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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Studies towards the selective inhibition of β-alanine pathways in Mycobacterium tuberculosis

Koekemoer, Lizbe 03 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2006. / The focus of this study was the pathways for β-alanine production in Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis. The major pathway for β-alanine production is the decarboxylation of L-aspartate by L-aspartate-α-decarboxylase (PanD). This enzyme is not essential for the survival for Mtb which implies that an alternative pathway for β-alanine production must exist. We postulated that such a secondary pathway may be based on the oxidation of various polyamines by a polyamine oxidase to give the β-alanine precursor 3-aminopropanal, and therefore set out to find data in support of this hypothesis. Based on sequence homology to the FAD-dependent Saccharomyces cerevisiae polyamine oxidase Fms1, Mtb AofH was identified as a likely candidate. The soluble expression and purification of AofH proved troublesome and lead to the investigation of various techniques to increase protein yield. These methods include fusion to various tags, coexpression with the protein chaperones, addition of scarce codon tRNA’s to the translation mixture and protein refolding. AofH was eventually purified as fusions to the Nus and MBP proteins and its activity determined by analysis of the enzymatic reactions by TLC, reverse phase HPLC, ESI-MS and LC-MS. TLC analysis indicated that 3- aminopropanol formed as a product during polyamine oxidation, but this could not be confirmed by any of the more sensitive analytical techniques. We set out to confirm the presence of the FAD cofactor in the enzyme by various methods and concluded that the AofH fusions did not contain FAD. Efforts to refold the protein in the presence of FAD also failed. From this study it is clear that the biochemical confirmation of the presumed activity of AofH will remain elusive until the enzyme can be purified in its active form, i.e. with FAD bound. A genetic test for activity based on functional complementation studies of Escherichia coli ΔpanD strains proved inconclusive since no difference in growth rate was found between cell transformed with the aofH gene and the negative control. We continued our studies of β-alanine biosynthesis by attempting the design of mechanism-based inhibitors for the PanD enzyme. Various structural analogues were identified and tested by qualitative and quantitative methods. Our results show that β- substituted aspartate analogues may be good potential inhibitors of Mtb’s PanD protein and can thus be used in rational drug design.

The development of biocatalytic methods for the production of CoA analogues

Van Wyk, Marianne 03 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2006. / This work focuses on the biocatalytic production of coenzyme A (CoA) analogues with different tether lengths in its pantetheine moiety, and on analogues where the cysteamine moiety has been replaced with a range of other amines. An attempt was made to develop a simple biocatalytic method for the optimum production of such CoA analogues by chemo-enzymatic means. Pantothenic acid ethyl thioesters with different tether lengths were first synthesized as substrates of the CoA biosynthetic enzymes, CoaA, CoaD and CoaE. The acceptability of these compounds as substrates for the pantothenate kinase (CoaA) from prokaryotic and eukaryotic organisms was investigated through kinetic studies. These substrates were subsequently exposed to CoaA, CoaD and CoaE to produce various general CoA synthons (ethyl pre-CoAs). Finally aminolysis of these ethyl pre-CoAs by cysteamine and homocysteamine gave the various CoA analogues of different tether lengths in their pantetheine moiety. The identical production of a second type of CoA synthon (phenyl pre-CoA) from pantothenic acid phenyl thioesters was also investigated as a means to increase reactivity of the thioester substrates. Aminolysis of the phenyl pre-CoA produced the corresponding CoA derivative, but reactivity was lower than expected. A second strategy was also developed where the pantothenic acid phenyl thioesters were first aminolyzed, resulting in various pantothenamide intermediates. Aminolysis was attempted with thiol-bearing amines such as cysteamine and homocysteamine as well as with amines without sulfhydryl functionalities. These pantothenamide intermediates were then used in the biosynthesis of the corresponding CoA analogues by addition of CoaA, CoaD and CoaE. The ideal method of CoA analogue production will utilize a continuous bioreactor system in which these analogues can be prepared on large scale. However, to construct a bioreactor the enzymes involved need to be immobilized on a matrix in order to transform substrate to product. The enzymes CoaA, CoaD and CoaE can be immobilized on cellulose via a cellulose binding domain (CBD) affinity tag. Various types of CBDs were investigated and used in the construction of suitable expression vectors. Optimum expression conditions to obtain soluble CBD-fused enzymes were developed.

Hybrid hydrogels based on RAFT mediated poly(N-vinyl pyrrolidone)

Eksteen, Zaskia-Hillet 12 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2009. / Thesis submitted in partial fulfilment for the degree of Master of Science (polymer science) at Stellenbosch University / ENGLISH ABSTRACT: The goal of this study was to synthesize hybrid hydrogels via a chemical crosslinking mechanism through use of chain end functional poly(N-vinyl pyrrolidone)(PVP) with various topologies. The crosslinking chemistries should be benign in nature i.e. at physiological pH ranges and at 37 °C. The degradation products should be biologically tolerable and renal clearance should be possible (< 30 000 g/mol PVP0. PVP of various topologies, controlled molar mass and quantitative chain end functionality was obtained via Reversible Addition Fragmentation chain Transfer (RAFT) mediated polymerization (PDI = 1.1- 1.4). The synthesized polymers were chain end functionalized to introduce thiol or aldehyde moieties. Thiol chain ends were obtained through post polymerization modification of xanthate functional PVP with either aminolysis or reduction. The aldehyde moiety was obtained by post polymerization modification of xanthate end functional PVP with sequential hydrolysis and thermolysis. Thiol functional four arm star PVP was reacted with acrylate difunctional poly(ethylene glycol) (DIAC PEG) crosslinker under standard Michael addition conditions. In order to obtain thioether crosslinked hydrogels from tetra functional star PVP molecules it was found that a minimum thiol functionalization of 30% and a molar ratio of acrylate:thiol of 1:1.1 is required. The Schiff base reaction was used to synthesize imine or secondary amine (after reduction) crosslinks with the lysine residues on either lysozyme or bovine serum albumin (BSA) or the primary amines of bis-(2-amino ethyl)amine). Hydrogels were obtained from aldehyde functionalized PVP molecules with a fraction of functional aldehyde chain ends of 0.88 for difunctional molecules and 0.50 for tetra functional star PVP molecules with lysozyme or BSA crosslinkers. The reaction rate was favoured by lowered pH (<6.0) and an optimum molar ratio of amine : aldehyde of 1:0.8. Hydrogels were analyzed by equilibrium swelling calculations to determine the molar mass between crosslinks and the estimated pore size. In both crosslinking systems the properties of the formed hydrogels were seen to be affected by molar ratio used and by the topology of the crosslinking agent. PVP BSA and PVP PEG hydrogels were tested for 24 h and 48 h cell viability by using H9C2 myoblast cells. A concentration range of 0.25 x 10(2) to 0.01 g/mL was studied. Cell mortality was tested by Trypan blue staining and results were verified with MTT assay. A very low cell death precentage (<37%)was observed. Cells even appeared to experience a stimulatory effect after 48 h of exposure at low concentrations of PVP PEG hydrogel treatments. The properties of the formed hydrogel could be tuned by the molar mass ratios of PVP and crosslinker. The functionality of the crosslinker directly affects the molar mass between crosslinks and thus indirectly the degradation profile. It was concluded that PVP molecules with various topologies, well-defined molar masses and chain end functionality could be obtained via RAFT mediated polymerization. Obtained polymers were successfully modified and crosslinked to obtain hydrogels with stoichiometrically tuneable properties i.e. initial swelling ratio, degradation time, molar mass between crosslinks. The hydrogels had very positive cell viability results that would definitely justify further research into these materials as “tissue-mimetic” materials. / AFRIKAANSE OPSOMMING: Die doel van die studie is om poli(N-viniel pirollidoon) (PVP) gebaseerde hibried hidrogelle te sintetiseer deur middel van kovalente kruisbindings met toepaslike kruisverbinder molekules. Die chemiese reaksies betrokke in die vorming van hierdie kovalente kruisbindings moet gematig van aard wees, by fisiologiese pH en by 37 °C plaasvind. Die degradasieprodukte van die hibried-hidrogel moet biologies verdraagsaam en ook uitskeibaar deur die endokrinologiese sisteem wees. PVP van verskillende topologieë, beheerde molêre massa en kwantitatiewe kettingendfunksionaliteit is berei deur ‘n omkeerbare addisie-fragmentasiekettingoordrag (OAFO)-beheerde polimerisasieproses (PDI = 1.1-1.4). Xantaat-kettingend-PVP is aangepas na thiol of aldehied kettingendfunksies. Thiolendfunksies is verkry deur middel van ‘n aminolisasie-reaksie. Xantaat kettingend-PVP is stapsgewys gemodifiseer deur hidroliese en verhittingstappe om die aldehied ketting-endfunksionaliteit te bekom. Thiol ketting-endfunksionele vier-armige ster-PVP is kovalent gebind aan difunksionele poly(etileen glikol) deur middel van die Michael-addisiereaksie. PVP PEG hidrogelle het slegs gevorm met vier-armige ster-PVP molekules wat oor ‘n minimum van 30 % thiol-funksionaliteit beskik het en ‘n optimale molêre massa verhouding van 1:1.1 vir ankrilaat to thiol. Die Schiff-basisreaksie is gebruik om hidrogel te sintetiseer wat met imiene of amiene (na redusering) kovalente bindings gekruisbind is. In hierdie sisteme het hidrogel slegs gevorm as die aldehied-PVP molekules oor ‘n fraksie funksionele kettingend-waarde van 0.88 vir dialdehied-PVP molekules en 0.5 vir vier armige ster-PVP molekules beskik het. Die reaksie snelheid van die Schiff-basis kovalente bindings is bevoordeel deur die pH te verlaag (≤ 6.0) en ‘n gunstige molêre massa verhouding van 1:0.8 vir die nukleofiel teen oor die akseptor molekule is waargeneem. Ewewigswel berekeninge is gebruik om die molêre massa tussen kruisbindings en die gemiddelde benaderde porieë binne die drie-dimensionele interne struktuur van die hydrogel te bepaal. Die seltoksisiteit van PVP-BSA en PVP-PEG hidrogelle is oor 24 h en 48 h in die teenwoordigheid van H9c2 mioblast-selle getoets. Die hydrogel behandelings is uitgevoer in ‘n konsentrasie reeks van 0.25 x 10(2) tot 0.01 g/mL. Selmortaliteit is getoets deur ‘n Trypan-blou verkleuringstudie. Hierdie resultate is ondersteun deur MTT sel-lewensvatbaarheidstoetse. ‘n Lae selmortaliteit (≤ 37 %) is waargeneem en, opspraakwekkend, het van die selle na 48 h verhoogde vitaliteit getoon in die teenwoordigheid van lae konsentrasies PVP-PEG hidrogelle. Dit is bevind dat hidrogel eienskappe deur stoichiometriese molêre massa verhoudings asook die keuse in die topologie van kruisverbinder beïnvloed word. Hierdie eienskappe het ‘n direkte effek op die degradasieprofiel van die gevormde hidrogel. Samevattend dus is PVP molekules met ‘n variasie van topologieë, spesifieke molêre massas en kettingfunksionalitete deur middel van OAFO-gemedieerde polimerisasies gesintetiseer. Xantaatkettingendfunksionele PVP-molekules kon suksesvol omgeskakel word na die kettingendfunksionaliteit van ons keuse om ‘n hibriedhidrogel met stoichiometries-manupileerbare eienskappe te sintetiseer. Die positiewe sel-lewensvatbaarbheidstudie resultate staaf verdere ondersoeke in hierdie PVPgebaseerde hibried hidrogelmateriaal as ‘n weefsel nabootsingsmateriaal.

Selective distal functionalization of resorcinarenes via an ortholithiation approach

Ngodwana, Lonwabo 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Resorcinarenes are tetramers, belonging to a class of [1]4metacyclophanes, easily formed by the acid- or base-catalysed condensation of resorcinol and aldehydes. Properties include, amongst others; bowl-like shape, adhesion to hydrophilic surfaces and formation of hexameric capsules. Their uses are, to mention some: components in liquid crystals, photoresistors, selective membranes, surface reforming agents, HPLC stationary phases, ion-channel mimics, metal-ion extraction, molecular switches and ligands for metal catalysts. Selective functionalization of resorcinarenes has been explored and achieved via relatively inefficient methodologies which limit studies, structural architecture and new applications. In this work, synthesis of C4v symmetric resorcinarene ethers was performed which were used as templates in undertaking studies towards selective derivatisation of resorcinarenes via an ortholithiation approach. Conditions for the efficient synthesis of distally substituted resorcinarenes using ortholithiation were optimized and tested with a range of electrophiles, lower rim lengths, scale, base equivalents, reaction times and solvent effects. Ortholithiation gave distally substituted resorcinarenes in reasonable yields (>80%). Ortholithiation and its ability to form distal-resorcinarene esters could possibly be used as a way to synthesize distal-chloromethyl resorcinarene precursors whose further functionalization would furnish a range of distal-resorcinarene imidazolium salts, a class of distal bidentate carbene ligand staring materials for transition metal coordination. / AFRIKAANSE OPSOMMING: Resorsinarene is sikliese tetramere, wat deel uitmaak van 'n klas van [1]4metasiklofane, en kan maklik gevorm word deur die suur- of basis-gekataliseerde kondensasie van resorsinol en aldehiede. Eienskappe sluit onder andere in: bak-vormig, adhesie aan hidrofiliese oppervlakke en die vormasie van heksameriese kapsules. Tipiese voorbeelde van gebruike sluit die volgende in: komponente van vloeistof kristalle, fotoresistors, selektiewe membrane, oppervlak hervormings agente, HDVC stationêre fases, ioon-kanaal nabootsers, metaal-ioon ekstraksie, molekulêre skakelaars en ligande vir metaalkatalise. Selektiewe funksionalisering van resorsinarene was al voorheen bestudeer, maar die metodologieë was beperkend ten opsigte van die struktuele argitektuur en nuwe toepassings wat daaruit gekom het. In hierdie werk was C4v simmetriese resorsinareen eters gesintetiseer wat gebruik was as uitgangstowwe om selektiewe funksionalisering deur orto-litiëring te bewerkstellig. Kondisies vir die effektiewe sintese van distaal gesubstitueerde resorsinarene, deur gebruik te maak van orto-litiëring, was bepaal en ge-optimaliseer deur gebruik te maak van 'n wye reeks elektrofiele, laer rand lengtes, reaksieskale, basis ekwivalente en reaksie tye. Deur dié proses was dit moontlik om distaal gesubstitueerde resorsinareen produkte te bekom in redelike opbrengste (>80%) met meeste funksionele goepe. Daar word voorsien om orto-litiëring, en sy vermoë om distale-resorsinareen esters te vorm,van gebruik te maak as 'n beginpunt in die sintese van distale-chlorometiel resorsinarene, wat op hulle beurt weer sal dien as uitgangstowwe vir die sintese van distale-resorsinareen imidasolium soute. Hierdie distale, bidentale soute kan gebruik word as karbeen ligande in oorgangsmetaal koordinasie.

Synthesis and evaluation of halogenated amino acid analogues as inhibitors of decarboxylase enzymes of selected pathogens

De Villiers, Jandre 03 1900 (has links)
Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The use of fluorine in medicinal chemistry has increased dramatically in the last 20 years. The addition of fluorine to a lead compound has various advantages such as the blocking of metabolic active sites, the increase of solubility and lipophilicity of a compound, acting as conformational probes for the active site of an enzyme, and influencing (in most cases increasing) the binding affinity of a compound to a target protein. Their use as mechanism based inhibitors is also well known. In this study we set out to synthesize hydroxyl- and fluorinated-amino acid analogues as potential inhibitors and probes towards the active site of various enzymes. The synthesis of the hydroxylamino acid analogues would precede the fluorinated analogues to serve as precursors with fuorination achieved via a fluoro-dehydroxylation reaction. These aims have successfully been achieved with the synthesis of the two enantiopure isomers of 3-fluoro-aspartic acid. The fluorinated aspartic acid analogues were subsequently used in a conformational analysis, with regards to substrate- and binding activity, which investigated the interaction of these compounds with aspartate decarboxylase (PanD). The synthesis of the 3- hydroxy-analogues of ornithine and diamino pimelic acid was also successfully achieved. These syntheses were done in a stereospecific manner to provide one enantiomer of the L-amino acid analogue. However, our efforts toward the synthesis of the other enantiomer of hydroxy analogues as well as our attempts at the conversion of the hydroxyl group to a fluorine were unsuccessful to date. Nevertheless, these results gave us a new direction towards the synthesis of the desired compounds and have led us to new strategies and ideas. Hopefully, the work done in this study will be part of the ground work towards new methodologies for the synthesis of desired halogenated amino acid analogues as small molecule inhibitors. / AFRIKAANSE OPSOMMING: The use of fluorine in medicinal chemistry has increased dramatically in the last 20 years. The addition of fluorine to a lead compound has various advantages such as the blocking of metabolic active sites, the increase of solubility and lipophilicity of a compound, acting as conformational probes for the active site of an enzyme, and influencing (in most cases increasing) the binding affinity of a compound to a target protein. Their use as mechanism based inhibitors is also well known. In this study we set out to synthesize hydroxyl- and fluorinated-amino acid analogues as potential inhibitors and probes towards the active site of various enzymes. The synthesis of the hydroxylamino acid analogues would precede the fluorinated analogues to serve as precursors with fuorination achieved via a fluoro-dehydroxylation reaction. These aims have successfully been achieved with the synthesis of the two enantiopure isomers of 3-fluoro-aspartic acid. The fluorinated aspartic acid analogues were subsequently used in a conformational analysis, with regards to substrate- and binding activity, which investigated the interaction of these compounds with aspartate decarboxylase (PanD). The synthesis of the 3- hydroxy-analogues of ornithine and diamino pimelic acid was also successfully achieved. These syntheses were done in a stereospecific manner to provide one enantiomer of the L-amino acid analogue. However, our efforts toward the synthesis of the other enantiomer of hydroxy analogues as well as our attempts at the conversion of the hydroxyl group to a fluorine were unsuccessful to date. Nevertheless, these results gave us a new direction towards the synthesis of the desired compounds and have led us to new strategies and ideas. Hopefully, the work done in this study will be part of the ground work towards new methodologies for the synthesis of desired halogenated amino acid analogues as small molecule inhibitors.

Studies towards high-throughput production of nanofiber yarns

Smit, Eugene A. 12 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2008. / ENGLISH ABSTRACT: Electrospinning is a simple yet versatile method used for producing nanofibers from various materials, including natural and synthetic polymers, polymer blends, ceramics and metals. The high specific surface areas, high aspect ratios (length/diameter), and the biomimicking nature of nanofibers make them ideally suited for application in diverse areas, including high-performance filtration, biomedical materials for tissue engineering scaffolds, wound dressings and controlled drug release, fiber-reinforced composites and highly sensitive nano-sensors. Two of the main requirements that need to be met for electrospun nanofiber materials to become commercially viable are: (1) a process for manufacturing continuous aligned nanofiber yarns, and (2) a drastic increase in the fiber production rate of the electrospinning process. The objectives of this study were to develop a scalable process for making continuous yarns of aligned electrospun nanofibers, and to develop a needleless electrospinning method for the high-throughput production of nanofibers. Three novel processes were developed while exploring innovative ways for making yarns from electrospun fibers. Finally, a fourth process, the so-called the NanoCaterpillar process, was developed. This process can be used as a scalable method for obtaining continuous yarns of aligned nanofibers. Advantages of the process include the requirement for relatively simple equipment, the simple process variations required for obtaining yarns of different linear densities, and the fact that, as a 'dry' process, it can be used to manufacture yarns from most materials that can be electrospun. The second goal of this study was to develop a needle-less electrospinning process, capable of making nanofibers at commercially viable throughput rates. The phenomenon of bubble electrospinning was discovered and developed further. Initial exploratory studies showed that bubble electrospinning could be employed to produce nanofibers from polymers in aqueous as well as organic solutions, and that the process follows similar trends to classic electrospinning such as the dependence of fiber diameter on polymer solution concentration and the possibility of including substances in the formed fibers by including them in the spin solution. A second, more in-depth study of the bubble electrospinning process revealed that fibers could be produced at very high rates with estimates, under ideal spinning conditions, reaching 5 kg/h per square meter of bubble bath surface. Furthermore it was found that fiber yield did not depend on the size or the lifetime per bubble. Investigations into several interesting aspects of bubble and jet behaviour during bubble electrospinning, such as child bubble formation and jet-splitting, led to predictive multiple linear regression models being fitted to the experimental data to describe process variables like yield per bubble, number of jets per bubble and fiber diameters. / AFRIKAANSE OPSOMMING: Die elektrospintegniek is 'n eenvoudige, dog veelsydige metode wat gebruik word om nanovesels van verskeie materiale, insluitend natuurlike en sintetiese polimere, polimeermengsels, keramieke, en ook metale te vervaardig. Die hoë spesifieke oppervlakareas, hoë lengte tot deursnee verhoudings en bio-naboodsende eienskappe van nanovesels maak hulle ideaalgeskik vir toepassing in uiteenlopende velde soos filtrasie, biomediese materiale vir weefselingenieurswese, wondbedekkings en beheerde vrystelling van geneesmiddels, veselversterkte saamgestelde materiale en hoogs-sensitiewe nanosensors. Twee van die hoofvereistes waaraan voldoen moet word, voordat elektrogespinde nanovesels kommersieël-lewensvatbaar kan word, is: (1) 'n proses vir die vervaardiging van kontinuë garings van gerigte nanovesels, en (2) 'n drastiese toename in die vervaardigingstempo van die elektrospinproses. Die doelwitte van hierdie studie was om 'n skalleerbare proses te ontwikkel vir die vervaardiging van kontinuë garings van gerigte nanovesels, en om 'n naaldlose elektrospinmetode te ontwikkel vir die hoë-uitset vervaardiging van nanovesels. Drie nuwe prosesse is ontwikkel tydens ondersoeke na innoverende maniere om garings van elektrogespinde nanovesels te vervaardig. Laastelik is 'n vierde proses, die sogenaamde NanoCaterpillar proses ontwikkel. Hierdie proses kan gebruik word as 'n skalleerbare metode vir die vervaardiging van kontinuë garings van gerigte nanovesels. Voordele van die proses sluit in dat relatief eenvoudige toerusting benodig word om die konsep toe te pas, dat slegs eenvoudige veranderinge aan die proses benodig word om garings van verskillende liniêre digthede te verkry, en dat die proses, synde 'n 'droë' proses, gebruik kan word om garings te maak van meeste materiale wat gespin kan word met die elektrospintegniek. Die tweede doelwit van hierdie studie was om 'n naaldlose elektrospinproses te ontwikkel wat nanovesels kon vervaardig teen kommersieël-lewensvatbare tempo's. Die borrelelektrospin verskynsel is ontdek en verder ontwikkel. Aanvanklike ondersoeke het getoon dat die borrelelektrospinproses gebruik kon word om nanovesels te vervaardig van polimere in water- sowel as organiese oplossings. Dit het ook getoon dat die proses soortgelyke tendense toon as die klassieke elektrospintegniek, soos die afhanklikheid van veseldeursnee aan polimeeroplossingkonsentrasie en die moontlikheid om ander stowwe in die gevormde vesels in te sluit deur dit aanvanklik in die spinoplossing in te sluit. 'n Verdere indiepte ondersoek van die borrelelektrospinproses het onthul dat vesels, onder geïdealiseerde omstandighede, vervaardig kon word teen baie hoë tempo's, na beraming 5 kg/h per vierkante meter borrelbadoppervlakarea. Verder is bevind dat die veselopbrengs nie afhanklik was van die borrelgrootte of -lewensduur nie. Ondersoeke na verskillende aspekte van die borrel- en polimeerstraalgedrag tydens die borrelelektrospinproses, soos die vorming van kind-borrels en polimeerstraalsplitsing, het gelei tot die passing van voorspellende meerfoudige liniêre regressiemodelle op die eksperimentele data, ten einde prosesveranderlikes soos opbrengs per borrel, aantal polimeerstrale, en vesels deursnee te kan beskryf.

Coaxial electrospinning of reversibly thermochromic fibres

Malherbe, Ilana 12 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2009. / Thesis presented in partial fulfilment of the requirements for the degree of Master of Science (polymer science) at the University of Stellenbosch / Embargo date 2010-03-31 plt 2010 / ENGLISH ABSTRACT: A novel method, herein referred to as ‘solvent facilitated coaxial electrospinning’, was used to produce reversibly thermochromic core-shell fibres with poly(methyl methacrylate) (PMMA) as shell and a thermochromic dye composite as core. The thermochromic dye composite consisted of combinations of 1-dodecanol, bisphenol A (BPA) and crystal violet lactone (CVL). In the ‘solvent facilitated coaxial electrospinning’ method, the thermochromic dye composite was dissolved in a suitable ‘facilitating solvent’ prior to spinning, instead of being spun into the fibres from the melt as previously described in literature. A low interfacial tension between the core and shell liquids, which is beneficial to effective core entrainment, was achieved by using a correctly chosen core ‘facilitating solvent’. The PMMA was dissolved to form the shell spinning liquid and by selecting the correct core and shell solvents, spinneret blockage and precipitation due to core and shell liquid interactions were eliminated. High molar mass PMMA was used to produce fibres with diameters in the range of 3–10 μm (larger than typical electrospun fibres) in order to minimize light scattering and subsequently allow visual observation of the thermochromic transitions, unlike the fibres that were produced in literature. The fibres were analyzed using SEM, TEM, TGA and DSC to investigate fibre morphology, dye composite thermal transition and fibre composition. Physical and chemical interactions between the thermochromic dye composite and the PMMA shell were identified as possible causes of differences between the thermochromic transition temperatures of the core-shell fibres and the bulk dyes, as well as of the instability of the colour developed state of certain thermochromic fibres. The spatial confinement of the dye composite inside the fibres and the extensive volume reduction (from bulk dye to small volume inside the fibres) affected the thermochromic behaviour of the thermochromic composite once it was entrained in the fibres. An excess BPA was used in the dye composition to allow the production of reversibly thermochromic fibres with a stable colour developed state. / AFRIKAANSE OPSOMMING: A nuwe metode, hierin beskryf as ‘oplosmiddel gefasiliteerde koaksiale elektrospinnery’, is gebruik om omkeerbare termochromiese kern-skil vesels met polie(metiel metakrilaat) (PMMA) as skil en ‘n saamgestelde termochromiese kleurmiddel as kern te vervaardig. Die saamgestelde termochromiese kleurmiddel het bestaan uit kombinasies van 1-dodekanol, bisfenol A (BPA) en kristal violet laktoon (CVL). Vir die ‘oplosmiddel gefasiliteerde koaksiale elektrospin’-metode, is die saamgestelde termochromiese kleurmiddel opgelos in ‘n gepaste ‘fasiliterende oplosmiddel’ voordat dit geëlektrospin is, eerder as om dit te smelt om dit sodoende in vesels te kan inspin soos beskryf in die literatuur. ‘n Lae raakvlakspanning tussen die kern- en skilspinvloeistowwe, wat voordelig is vir doeltreffende kern insluiting, is bereik deur gebruik te maak van sorgvuldig verkose ‘fasiliterende oplosmiddels’ vir die kern. Die PMMA is opgelos om die skilspinoplossing te vorm en, deur die keuse van die korrekte kern- en skiloplosmiddels, kon spinneret blokkasie en neerslag van die polimeer as gevolg van kern- en skilvloeistofinteraksies elimineer word. Hoë molekulêre massa PMMA is gebruik om vesels te vervaardig met deursnee in die omtrek van 3–10 μm (groter as tipiese elektrogespinde vesels) om sodoende lig-verstrooiing te verminder en daardeur visuele waarneming van die termochromiese oorgange moontlik te maak, in teenstelling met die vesels wat in die literatuur gevorm is. Die vesels is ge-analiseer met SEM, TEM, TGA en DSC om veselmorfologie, termiese omskakelinge van saamgestelde kleurmiddels en veselsamestelling te bestudeer. Fisiese en chemiese interaksies tussen die saamgestelde termochromiese kleurmiddel kern en die PMMA skil is geïdentifiseer as moontlike oorsake van verskille tussen die termochromiese oorgangstemperature van die kern-skil vesels en die kleurmiddels in grootmaat, asook van die onstabiliteit van die gekleurde toestand van sommige termochromiese vesels. Die ruimtelike inperking van die saamgestelde kleurmiddel binne in die vesels asook die beduidende volume verkleining (van grootmaat kleurmiddel tot klein volume binne in die vesels) het die termochromiese gedrag van die saamgestelde kleurmiddel binne die vesels beïnvloed. ‘n Oormaat BPA is in die saamgestelde termochromiese kleurmiddel gebruik om die produksie van omkeerbaar termochromiese vesels met ‘n stabiele gekleurde toestand toe te laat.

Oxazoline directed lithiation of Calix[4]arene and Ferrocene

Herbert, Simon Anthony 12 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The use of chiral oxazoline directed lithiation provides a highly diastereoselective (up to >99% de) route to meta functionalised inherently chiral calixarenes. This methodology can be used on both the butylated and debutylated calixarene systems and is tolerant of a wide range of different electrophillic quenches allowing access to a structurally diverse range of inherently chiral metafunctionalised calixarenes. The oxazoline directing group can be removed via hydrolysis, generating a range of functionalised calixarene carboxylic acids in high ee. We also demonstrate that the use of derivative alkyllithiums such as cyclopentyl lithium can provide significantly enhanced diastereoselectivity over the conventional organolithiums such as sec-butyl lithium, when employed in ortholithiation reactions of this nature. The differences in diastereoselectivity associated with the different alkyllithiums can be tied, in certain cases, to the steric bulkiness associated with the individual reagents. In this regard we have found that the use of the so called Tolman angle or cone angle approach allows quantification of the relative steric bulk of the alkyllithium. We also detail that the oxazoline directing group provides a hitherto unknown ability to be diastereoselectively tuned through the choice of the ligand system in the ortholithiation reaction. In this regard the development of a series of diglyme based ligands have proved to provide a highly diastereoselective means of inverting the chirality from that which the use of the conventional TMEDA ligand is able to generate (up to –92% de). The use of diglyme ligands to invert the sense of chirality is also shown to occur on the ferrocenyloxazoline system and presents an apparently general and hitherto unknown facet of asymmetric oxazoline directed ortholithiation. This diglyme induced inversion has been shown to be controlled through a secondary nitrogen coordinated mechanism that is able to operate with chiral oxazolines. / AFRIKAANSE OPSOMMING: Die gebruik van chirale oksasoliengerigte litiëring verskaf ’n hoogs diastereoselektiewe (tot en met >99% do) roete om metagefunksionaliseerde, inherente chirale calixareen produkte te sintetiseer. Deur gebruik te maak van verskillende elektrofiele kan die metodologie toegepas word op beide gebutileerde en de-gebutileerde calixareen sisteme om ’n reeks uiteenlopende inherente chirale, meta-gefunksionaliseerde calixareen produkte te vorm. Die oksasolien groep kan daarna verwyder word deur hidroliese om ’n reeks gefunksionaliseerde calixareenkarboksielsure te vorm in baie hoë eo. Ons het ook gedemonstreer dat die gebruik van afgeleide alkiel-litiums, soos siklopentiel-litium, kan bydrae tot aansienlik verhoogde diastereoselektiwiteit as dit vergelyk word met meer algemene organolitiums soos sekbutiellitium, tydens ortolitiëring reaksies van hierdie natuur. Die verskille in diastereoselektiwiteit kan verbind word, in sekere gevalle, tot die steriese bonkigheid van die individuele reagense. Deur gebruik te maak van die sogenaamde Tolmanhoeke of die koniesehoek benadering is dit moontlik om die relatiewe steriese bonkighied van alkiellitiums te kwantifiseer. Daar was ook bepaal dat die oksasoliengroep die ongekende vermoë besit om die diastereoselektiwiteit van die produk te stem deur die keuse van verskillende ligand sisteme tydens die ortolitiëring reaksie. Daar was bepaal dat die chiralitiet van die produkte omgekeer kan word op ’n hoogs diastereoselektiewe manier, deur gebruik te maak van ’n reeks ontwikkelde diglymegebaseerde ligande, indien dit vergelyk word met die produkte wat deur die konvensionele TMEDA gegenereer was (tot en met –92% do). Die gebruik van diglyme ligande was ook getoets op ferroseenoksasolien sisteme en dit was bevind dat dieselfde omkering in chiraliteit ook plaasvind wat aanleiding kan gee tot 'n oënskynlik algemene en tot nou toe onbekende faset van asimmetriese oksasoliengerigte orto-litiëring. Dit is bepaal dat hierdie diglyme geïnduseerde omkering in chiraliteit beheer word deur middel van 'n sekondêre stikstofgekoördineerde meganisme, wat in staat is om saam te werk met chirale oksasoliene.

Novel electrospun fibres of amphiphilic organic-inorganic graft copolymers of poly(acrylonitrile)-graftpoly( dimethylsiloxane) for silicone composite reinforcement

Bayley, Gareth Michael 12 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Novel silicone nanocomposites were prepared using poly(acrylonitrile) (PAN) based reinforcing fibres as well as multi-walled carbon nanotubes (MWCNTs). Compatibility of the fibre fillers with the silicone matrix required the synthesis of novel amphiphilic, organic–inorganic graft copolymers of PAN and poly(dimethylsiloxane) (PAN-g-PDMS). These fibre precursor materials were synthesised via the “grafting through” technique using conventional free radical copolymerisation. The PDMS macromonomer content in the feed was varied from 5 wt% to 25 wt% and the molecular weights of the macromonomer were 1000 g.mol-1 and 5000 g.mol-1. The solvent medium of the precipitation reaction was optimised at a volume ratio of 98% benzene to 2% dimethylformamide (DMF). Successful incorporation of PDMS yielded graft copolymer blend materials of PAN-g-PDMS, blended with PAN homopolymer and unreacted PDMS macromonomer. A gradient elution profile was developed to track the successful removal of the PDMS macromonomer via hexane extraction. The gradient profile showed that as the PDMS content in the feed increased, the number of graft molecules in the blend increased relative to the number of PAN homopolymer molecules. The crystallisability of the PAN segments was shown to decrease as the PDMS content increased. The synthesised polymer was used as precursor material for the electrospinning of fibre fillers. The electrospinning of the precursor material was successfully achieved using 100% DMF as electrospinning solution medium. The amphiphilic nature of the precursor material in DMF resulted in self-assembled aggregate structures in the electrospinning solution. An increasing PDMS content was shown to affect the aggregation of the precursor material, and resulted in an increase in the solution viscosity. The “gel-like” solutions limited the achievable fibre morphological control when altering conventional electrospinning parameters such as voltage, tip-to-collector distance, and solution concentrations. The rapid evaporation and stretching of the solution during electrospinning, combined with the phase segregated amphiphilic molecules in solution and the crystallisation of the PAN segments resulted in (non-equilibrium morphology) fully porous fibres. The crystallinity was shown to decrease after electrospinning of the fibre precursor materials. Successful incorporation of surface oxidised MWCNTs into the electrospun fibres was achieved. The content of nanotubes was varied from 2 wt% to 32 wt%. The MWCNTs reduced the mean fibre diameters by acting as cross-linkers between the PAN segments and increasing the solution conductivity. The nanotubes dispersed well throughout the porous structure of the fibres and aligned in the direction of the fibre axis. Fabrication of silicone composites containing nonwoven and aligned fibre mats (with 8 wt% MWCNTs in the fibres, and without) was successfully achieved. The compatibilisation of the PDMS surface segregated domains allowed excellent dispersion and interaction of the PAN based fibre fillers with the silicone matrix. Mechanical analysis showed improved properties as the PDMS content in the fibre increased. The highest PDMS content fibres did, however, exhibit decreased properties. This was ascribed to increased PDMS (soft and weak) content, decreased crystallinity and increased fibre diameter (lower interfacial area). Dramatic improvements in strength, stiffness, strain and toughness were achieved. The most significant result was an increase in strain of 470%. The mechanical results correlated with results of SEM analysis of the fracture surfaces. The dramatic improvements in properties were a result of the fibre strength and ductility, as well as the mechanism of composite failure. / AFRIKAANSE OPSOMMING: Nuwe silikonnanosamestellings is berei deur gebruik te maak van poli(akrilonitriel) (PAN) gebaseerde versterkende vesels wat multi-ommuurde koolstof nanobuisies bevat het. Versoenbaarheid van die vesels met die silikonmatriks het die sintese van nuwe amfifiliese, organies–anorganiese ent-kopolimere van PAN en poli(dimetielsiloksaan) (PAN-g-PDMS) benodig. Die vesel voorlopermateriaal is deur middel van ‘n “ent-deur” vryeradikaalkopolimerisasie gesintetiseer. Die inhoud van die PDMS makromonomeer in die reaksie het gewissel vanaf 5% tot 25%. Die gebruik van twee verskillende molekulêre massas makromonomere is bestudeer (1000 en 5000 g.mol-1). Die optimale oplosmiddelmengsel vir die neerslagreaksie was 'n volume verhouding van 98% benseen tot 2% dimetielformamied (DMF). Suksesvolle insluiting van PDMS het versnitmateriale van PAN-g-PDMS kopolimere gemeng met PAN homopolimere en ongereageerde PDMS makromonomere gelewer. 'n Gradiënteluering- chromatografiese profiel is ontwikkel om die suksesvolle verwydering van die PDMS makromonomere via heksaanekstraksie te bepaal. Die gradiëntprofiel het aangetoon dat indien die PDMS inhoud in die reagense verhoog is, die aantal entmolekules relatief tot PAN homopolimeermolekules ook verhoog het. 'n Toename in PDMS inhoud het egter 'n afname in kristallisasie van die PAN segmente tot gevolg gehad. Die gesintetiseerde polimeer is gebruik as die beginmateriaal vir die elektrospin van veselvullers. Die elektrospin van die beginmateriaal was suksesvol wanneer 100% DMF as elektrospinoplosmiddel gebruik is. Die amfifiliese aard van die beginmateriaal in DMF lei tot outokonstruksie van aggregaatstrukture in die elektrospinoplossing. Toenemende PDMS inhoud beïnvloed die outokonstruksie van die molekules in oplossing en het gelei tot 'n toename in die oplossings se viskositeit. Die "gelagtige" oplossings beperk die haalbare vesel se morfologiese beheerbaarheid wanneer konvensionele elektrospin parameters soos elektriese spanning, punt-tot-versamelaar afstand, en oplossingkonsentrasies gewysig word. Die vinnige verdamping en strek van die oplossing tydens elektrospin, gekombineer met die fase-geskeide amfifiliese molekules in oplossing en die kristallisasie van die PAN segmente, het gelei tot (nie-ewewig morfologie) volledige poreuse vesels. Die kristalliniteit van die veselbeginmaterial het afgeneem nadat elektrospin toegepas is. Die insluiting van die oppervlak-geoksideerde multi-ommuurde koolstof nanobuisies in die elektrogespinde vesels was suksesvol. Die inhoud van die nanobuisies het gewissel van 2 wt% tot 32 wt%. Die MWCNTs het die gemiddelde veseldeursnit verminder deur op te tree as kruisbinders tussen die PAN segmente van die molekules. Die nanobuisies was goed versprei deur die poreuse struktuur van die vesels en dit was gerig in die rigting van die vesel-as. Bereiding van die silikonsamestellings bestaande uit nie-geweefde en gerigte veseloppervlakke (met en sonder 8 wt% multi-ommuurde koolstof nanobuisies in die vesel) was suksesvol. Die versoenbaarheid tussen die oppervlak van die PDMS-geskeide gebiede en die silikonmatriks laat uitstekende verspreiding en interaksie van die PAN-gebaseerde veselvullers met die silikonmatriks toe. Meganiese analise het aangetoon dat die fisiese eienskappe verbeter het namate die PDMS inhoud in die vesel vermeerder het. Die vesels met die hoogste PDMS inhoud het egter verswakte eienskappe getoon. Dit is toegeskryf aan ‘n verhoogde PDMS inhoud (sag en swak), ‘n afname in kristalliniteit en ‘n verhoogde veseldeursnit (laer grensoppervlakke). Dramatiese verbeterings in sterkte, styfheid, verlengbaarheid, vervorming en taaiheid is bereik. Die mees betekenisvolle gevolg was 'n toename in die verrekking van 470%. Die meganiese resultate is gekorreleer met SEM ontleding van die brekingsoppervlakke. Die veselkrag en vervormbaarheid, sowel as die meganisme van die splyting van die samestellings, het tot die dramatiese verbeterings in die meganiese eienskappe gelei.

Synthesis and characterization of multiphase copolymers

Elhrari, Wael K. S. 12 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Multiphase copolymers generally consist of copolymers where the disparate natures of each of the segments results in complex phase-segregated morphologies in the solid state. The outstanding properties and wide range of applications of multiphase copolymers has led to the need for more sophisticated synthesis methods to produce copolymers with controlled structures. Associated with developments in synthetic methods is the need to develop suitable techniques to characterize these materials in order to obtain a better understanding of their structure–property relationships. The synthesis of multiphase copolymers presents many challenges. These are related to the nature of the molecular requirements, were the monomers of each of the different components may not be polymerized by all available polymerization techniques. This has led to the need to combine different polymerization techniques to overcome such limitations. The focus of this study is the combination of living controlled polymerization techniques, namely anionic polymerization and RAFT polymerization, with hydroboration/autoxidation, to produce non-polyolefin block and graft copolymers. Block copolymers were synthesized by coupling anionic polymerization and hydroboration/autoxidation reactions. The first block segment was prepared via anionic polymerization, and then end-functionalized with a suitable functional group (e.g. an allyl group). A hydroboration/autoxidation reaction was then used to initiate the polymerization of the second block by the slow addition of oxygen at room temperature. Graft copolymers were synthesized using the 'grafting from' technique, by coupling RAFT copolymerization with hydroboration/autoxidation reactions. The backbone polymer was synthesized via RAFT copolymerization of symmetric and asymmetric monomer, after which a hydroboration/autoxidation reaction was carried out to produce graft copolymers. The hydroboration/hydroxylation reaction could also be used to modify an unsaturated polymer chain. The EPDM rubber chain was modified by transforming the double bond into an hydroxyl group, which could undergo an esterification reaction with an acid chloride RAFT agent to produce the multifunctional RAFT polymer. This was used for the controlled living free radical polymerization of the graft chains. Significant amounts of homopolymerization in addition to graft formation were obtained. Solid state NMR (SS NMR) and positron annihilation lifetime spectroscopy were used to determine the compositional phase segregation point in the graft copolymers. The spin diffusion data from the SS NMR provided insight into the seemingly anomalous positron data at the phase segregation point. It is demonstrated how these two techniques can provide complimentary data on the solid state morphology of these multiphase materials. / AFRIKAANSE OPSOMMING: In die algemeen bestaan multifase kopolimere uit segmente van verskillende aard wat komplekse fase-geskeide-morfologie in die vastetoestand tot gevolg het. Die uitstekende eienskappe en wye reeks toepassings van multifase kopolimere het daartoe gelei dat meer gesofistikeerde sintesemetodes vir die bereiding van kopolimere met gekontrolleerde strukture nodig was. Gepaardgaande met verwante ontwikkelings op die gebied van sintesemetodes was dit nodig om gepaste analitiese tegnieke te ontwikkel vir die karakterisering van hierdie verbindings, ten einde die struktuur–eienskap verwantskap van hierdie materiale beter te verstaan. Daar is egter baie uitdagings m.b.t. die sintese van hierdie multifase kopolimere. Dit is afhanklik van die aard van die molekulêre vereistes waar die monomere van elk van die verskillende komponente nie deur alle beskikbare polimerisasietegnieke gepolimeriseer kan word nie. Dit het daartoe gelei dat verskillende polimerasietegnieke gekombineer is ten einde hierdie beperkinge te oorbrug. Die fokus van hierdie studie is die kombinering van lewende vry-radikaal gekontrolleerde polimerisasietegnieke, naamlik anioniese polimerisasie en RAFTpolimerisasie, met hidroborering/outoksidasie, om nie-olefiniese blok- en entkopolimere te berei. Blok-kopolimere is berei deur die koppeling van anioniese polimerisasie en hidroborering/outoksidasie reaksies. Die eerste bloksegment is berei via anioniese polimerisasie en daarna is endfunksionering met 'n geskikte funksionele groep (bv. 'n allielgroep) bewerkstellig. Daarna is 'n hidroborering/outoksidasie reaksie gebruik om die polimerisasie van die tweede blok te inisieer d.m.v. die stadige toevoeging van suurstof by kamertemperatuur. Entkopolimere is berei deur gebruik te maak van die 'ent-vanaf' tegniek, d.m.v. die koppeling van RAFT-kopolimerisasie met hidroborering/outoksidasie reaksies. Die rugraatpolimeer is berei d.m.v. kopolimerisasie van simmetriese en nie-simmetriese monomere waarna die hidroborering/outoksidasie reaksie uitgevoer is om sodoende entkopolimere te vorm. Die hidroborering/hidroksilasie reaksie kon ook gebruik word om 'n onversadigde polimeerketting te wysig. Die EPDM rubberketting is gewysig deur die omskakeling van die dubbelbinding in 'n hidroksielgroep, wat dan 'n esterifikasie reaskie kon ondergaan met 'n suurchloried-RAFT verbinding, om sodoende die multifunksionele RAFT-polimeer te vorm. Dit is gebruik vir die gekontrolleerde lewende vry-radikaalpolimerisasie van die entkettings. Behalwe entvorming is 'n hoë mate van homopolimerisasie waargeneem. Vastetoestand KMR (VS KMR) en positronvernietigingsleeftydspektroskopie is gebruik om die saamgestelde faseskeidingspunt in die entkopolimere te bepaal. Die spindifffusie data van VS KMR het insig verleen aan die oënskynlik onreëlmatige positrondata by die faseskeidingspunt. In die studie is bewys hoe hierdie twee tegnieke komplimentêre data kan lewer m.b.t. die vastetoestandmorfologie van hierdie multifase materiale.

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