Doktorsgrade wat in die Fakulteit Natuurwetenskappe toegeken word, Desember 2011
BADENHORST, Daleen (Zoology)
Rattini (Rodentia, Murinae) species relationships and involvement as reservoirs for scrub typhus: a comparative molecular cytogenetic and gene expression approach
A comparative molecular cytogenetic investigation of Rattini identified a constrained pattern of chromosome evolution in these rodents. This study presents the first hypothesis of the Rattini ancestral karyotype that may prove useful in directing the selection of species for future large-scale investigations of rodent genome organization. A qPCR approach designed to examine the role of Rattini species as reservoirs of scrub typhus (infectious febrile disease) identified a link between the underexpression of syndecan-4 in Murini and seropositive Rattini rodents that may explain the poor carrier status of Murini, and increased longevity of Rattini host species.
Supervisor: Prof TJ Robinson
Co-supervisor: Dr G Dobigny
BAYLEY, Gareth (Polymer Science)
Novel electrospun fibres of amphiphilic organic-inorganic graft copolymers of poly(acrylonitrile)-graft-poly(dimethylsiloxane) for silicone composite reinforcement
The inclusion of nano-scale fillers in polymer materials has the potential to dramatically modify the physical and mechanical properties of these materials. A new amphiphilic graft copolymer of acrylonitrile and poly(dimethylsiloxane) was developed for use as a novel filler for silicone elastomer. This unique material was designed to provide mechanical strength from the acrylonitrile segments while simultaneously being compatible with the silicone matrix. The material also allows for the inclusion of carbon nanotubes in the silicone matrix. The material was processed into nano-fibres using the electrospinning technique. The amphiphilic nature of the material lead to highly porous fibre morphology due to the self-assembly of the polymer in the electrospinning solution. The inclusion of this material in a silicone matrix leads to a dramatic increase in the strain at break (elongation) in silicone elastomers.
Supervisor: Prof PE Mallon
BOONZAAIER, Leandro (Physics)
Confined counterions surrounding a macroion – a field theoretic approach
A field-theoretic approach was developed to study Coulomb interactions of confined counterions with a charged spherical macroion of variable radius. The linearised field-theory shows bound states in the spectrum under all conditions which lead to non-perturbative effects. The fluctuation part of the free energy favours a decrease in the free energy upon expansion of the macroion inside the fixed confinement volume nor does it dominate the free energy at high, but finite, temperatures. A novel regularisation scheme for computing the relevant functional determinant was introduced and the associated cut-off could be specified unambiguously in terms of physical parameters.
Supervisor: Prof KK Müller-Nedebock
Co-supervisor: Prof FG Scholtz
CLOETE, Valeska (Polymer Science)
Impact of molecular structure on water vapour sorption properties in nanostructured polymeric films
Three aspects related to the transport behaviour of water vapour in polymeric films were investigated: the effects of reactive surfactants, clay-based organophilic modifiers, and polymer crystallinity. The inclusion of reactive surfactants resulted in the rapid diffusion of water vapour through latex films. Low permeability associated with increased diffusion path tortuosity for water vapour could be eliminated by the increased solubility associated with the presence of intercalated clays. Polymer chain mobility could reduce diffusion kinetics to a greater extent than crystalline domains in polymer films. These findings could be used in the engineering of protective coatings with controlled water vapour sorption behavior, for the packaging materials industry, in particular where specific transport behaviour of water vapour is required.
Supervisor: Prof H Pasch
Co-supervisor: Dr P Hartman
ELHRARI, Wael (Polymer Science)
Synthesis and characterisation of multiphase copolymers
Graft copolymers were synthesised using a combination of various living polymerisation techniques and boron chemistry. Multifunctional polymer backbones were synthesised and then functionalised using boron chemistry to form multifunctional macroinitiators. These were subsequently used in “grafting from” polymerisation reactions. Two advanced analytical techniques were used to examine the phase morphology of the graft copolymers: solid state Nuclear Magnetic Resonance and positron annihilation lifetime spectroscopy. Complimentary information on the compositional phase segregation point of the graft copolymers was obtained. Analysis of the spin diffusion from the Nuclear Magnetic Resonance data enabled the interpretation of the seemingly anomalous results in the positron data observed at the phase segregation point.
Supervisor: Prof PE Mallon
GREESH, Nagi (Polymer Science)
Preparation of polymer-clay nanocomposites by dispersion polymerisation using tailor-made polymeric surface modifiers
Clays are a good example of naturally occurring precursors of nano-materials. When delaminated, 2:1 smectite clays separate into 1-nm thick layers, which, when incorporated into polymer materials, improve the mechanical properties, thermal stability, and gas barrier properties. A new method of preparation of polymer-clay nanocomposites, involving dispersion polymerization, was developed. The incompatibility between the inorganic clay and the organic polymer matrix was overcome by using various clay organic modifiers. Intercalated to semi -exfoliated materials with enhanced latex stability and coating properties were obtained. This approach has possible direct application in the coatings industry.
Supervisor: Dr P Hartmann
Co-supervisor: Prof RD Sanderson
HEIDT Alexander (Laser Physics)
Novel coherent supercontinuum light sources based on all-normal dispersion fibers
The highly topical issue of supercontinuum generation, which finds wide application in diverse research fields and technology applications, is addressed in the research. The focus is on utilizing new developments in photonic crystal fibers to produce highly coherent ultra-broadband spectral output with reduced pump source requirements compared to traditional zero dispersion wavelength fiber setups. New photonic crystal fibers were designed to operate in the normal dispersion regime, in order to maintain excellent temporal coherence. A broad band super continuum with temporal coherence which allowed for pulse compression to a few cycle ultrashort pulse was produced for the first time.
Supervisor: Prof EG Rohwer
Co-supervisor: Prof H Bartelt
HERBERT, Simon (Chemistry)
Oxazoline directed lithiation of calixarene and ferrocene
This research pioneered the first synthetic method to inherently chiral calixarenes, which although theoretically known for over three decades, have only now been permitted in targeted asymmetric synthesis. It opens the door to further investigation into new catalyst motifs that may have a significant role to play in the fine chemical arena. This research also demonstrated that the chiral oxazoline directing group could be tuned to deliver either diastereomer of the product thus extending the versatility of the method. This unusual effect was also demonstrated on ferrocene systems, confirming its wider applicability to other chiral systems.
Supervisor: Dr GE Arnott
KALEME, Prince (Zoology)
Habitat fragmentation, patterns of diversity and phylogeography of several small mammals species in the Albertine Rift
The genus Praomys was used as a model to investigate different hypotheses underlying the patterns of diversification for small mammals in the Albertine Rift. Molecular and morphological characters where combined to unravel systematic and phylogeographic relationships, and placed the divergence of the Praomys group in the Pliocene. It is speculated that past climate cycling, habitat fragmentation as well as species-specific ecological requirement where causal in the patterns of diversification. The results contribute to our understanding of the biogeography of the Albertine Rift as well as the mechanisms underlying species distribution and diversification.
Supervisor: Prof B Jansen van Vuuren
Co-supervisors: Proff RCK Bowie, J Bates
KOEKEMOER, Lizbé (Chemistry)
|Characterization of prokaryotic pantothenate kinase enzymes and the development of type-specific inhibitors|
Pantothenate kinase (PanK) enzymes catalyse the first reaction in the five step biosynthesis of the essential cofactor coenzyme A. Importantly, PanKs exhibits a unique diversity between different organisms, therefore highlighting it as a potential drug target. In this study the type III PanK of specifically pathogenic bacteria were characterized with the goal of developing type-specific inhibitors. Several questions about the activity of the Mycobacterium tuberculosis enzyme were answered. The first inhibitors that are competitive to the pantothenate binding site were designed, synthesised and tested against the Pseudomonas aeruginosa enzyme. This resulted in the discovery of the most potent inhibitors of this PanK type to date.
Supervisor: Prof E Strauss
LOKUGALAPPATTI, Sampath (Zoology)
Climatic perturbation and speciation of Southern and eastern African Bulbul/Greenbul species (Family Pycnonotidae)
African greenbuls are an ideal model system to explore different hypothesis that may underlie observed patterns of lineage diversification. Phylogenetic and population genetic methods to reconstruct the diversification history of three African greenbul species/species complexes (Pycnonotidae) in southern and east Africa were used. Most of the greenbul diversification took place in Plio-Pleistocene and the primary mechanism appears to be climatic cycling, yet dispersal and vicariance have also shaped the population genetic structure. The pattern of diversification observed in the three study taxa/species complexes differs substantially and can mostly be explained by the Pleistocene refuge hypothesis.
Supervisor: Prof B Jansen van Vuuren
Co-supervisor: Proff RCK Bowie, J Fjeldså
OMBINDA LEMBOUMBA, Saturnin (Laser Physics)
Femtosecond Pump probe Spectroscopy of Light harvesting Complexes and Phthalocyanines
On the development and application of a time resolved spectroscopic technique for analysis of ultrafast photo-induced processes on the timescale of hundreds of femtoseconds to hundreds of picoseconds are reported. This pump probe spectroscopy setup was fully characterized and applied to study natural photosynthetic complexes as well as an artificial complex, Zn phthalocyanine, used in photodynamic cancer therapy. The study of photosynthetic complexes is motivated by the need for artificial light harvesting complexes for solar cells. The new results on Zn phthalocyanine add to the understanding of the ultrafast dynamics of this drug in cancer therapy.
Supervisor: Dr A du Plessis
Co-supervisors: Dr CM Steenkamp, prof EG Rohwer
SPICER, Esmé (Geology)
The low-pressure partial-melting behaviour of natural boron-bearing metapelites from the Mt Stafford area, central Australia
The unusual, low-pressure partial melting behaviour of metasediments from Mt Stafford, central Australia, has been experimentally investigated. The findings indicate that muscovite melted at unusually high temperatures, between 750 and 800 ºC at 0.3GPa, due to the incorporation of Ti and F in the structure, as well as the absence of plagioclase in the rocks. The notion that boron fluxed melting in these rocks is found to be untrue as tourmaline was resistant to melting, additionally; andalusite persisted as a metastable mineral into the anatectic zone creating the impression of incongruent melting at low pressure and temperature.
Supervisor: Prof G Stevens
Co-supervisor: Prof IS Buick
Van der Westhuizen, Katriena (Chemistry)
Comprehensive multidimensional GC for the analysis of Fischer-Tropsch products
The analysis of Fischer-Tropsch-derived synthetic crude and derived products is very challenging because of the highly complex nature of these products. Comprehensive multidimensional gas chromatography (GCxGC) was found to be invaluable for the analysis of these complex mixtures. GCxGC was used in combination with micro-reactor experiments to simulate the Sasol Secunda plant operation to assist with the development of accurate engineering models and was instrumental in Sasol obtaining international approval for use of their fully synthetic jet fuel as Jet A1 fuel.
Supervisor: Prof P Sandra
Co-supervisor: Dr AJ de Villiers