No description available.

This study deals with the effects of branching in some high polymers on viscosity behaviour and on the rela­tionship between intrinsic viscosity and molecular weight. First, the preparation of branched polymers by means of graft polymerization is described. Evidence for the occurrence of grafting, which would result in the growth of polystyrene branches on a min, or backbone, chain com­posed of polystyrene or a copolymer of styrene and li-vinyl-cyclohexene-1, is reviewed, Next, the design, construction, and calibration of a flexible light scattering photometer is described. The performance of this instrument is shown to bs adequate for the determination of the molecular weight and size, as well as the second virial coefficient, of a high polymer in solu­tion. Finally, properties of the graft polymers in solu­tion are compared, with the corresponding properties of linear polystyrene. The relationships found between intrin­sic viscosity and molecular weight, as well as the values obtained for the second virial coefficient, confirm the ex­istence of branching in the graft polymers. On the other hand, it is shown that the viscosity slope constant, Huggins' k', the of which, is often used as an indication of branching, is not affected significantly by the branching present in the graft polymers. / Thesis / Doctor of Philosophy (PhD)

Thesis (PhD (Chemistry and Polymer Science))--Stellenbosch University, 2008. / Polysaccharides were modified with selected polymers via the grafting technique. Both anionic and cationic polysaccharides were prepared. Random and crosslinked graft copolymers were also prepared. The percentage grafting was determined by gravimetric analysis and results were confirmed by cross-polarization magic angle spinning carbon-13 nuclear magnetic resonance microscopy (CP/MAS 13C NMR). These modified biodegradable polymers were then used to flocculate precipitated calcium carbonate (PCC). The effects of pH, percentage grafting, crosslinker concentration and polysaccharide concentration on PCC flocculation were evaluated. Furthermore, the effects of anionic and cationic starch, either added to PCC sequentially or simultaneously, on PCC flocculation were also investigated. Generally, anionically modified starch showed excellent flocculation properties, which are desirable for the end application of PCC retention. The effect of polyacrylic acid (PAA) and polyacrylamide (PAM) modified cellulose fibers on calcium carbonate crystal nucleation and growth modification was investigated. When the heterogeneous crystallization of CaCO3 was carried out in the presence of modified cellulose fibers the CaCO3 crystals were found to be residing on the surface of the fibers. The morphologies of the crystallized CaCO3, polymorph and fiber surface coverage were different for cellulose materials grafted with polymers of different functionalities, meaning that there is interaction between the crystal growth modifier and the growing nuclei. The effect of the modified starch on the crystallization of calcium carbonate gave useful insight into designing CaCO3 filler morphologies. It was found that the filler size, morphology and surface properties of fillers can be tailor-made by choosing suitable CaCO3 crystallization conditions as well as a suitable crystal growth modifier. The crystallized CaCO3 had a negatively charged surface. Results of fluorescence studies showed that the PAA modified starch (polymeric additive used) resided on the surface of the crystals. Thus the presence of the polysaccharide on the surface of a filler could be advantageous for strengthening fiber–filler bonding in paper applications. Anionic starch materials were also used to prepare anionic-starch-coated starch particles. Both the anionic starch and anionic-starch-coated starch particles were evaluated for PCC retention and other properties of hand sheets. When anionic-starch-coated starch particles were used there was generally an improvement in the PCC retention, while the other paper properties remained desirable. The success achieved with the use of anionic-starch-coated starch particles now opens the way for the further preparation and testing of various modified starch particles, for optimization of filler retention.

Conventionally, adherent cells are cultured in vitro using flat 2D cell culture trays. However the 2D cell culture method is tedious, unreliable and does not replicate the complexity of the 3D dynamic environment of native tissue. Nowadays 3D scaffolds can be used to culture cells. However a number of challenges still exist, including the need for destructive enzymes to release confluent cells. Poly(Nisopropylacrylamide) (PNIPAAm), a temperature responsive polymer, has revolutionised the cell culture fraternity by providing a non-invasive means of harvesting adherent cells, whereby confluent cells can be spontaneously released by simply cooling the cell culture medium and without requiring enzymes. While PNIPAAm monolayer cell culturing is a promising tool for engineering cell sheets, the current technology is largely limited to the use of flat 2D substrates, which lacks structural and organisational cues for cells. The aim of this project was to develop a 3D PNIPAAm scaffold which could be used efficiently for non-invasive 3D culture of adherent cells. This project was divided into three phases: Phase 1 (preliminary phase) involved development and characterisation of cross-linked PNIPAAm hydrogels; Phase 2 involved development and characterisation of PNIPAAm grafted 3D non-woven scaffolds, while Phase 3 focused on showing proof of concept for non-invasive temperature-induced cell culture from the 3D PNIPAAm grafted scaffolds. In Phase 1, PNIPAAm was cross-linked with N,N’-methylene-bis-acrylamide (MBA) using solution free-radical polymerisation to form P(PNIPAAm-co-MBA) hydrogels. A broad cross-link density (i.e. 1.1 - 9.1 Mol% MBA) was investigated, and the effect of using mixed solvents as the co-polymerisation medium. The P(PNIPAAm-co-MBA) gels proved unsuitable as a robust cell culture matrix, due to poor porosity, slow swelling/deswelling and poor mechanical properties. Subsequently, in Phase 2, polypropylene (PP), polyethylene terephthalate (PET), and nylon fibers were processed into highly porous non-woven fabric (NWF) scaffolds using a needle-punching technology. The NWF scaffolds were grafted with PNIPAAm using oxyfluorination-assisted graft polymerisation (OAGP). The OAGP method involved a 2 step process whereby the NWF was first fluorinated (direct fluorination or oxyfluorination) to introduce new functional groups on the fibre surface. The functionalised NWF scaffolds were then graft-polymerised with NIPAAm in an aqueous medium using ammonium persulphate as the initiator. Following oxyfluorination, new functional groups were detected on the surface of the NWF scaffolds, which included C-OH; C=O; CH2-CHF, and CHF-CHF. PP and nylon were both easily modified by oxyfluorination, while PET displayed very little changes to its surface groups. Improved wetting and swelling in water was observed for the oxyfluorinated polymers compared to pure NWF scaffolds. PP NWF showed the highest graft yield followed by nylon and then PET. PNIPAAm graft yield on the PP NWF was ~24 ±6 μg/cm2 on grafted pre-oxyfluorinated NWF when APS was used; which was found to be significantly higher compared to when pre-oxyfluorinated NWF was used without initiator (9 ±6 μg/cm2, p= 1.7x10-7); or when grafting was on pure PP with APS (2 ±0.3 μg/cm2, p = 8.4x10-12). This corresponded to an average PNIPAAm layer thickness of ~220 ±54 nm; 92 ± 60 nm; and 19 ± 3 nm respectively. Scanning electron microscopy (SEM) revealed a rough surface morphology and confinement of the PNIPAAm graft layer to the surface of the fibers when oxyfluorinated NWF scaffolds were used, however when pure NWF scaffolds were used during grafting, homopolymerisation was observed as a loosely bound layer on the NWF surface. The OAGP method did not affect the crystalline phase of bulk PP as was determined by X-ray diffraction (XRD), however, twin-melting thermal peaks were detected from DSC for the oxyfluorinated PP and PP-g-PNIPAAm NWF which possibly indicated crystal defects. Contact angle studies and microcalorimetric DSC showed that the PP-g-PNIPAAm NWF scaffolds exhibited thermoresponsive behaviour. Using the 2,2-Diphenyl-1-1-picrylhydrazyl (DPPH) radical method and electron-spin resonance (ESR), peroxides, as well as trapped long-lived peroxy radicals were identified on the surface of the oxyfluorinated PP NWF, which are believed to be instrumental in initiating graft polymerisation from the NWF. A free radical mechanism which is diffusion controlled was proposed for the OAGP method with initiation via peroxy radicals (RO•), as well as SO4•- and OH• radicals, whereby the latter result from decomposition of APS. In Phase 3 of this study, proof-of-concept is demonstrated for use of the PNIPAAm grafted NWF scaffolds in non-invasive culture of hepatocytes. Studies demonstrated that hepatocyte cells attached onto the 3D PNIPAAm scaffolds and remained viable in culture over long periods. The cells were released spontaneously and non-destructively as 3D multi-cellular constructs by simply cooling the cell culture medium from 37°C to 20°C, without requiring destructive enzymes. The PP-g- PNIPAAm NWF scaffolds performed the best in 3D cell culture. Additionally the CSIR is developing a thermo responsive 3D (T3D) cell culturing device, whereby the 3D thermo responsive NWF scaffolds are used in the bioreactor for cell culture. Temperature-induced cell release was also verified from the 3D Thermo responsive scaffolds in the bioreactor. This technology could lead to significant advances in improving the reliability of the in vitro cell culture model. Please cite as follows: Chetty, AS 2012, Thermoresponsive 3D scaffolds for non-invasive cell culture, PhD thesis, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-06112013-151344/ > D13/4/713/ag / Thesis (PhD)--University of Pretoria, 2012. / Chemical Engineering / unrestricted

<p>Liquid Chromatography (LC) and Capillary Electrophoresis (CE) are modern analytical techniques that play very important roles in many areas of modern science such as life science, biotechnology, biomedicine, environmental studies, and development of pharmaceutics. Even though these two techniques have existed and been subjected to studies for several decades, the developments of new separation materials for them are still very important till now in order to meet the different new demands for improvement from other disciplines in science.</p><p>In this doctoral thesis, several novel covalently bonded sulfobetaine type zwitterionic separation materials are synthesized for the application in LC and CE. These materials carry both positively charged quaternary ammonium groups and negatively charged sulfonic groups, which result in a very low net surface charge compared to conventional separation materials with only anionic or cationic functional groups. Consequently, it is possible to employ these materials for separation of different ionic species under mild conditions. The surface properties have also been characterized, mainly by elemental analysis, sorption isotherm, ζ-potential measurements, and spectroscopic methods.</p><p>By using packed zwitterionic columns for liquid chromatography, small inorganic anions or cations, and acidic or basic proteins can be independently and simultaneously separated in a single run using optimal sets of separation conditions. This is a unique property compared to conventional ionic separation material for LC. When fused silica capillaries coated with zwitterionic polymer are used for capillary electrophoresis, good separations can be achieved for solutes as different as inorganic anions, peptides, proteins, and tryptically digested proteins.</p>

Liquid Chromatography (LC) and Capillary Electrophoresis (CE) are modern analytical techniques that play very important roles in many areas of modern science such as life science, biotechnology, biomedicine, environmental studies, and development of pharmaceutics. Even though these two techniques have existed and been subjected to studies for several decades, the developments of new separation materials for them are still very important till now in order to meet the different new demands for improvement from other disciplines in science. In this doctoral thesis, several novel covalently bonded sulfobetaine type zwitterionic separation materials are synthesized for the application in LC and CE. These materials carry both positively charged quaternary ammonium groups and negatively charged sulfonic groups, which result in a very low net surface charge compared to conventional separation materials with only anionic or cationic functional groups. Consequently, it is possible to employ these materials for separation of different ionic species under mild conditions. The surface properties have also been characterized, mainly by elemental analysis, sorption isotherm, ζ-potential measurements, and spectroscopic methods. By using packed zwitterionic columns for liquid chromatography, small inorganic anions or cations, and acidic or basic proteins can be independently and simultaneously separated in a single run using optimal sets of separation conditions. This is a unique property compared to conventional ionic separation material for LC. When fused silica capillaries coated with zwitterionic polymer are used for capillary electrophoresis, good separations can be achieved for solutes as different as inorganic anions, peptides, proteins, and tryptically digested proteins.

No description available.

Controlled radical polymerization (CRP) can be viewed as the middle ground between living anionic polymerization (LAP) and conventional free radical polymerization (FRP). It combines the precise control over polymer structure offered by LAP, under a tolerant reaction condition similar to FRP. One of the most studied CRP is atom transfer radical polymerization (ATRP), with over 10,000 papers published since its introduction in 1995. Despite the numerous studies, knowledge on its fundamental mechanism is still lacking, as evident from the lack of expression for full MWD and polydispersity that account for termination reaction. Since termination is unavoidable in ATRP, the existing expressions give inaccurate predictions as dead chains accumulate. In this study, we derived expressions for full MWD at low conversion and for polydispersity. These expressions allow us to quantify and gain better understanding on the contribution of termination. In addition, the resulting polydispersity expression shows better agreement than the existing equation when correlated with experiment data. In addition to the aforementioned questions, there are also controversies regarding the kinetics of surface-initiated ATRP, with researchers divided into two schools of theories. We evaluated the validity of these theories by comparing their predictions to experimental trends. Both theories were found to be inadequate in explaining all the experimental observations, thus triggering an investigation of the graft density. Graft density is an important determining property for polymer brushes, yet little is known about what affects its final value. Through simulations, we investigated the effect of experiment factors on the grafting density. A decrease in the amount of deactivator is found to decrease the grafting density, which could be explained by an increase in the number of monomers added per activation cycle. This knowledge allows us to explain the conflicting experiment observations regarding the growth trends of polymer layers reported in the literatures. / Thesis / Doctor of Philosophy (PhD) / Polymer materials are used almost everywhere in our daily life from clothing to water bottle. This wide range of applications owes to the nearly infinite possible properties that polymer can possess. Different polymerization processes to synthesize polymers have their own weaknesses and strengths. Herein we investigated the fundamental mechanism of one of the currently most attractive polymerization systems, atom transfer radical polymerization (ATRP). This process allows the synthesis of polymers with precisely tailored chain microstructures, making it possible to create polymer with sophisticated properties. Using modeling approaches, we derived explicit expressions for determining chain properties, allowing detailed investigation of how various factors affect these properties. Through these investigations, we obtained better understanding on the mechanism of ATRP in solution and on surface. This knowledge is crucial in providing insight and guiding experimental designs for better control over the material properties.

Les maladies ostéoarticulaires représentent environ 10% de l'ensemble des pathologies identifiées en France chaque année. Pour l'instant aucun traitement permettant la réparation du tissu cartilagineux n'est vraiment disponible, hormis la pose d'un implant articulaire. Mais, malgré de nombreux efforts pour développer de nouveaux matériaux pour les implants articulaires leur durée de vie in vivo s'avère souvent très décevante par rapport aux extrapolations faites à partir de simulations ex-vivo. Les discordances entre les durées de vie in vivo et ex vivo sont principalement imputées aux conditions d'essais ex vivo insuffisamment réalistes vis-à-vis des propriétés physico-chimiques des lubrifiants biologiques. Dans ce contexte, ce travail vise à agir sur la réactivité physicochimique des surfaces frottantes des implants articulaires en UHMWPE afin de maîtriser l'accrochage des molécules lubrifiantes de type phospholipidique et ainsi d'augmenter leurs performances tribologiques. Les résultats montre que l'activation physichochimique des surfaces de UHMWPE par des couche de MPC peut diminuer l'usure des surfaces polymères d'implant mais cela nécessite un contrôle de la qualité de la couche MPC greffée (densité surfacique, épaisseur, accrochage chimique, adsorption physico-chimique) afin de garantir une bonne tenue mécanique et tribologique. D'autre part il a été montré que la présence de lubrifiant biologique (substitut du fluide synovial à base de liposomes) réduit l'usure des surfaces de UHWPE même si la couche de MPC est peu dense et peu épaisse