Na/electrolyte interface. No evidence of crack initiation nor internal damage was found on the cathode/electrolyte interface. The cracks through the beta-alumina electrolyte wall were found to have sealed by the formation of a plug consisting largely of Na and Cl. On the basis of EDS analyses of the fracture surface of the sealed crack possible sealing mechanisms are proposed.

Malaria parasite infects more than 200 million people and about 435 000 succumb to the illness annually (WHO, 2019). Victims are mostly young children and pregnant women. It is transmitted by the bite of the infected female Anopheles mosquitoes. Indoor protection is provided by bed nets and residual spraying of insecticides. Mosquitoes typically bite ankles and feet most of the time (93%) whilst in outdoor settings. Long lasting insect-repellent anklets/bracelets/footlets may provide a strategy for reducing mosquito bites outdoors in the lower limb regions. This study considered long-lasting repellent anklets that may be used for outdoor protection against mosquito bites. Experiments were performed to investigate the incorporating of mosquito repellents into the thermoplastic polymers, poly(ethylene-co-vinyl acetate) (EVA) and linear low-density polyethylene (LLDPE). Two different mosquito repellents, namely DEET and Icaridin, were employed. The target was to develop cost-effective bracelets with long-lasting efficacy, i.e., slow release of the active ingredient over extended periods. In this way, it is expected to protect people from acquiring mosquito-borne diseases during the time they spend outdoors. The proposed concept utilises microporous polymer strands manufactured via conventional plastic extrusion processes. The internal open-cell polymer foam structure serves both as a reservoir and a protective environment for the active ingredient trapped inside. An outer dense skin layer covering the strands may provide the necessary diffusion barrier that controls the release of repellent at effective levels over a considerable period. The objective was achieved by phase separation via spinodal decomposition (SD), triggered by extruding the molten strands directly into ice-cold water. Thermogravimetric analysis (TGA) and solvent extraction confirmed that all of the repellents were embedded in the polymer matrices. Scanning Electronic Microscopy (SEM) confirmed the porous co-continuous repellent-polymer microstructure. The stability of the polymer matrix was studied by estimating the swelling and shrinkage of the polymer matrix. The release of the active ingredient in the polymer/repellent system was followed as a function of oven-ageing temperature and time. The kinetics of the release rate of the repellent from microporous polymer matrix strands was mathematically modelled using semi-empirical models. The performance of the repellent-based strands was evaluated using foot-in-cage repellence testing. Finally, an attempt was made to predict the phase diagrams of the LLDPE/repellent system on the basis of alkane/repellent systems data. The results confirmed that EVA and LLDPE are suitable scaffold matrices, acting as reservoirs, for liquid repellents that were released at a constant rate. As expected, the repellent swelled EVA more than LLDPE. As a result, it also shrank significantly more when the repellent was released, i.e. EVA showed poor dimensional stability compared to LLDPE. The semi-empirical repellent release models were found valuable as they provided insights into the way that the repellent was being released. They allowed differentiating between diffusion and relaxation mechanisms. It was found that repellence efficacy can be maintained for more than 90 days. Future developments of sandals and anklets based on this approach may assists in preventing outdoor mosquito bites, thereby decreasing malaria infection rates. / Thesis (PhD)--University of Pretoria, 2019. / Deutsche Forschungsgemeinschaft (DFG) Grant AN 212/22-1 / Chemical Engineering / PhD / Unrestricted

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