Doctoral Degrees (Chemistry and Polymer Science)
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- ItemDeveloping marker technologies to probe complex polymers and products(Stellenbosch : Stellenbosch University, 2022-11) Liprini, Marehette Suzanne; Van Reenen, Albert; Lutz, Marietjie; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Fluorescent imaging has proven to be a valuable tool across a wide variety of applications. It can help with detecting, locating and observing the interactions between different molecules. This study used fluorescence to evaluate interactions, provide detection and trace different components within complex polymer systems. Cellulose nanowhiskers (CNW) were isolated and characterized, then labelled using fluorescent labelling with fluorescein 5(6)-isothiocyante (FITC) and rhodamine B (RhB). The attachment of the fluorescent dyes was easily observed with fluorescence microscopy. Fluorescently labelled cellulose nanowhiskers (CNW) were incorporated into different crystalline fractions of two different types of polypropylene impact copolymers (CMR 648 and CMR 348). This part of the study focused on the visually assessing the interactions between the different crystalline fractions within polypropylene impact copolymers. The different fractions of the copolymers were first evaluated in the form of solvent-cast films. The solvent-cast films showed strong associations between the 30 oC fraction and the 60 oC fraction, as well as between the 60 oC fraction and the 80 oC fraction. The fluorescent images showed no strong association between the 30 oC fraction and the 80 oC fraction in the absence of the 60 oC fraction. The fraction interactions were also evaluated after mechanical agitation in the melt, followed by injection moulding. The injection-moulded samples showed the same interactions visually as observed with the solvent-cast films. The visual assessment after extracting the labelled 30 oC fraction and 60 oC fraction also showed that the labelled CNW did not migrate out of the fractions into which they were incorporated. The next section of this study focused on the detection of fluorescently labelled markers within paint samples. CNW labelled with fluorescent dyes were incorporated into two types of paint. The paint films were evaluated with confocal fluorescence microscopy and the different paint samples could be identified. The fluorescent markers could still be detected within the paint films even after accelerated weather testing by implementing confocal fluorescence microscopy (CFM), lambda scanning, and unmixing. The last part of the study focused on the release of ethyl formate from different polymer films of polylactic acid (PLA) and polyethylene glycol (PEG). The intrinsic fluorescence associated with the ethyl formate precursor, PLA and PEG was used to track the release of the ethyl formate after exposure to humidity and humidity with citric acid. The release could easily be tracked visually with CFM.
- ItemBonding interactions in congested molecules: a study of the interatomic forces and the molecular electrostatic potential(Stellenbosch : Stellenbosch University, 2022-09) von Berg, Stuart Raymond Colenzo; Dillen, Jan; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The purpose of this research is to apply the quantum theory of atoms in molecules (QTAIM) to the molecular electrostatic potential (MEP) field and use the topology of the MEP to determine whether a stabilising interaction occurs between two hydrogens in a congested molecule. A method for comparing bond strength using the ratio of the nuclear and electronic components of the MEP is developed and applied to the congested molecules. The MEP ratio was used to associate the bond strength of the hydrogen-hydrogen interaction in congested molecules to that of a hydrogen bond between water molecules. Despite this result, analysis of the electron density, laplacian and kinetic energy created an equally compelling argument against the interaction being stabilising.
- ItemPolymer-coated magnetic nanoparticles and polymer nanoparticles for the treatment of Mycobacterium tuberculosis (Mtb)(Stellenbosch : Stellenbosch University, 2022-04) Smit, Marica; Lutz, Marietjie; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Tuberculosis (TB) can be classified as a neglected disease where an estimated one fourth of the world’s population could be infected with Mycobacterium tuberculosis (Mtb) in the form of latent TB. Combinations of three or more anti-tuberculosis (anti-TB) drugs are required during a long treatment period (between 6 months and 2 years) to effectively eliminate Mtb. The long treatment duration with concentrated anti-TB drugs has lead to side-effects, low patient adherence and resulting possible drug resistance. Orally administered anti-TB drugs have difficulty effectively reaching the lung and alveolar macrophages. Concentrated anti-TB drugs are thus orally administered daily in tablet form but anti-TB drug loaded polymer nanoparticles could possibly prevent rapid drug degradation via sustained release. There is thus a need to decrease the necessary concentration of the administered anti-TB drugs, which could be achieved via pulmonary inhalation which directly treats TB in the lungs. Polymer coated superparamagnetic iron oxide nanoparticles (SPMNs) could possibly enable targeted drug delivery via injection. The nanoparticles can be controlled with an external magnetic field to Mtb infected areas, followed by drug release from the anti-TB drug loaded polymer coating. In this thesis, biocompatible polymers namely chitosan, carrageenan, alginate, dextran sulfate and poly(lactide-co-glycolide) (PLGA) were utilized for anti-TB drug loading. Quaternary ammonium chitosan (CS-qC12) and quaternary ammonium poly(styrene-alt- maleic anhydride) (SMI-qC12) were also synthesized due to the known improvement in antimicrobial activity and mucoadhesion, due to the quaternary ammonium functional groups, compared to pristine chitosan and poly(styrene-alt-maleic anhydride). Several commonly administered anti-TB drugs such as isoniazid (INH), rifampicin (RIF), ethambutol (EMB), streptomycin (STM), ethionamide (ETA) and ofloxacin (OFX) were utilized for anti-TB drug loading. Chitosan (CS) based anti-TB drug loaded nanoparticles were synthesized via ionic gelation where polymer and anti-TB drug is dissolved, followed by the addition of crosslinking agent or polymer to prepare the nanoparticles (distributing anti-TB drug throughout the polymer matrix). PLGA nanoparticles were prepared via an “oil-in-water” emulsion followed by solvent evaporation. Sustained drug release (aqueous acetic acid solution, pH 5, UV-Vis spectrophotometry) over 7 days was seen for all the drug loaded nanoparticles, except with ofloxacin loading. The SPMNs were produced via co-precipitating with Fe2+ and Fe3+ in one step (CS SPMNs and CS-qC12 SPMNs) or two steps (chitosan-alginate-carrageenan (CS-Al-Car) SPMNs and chitosan-dextran sulfate (CS-DS) SPMNs). PLGA SPMNs and SMI-qC12 SPMNs were synthesized by activating the pristine iron oxide nanoparticles with oleic acid and (3-aminopropyl)triethoxysilane (3-APTES), respectively, before polymer coating. The polymer coated SPMNs were ex situ drug loaded by dispersing the SPMNs in anti-TB drug solution. Sustained drug release over 8 days was observed for the INH, ETA and RIF loaded polymer coated SPMNs. The resazurin microtiter assay (REMA) against TB mimic Mycobacterium Smegmatis (M. Smeg) was utilized to quantify the antimicrobial activity, via minimum inhibition concentration (MIC) determinations. The CS-DS nanoparticles were determined to be the optimal drug carrier with lower MIC values (CS-DS-OFX = 0.2441 μg/mL) compared to the free drugs (OFX = 0.5859 μg/mL).
- ItemSynthesis and characterization of fluorous-stabilized metal nanoparticles for evaluation in fluorous biphasic catalysis(Stellenbosch : Stellenbosch University, 2022-04) Hensberg, Joshua; Malas-Enus, Rehana; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: A fluorous biphasic approach as a green strategy for the facile recycling and re-use of expensive catalysts, has been probed. A series of fluorous-stabilized Au NPs were successfully synthesized using a micelle-template strategy. The strategy entailed modifying a hydrophilic G3-DAB PPI-NH2 dendrimer to include peripheral palmitoyl groups yielding an amphiphilic unimolecular micelle (referred to as the modified dendrimer in this work). The modified dendrimer was characterized by FT-IR spectroscopy and 1H NMR spectroscopy; and displayed complete solubility in CHCl3. Using the modified dendrimer as a template, organic- soluble Au DENs were prepared by the encapsulation of Au ions into the interior of the dendrimer, followed by reduction. These Au DENs were extracted from the organic phase into a fluorous phase (S1 or S2) with the use of fluorous ligands (L1 and L2). This extraction step was found to be the most challenging and much effort was placed on optimizing the extent of extraction into the fluorous phase. In instances incorporating high Au quantities, little or no extraction was observed and was ascribed to the larger size of the Au DENs making phase transfer more unlikely. It was identified that for the formation of small, uniform Au DENs, it was necessary that we identify the maximum quantity of Au ions which could be encapsulated by the dendrimer. Failure to determine this value could lead to overloading the dendrimer and subsequent reduction would form DSNs. For the purposes of this research, it was critical to prepare organic DENs and prevent the formation of DSNs, therefore an additional study was executed to identify the endpoints in a series of UV-Vis spectrophotometric titrations involving the dendrimer and metal salt being investigated. In the study, two dendrimers were investigated and included an unmodified, hydrophilic G3-DAB-PPI-NH2 dendrimer and the aforementioned modified dendrimer. The metal loading capacities of these dendrimers were determined for a range of metal ions in triplicate, which include; Cu(II), Ni(II), Co(II), Zn(II), Cd(II), Pb(II), Ru(III), Rh(III), Pd(II), Pt(II) and Au(III). The results showed that the unmodified dendrimer, in most cases, housed fewer metal ions in comparison to the analogous modified dendrimer. This was attributed to the improved solubility of the modified dendrimer in organic solvents. This, in effect, causes the loading interaction to be driven by solubility differences between the hydrophilic interior of the modified dendrimer and the hydrophobic solvent as opposed to fixed stoichiometric ratios. Subsequently, eight unique, spherical, monodisperse and small fluorous-stabilized Au NPs were generated and characterized by UV-Vis spectroscopy, TEM and ICP-OES analysis. Systems incorporating both L1 and L2 as the fluorous stabilizer were produced. Moreover, the use of perfluoro-1,3- dimethylperfluorocyclohexane (S2) provides smaller fluorous-stabilized Au NPs in comparison to the use of perfluoromethylcyclohexane (S1) in the extraction step. It was found that an increase in temperature during the extraction did not aid it but instead promoted the oxidation of the NPs or accelerated agglomeration in the organic phase. Thus it was discovered that the extraction of the organic soluble DENs into the fluorous phase was highly dependent on their size which in turn was dependent on the outcome of the reduction step. An alternative novel synthetic method (called the direct method) was designed and optimized for the preparation of fluorous-stabilized Au NPs stabilized by L1. Not only did this method offer a significantly reduced preparation time, but it also entailed a fluorous-aqueous biphasic reduction to yield the fluourous-stabilized Au NPs. Furthermore, it was shown by way of this method that it was possible to tailor different sizes of NPs by varying the Au: L1 ratio. It was found that increasing the quantity of ligand to gold resulted in smaller fluorous-stabilized Au NPs. The ratios of Au: L1; with Au = 1 eq. and L1 = 0.28 eq.; 0.56 eq.; 1.12 eq. and 2.24 eq. yielded Au NPs of sizes; 43.4 ± 22.2 nm, 17.8 ± 13.7 nm, 11.8 ± 15.8 nm and 2.0 ± 0.3 nm, respectively. From this work, it has been shown that a simple strategy exists to produce fluorous-stabilized Au NPs within 3 h at ambient temperatures using L1. Other attempts were made with the use of other fluorous ligands such as L2, L3 and L4. In these cases, no fluorous-stabilized Au NPs were attained. Ten fluorous-stabilized Au NP catalyst systems were prepared using both methods, using varying ratios of Au: L1 or L2, in S1 or S3, respectively. These systems were assessed as catalysts in the biphasic catalytic oxidation of 1-octene under the optimized catalytic reaction conditions. It was found that all the fluorous-stabilized Au NPs which were examined in these aforementioned experiments, were active in the fluorous biphasic catalytic oxidation of 1-octene. Not only this, but even after recycling up to five times, the catalyst continued to show steady activity and always performed better than the blank reaction (without catalyst) in terms of % conversion of substrate. There appeared to be a distinct relationship between the average particle diameter (nm) of the Au NPs in the system and the conversion of 1-octene. This was demonstrated for JHD12 and JHD16 which comprised the largest NPs and afforded the lowest conversions of 1-octene in relation to the other catalyst systems tested at this constant metal loading (Cf (Au)) of 5 × 10-7 mol/mL and optimized experimental conditions. Although not tested at the same metal loading as the other catalyst systems, JHD15, was found to be the most active catalyst. This is because the catalyst was tested at a concentration ten times less than all the other catalysts and still provided a higher conversion of 1-octene. The high activity was attributed to the size of the Au NPs of JHD15 which were 2.0 ± 0.3 nm being much smaller than those associated with the other catalyst systems. GC-FID was employed to quantify the relevant chemical species after the catalysis runs. The recyclability and re-use of the catalysts was also investigated. In each case the epoxy product was the major product.
- ItemSolid-state interactions of materials: Applications-based solid-state NMR spectroscopy(Stellenbosch : Stellenbosch University, 2022-04) Matthews, Megan Esme; Van Reenen, Albert; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The solid-state interactions of various polymeric materials were investigated in this study to determine how these interactions affect the macroscopic properties of the solid. The work presented in this dissertation was classified into three commercial problems which were solved using solid-state NMR and a variety of other solid-state techniques. First, vis- breaking was performed in-situ on heterophasic ethylene-propylene copolymers (HEPCs) to determine the effect on HEPCs of increasing ethylene contents. Solution 13C NMR and solid- state NMR was used to determine the effect of the peroxide on the structure and morphology of the HEPCs. The extent of vis-breaking was found to be dependent on the homogeneity of chain sequences. In all cases vis-breaking increased the mobility of the crystalline domains forming a disordered crystal phase within the polypropylene α-structure. Second, the possibility of using impact copolymers as the polymer component of hot-melt adhesives (HMAs) was investigated. A high ethylene-content impact copolymer was blended with three types of waxes and characterised by various solid-state techniques. It was found that the ethylene-rich regions of the impact copolymer displayed strong interactions with the waxes and the extent of the interaction was dependent on the composition of the wax. The bond strength results of the HMAs were found to be comparable to that of an industry standard HMA and the failure mechanism was dependent on the wax type and concentration. Finally, the mechanism of oil entrapment by Fischer-Tropsch (FT) waxes was explored to determine whether wideline NMR is an appropriate method for determination of oil content of waxes. Benchtop solid-state NMR instruments apply wideline methods to determine oil contents commercially in cases where vast differences in mobility exist between phases. Blends of a standard oil and waxes of various melting temperatures were prepared and characterised by various techniques. The higher melting FT waxes were found to trap oil in small pools and limit the mobility of the oil making it difficult to detect by solid-state NMR. Linear correlations could only be obtained by using variable temperature methods and by simplifying the oil composition, however, the accuracy of the correlations was not high. The focus of this study was, therefore, not on the properties of a specific material but on the development of solid-state NMR methods with unique applications to solve industrial problems.