Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The work presented in this dissertation describes the use of coaxial electrospinning of two miscible polymer solutions in the preparation of core-shell fibres with a semicrystalline core of poly(L-lactic acid) and an amorphous shell of poly(D,L-lactic acid). Additionally it describes using these fibres in the production of single polymer composite yarns and films. These materials can potentially be used in applications that require biodegradable and fibre-reinforced composite materials e.g. suture yarns. Traditional methods, such as TEM, when used to analyse the core-shell fibre structure were not successful with the polymer system used. A method was developed to validate core-shell fibre structure by selective dissolution of the shell material and correlating analyses of the separated components using differential scanning calorimetry (DSC) and optical rotation. The effects of solution concentration and flow rate on resultant core-shell fibre diameters were investigated. Core-solution flow rate was shown to be the most significant variable affecting the core and core-shell fibre diameters. A thermal processing window exists between the glass transition temperature of the amorphous PDLLA shell material and the crystalline melting temperature of the semicrystalline PLLA core material. Heating these core-shell fibres within this processing window resulted in flow and fusion of overlapping shells of these core-shell fibres at their surfaces while the fibrous core components remained intact and reinforced the resulting composite material. This approach was used to produce single polymer composite yarns and films. Thermal treatments on yarns were shown to increase crystallinity of the cores and therefore the modulus and yield strength of the fibres when compared to untreated yarns. / AFRIKAANSE OPSOMMING: Die werk wat in hierdie tesis voorgelê word, beskryf die gebruik van koaksiale elektro-spinnery van twee wedersydsoplosbare polimeeroplossings in die vervaardiging van kern-skilvesels met ‘n gedeeltelik-kristallyne kern van poli-Lmelksuur (PLLA) en ‘n amorfe skil van poli-D,L-melksuur (PDLLA). Verder beskryf dit die gebruik van hierdie vesels in die vervaardiging van enkel-polimeer saamgestelde-materiaalgarings en -films. Hierdie materiale kan moontlik gebruik word in toepassings wat bio-afbreekbare en veselversterkte saamgestelde materiale vereis, soos byvoorbeeld chirurgiese steke. Die polimeervesels wat in hierdie studie gebruik is, kon nie gekaraktiseer word met behulp van transmissie-elektronmikroskopie (TEM), wat gewoonklik gebruik word in die bestudering van kern-skilvesels, nie. ‘n Alternatiewe karakteriseringsmetode is ontwikkel om te bevestig dat die gevormde vesels wel ‘n kern-skilstruktuur het. Hierdie metode het behels dat die skil van die kern-skilvesel selektief opgelos is, waarna beide komponente (d.i. kern en skilmateriale) bestudeer is met behulp van differensiële skanderingskalorimetrie en optiese rotasie. Die invloed van polimeeroplossingskonsentrasie en –vloeitempo op die gevormde kern-skilvesels is daarna ondersoek en die vloeitempo van die kernoplossing is geïdentifiseer as die prosesinsetwaarde wat die mees beduidende invloed het op die deursnee van die vesels. Daar bestaan ‘n temperatuurinterval tussen die glasoorgangstemperatuur van die amorfe PDLLA skilmateriaal en die kristallyne smeltpunt van die gedeeltelikkristallyne PLLA kernmateriaal. Verwerking van die kern-skilvesels by ‘n temperatuur binne hierdie interval het gelei tot vervloeiing en samesmelting van die oorvleuelende skille van die kern-skilvesels, terwyl die veselagtige kernkomponente van die vesels heel gebly het en opgetree het as versterking binne-in die saamgestelde materiaal wat gevorm het. Hierdie benadering is gebruik om enkelpolimeersaamgestelde- materiaalgarings en –films te vervaardig. ‘n Vergelykende studie tussen onbehandelde en hitte-behandelde enkel-polimeer-saamgesteldemateriaalgarings het aangetoon dat die hittebehandeling gelei het tot ‘n verhoging in die kristalliniteit van die veselkerne en daarom ook ‘n verhoging in die modulus en breeksterkte van die garings tot gevolg gehad het.

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.

Philosophiae Doctor - PhD / The application of porous materials into industrial hydrogen (H2) storage systems is based on their use in combination with high-pressure cylinders. The processing of metal-organic frameworks (MOF) powders into shaped forms is therefore imperative in order to counteract the adverse effects of poor packing of powders in cylinders. The fabrication of shaped MOFs has, however, been shown to be accompanied by compromised properties such as surface areas, gravimetric and volumetric H2 capacities, and also the working/deliverable H2 capacities in comparison to MOF powders.

No description available.

The aim of this study is to demonstrate the possibility of production of nanocables as an alternative to the other one dimensional metal/polymer composite structures like nanowires and nanorods. There is no certain definition of nanocables / however they could be considered as assemblies of nanowires. Nanocable structure can be defined as a core-shell structure formed by a polymeric shell and a metal core that runs continuously within this shell. To produce nanocables, two main steps were carried out. Firstly, monodispersed silver metal nanoparticles to be aligned within the cable core were produced. Investigations on reduction reactions in the presence of strong and weak reducing agents and different capping agents revealed the importance of the kinetics of reduction in the production of monodispersed nanoparticles. Use of capping agents to give a positive reduction potential, resulted in the slow reduction rates that was critical for fine tuning of the final particle sizes between 1-10 nm. Hydrazine hydrate and oleylamine/ oleic acid systems were used as strong and weak reducing agents, respectively. By using weak reducing agent, monodisperse spherical silver nanoparticles with the diameter of 2.7 nm were produced. It was shown that particles with controlled diameter and size distribution can be obtained by tuning the system parameters. Secondly, particles produced as such were electrospun within the core of the polymer nanofibers and long continuous nanocables were produced. Polyvinyl pyrrolidone and polycaprolactone were used in shell part of nanocables. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), photon correlation spectroscopy (PCS), X-ray diffraction (XRD) and surface plasmon resonance spectroscopy (SPR) analyses were carried out in order to understand the mechanism by which the nanoparticles were reduced and for further characterization of the product.

Platelet preparations are a source of various autologous growth factors and have numerous applications in tissues engineering. The aim of this work was to development electrospun nanofiber scaffolds with platelet preparations. Scaffolds based on the adhesion of platelets on nanofiber meshes were developed. The scaffolds were able to enhance chondrocyte proliferation in vitro. The main disadvantage of this system is the burst release of growth factors immediately after adhesion. To overcome this, we developed coaxially electrospun scaffolds with incorporated alpha granules. Alpha granules are novel platelet preparations with high amounts of growth factors. This system was able to stimulate chondrocyte proliferation and maintain TGF- 1 concentrations for 7 days. Additionally, a novel drug delivery system with coaxially incorporated liposomes was developed. Liposomes incorporated into nanofibers remain intact and can be used for the delivery of various molecules. The ability to maintain HRP activity was compared to systems based on coaxial electrospinning with liposomes, coaxial electrospinning without liposomes and blend electrospinning. When compared to other systems, coaxial electrospinning with liposomes preserves enzyme activity twice as long. These results clearly indicate the potential of...

Self-assembly is ubiquitous in nature. A diverse range of materials with exceptional properties are accessed from a limited number of sub-units, through controlling structural order on all length-scales. Achieving the same level of control to access functional materials akin to those in nature is a key challenge in chemistry. Self-assembly of block copolymers (BCPs) offers a valuable bottom-up route, governed by non-covalent interactions, to access ordered assemblies on the nanoscale. Anisotropic nanostructures, such as one- and two-dimensional (1D and 2D) micelle morphologies, are of particular interest for various applications including those in biomedicine, catalysis, optoelectronics, and materials engineering. Crystallization-driven self-assembly (CDSA) of BCPs containing a crystallizable core-forming segment presents a robust route to preparing 1D and 2D micelles. Significantly, the use of pre-existing seed micelles in a process termed living CDSA allows access to 1D and 2D nanostructures of controlled size and low size-dispersity. Although CDSA protocols represent powerful tools for the formation controlled 1D and 2D nanostructures, key challenges associated with scale-up of these processes remain. In most cases, increasing the concentration at which living CDSA is performed results in competitive self-nucleation, compromising micelle size-control and dispersity. Living polymerization-induced crystallization-driven self-assembly (PI-CDSA) has been presented as a promising alternative route to accessing scalable 1D micelles. In this case, the polymerization, self-assembly, and seeded growth of a BCP containing a crystallizable core-forming segment occur in situ. However, the scope of living PI-CDSA is currently limited to the use of polyferrocenylsilane (PFS)-based BCPs. Owing to the diverse range of crystalline core chemistries compatible with CDSA protocols, and therefore various promising applications of 1D and 2D micelles, scale-up is essential to facilitate their further investigation and application. The work presented in this thesis focusses on upscaling the preparation and processing of controlled 1D and 2D micelles with a crystalline core. The scalable preparation of low dispersity 2D platelet micelles by living CDSA of a charge-terminated PFS homopolymer with surfactant counteranions is presented in Chapter 2. Here, fundamental insight into the effects of living CDSA concentration on platelet dimensions, structure fidelity, and aggregation behaviour is provided. In Chapter 3, the scope of living PI-CDSA is extended to access scalable length-controlled low dispersity 1D nanofibers containing a biodegradable poly(fluorenetrimethylenecarbonate) (PFTMC) crystalline core. PFTMC-based 1D fibers are of interest for biomedical applications, hence, in this work, it is demonstrated that living PI-CDSA can be used to prepare fibers exhibiting biologically-relevant lengths at scalable concentrations. In Chapter 4, the scalable formation of low dispersity 1D micelles by living CDSA of a PFS-based BCP in a continuous flow setup is explored. Processing of 1D micelles into microfibers using simple, low cost, and high throughput electrospinning techniques is demonstrated in Chapter 5. Finally, Chapter 6 summarises the contribution of this thesis to improving the scalability of CDSA protocols and provides future directions for this work. / Graduate / 2023-04-12