Browsing by Author "Murima, Douglas"
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- ItemDevelopment of advanced methods for the analysis of star and highly branched polymers(Stellenbosch : Stellenbosch University, 2020-03) Murima, Douglas; Pasch, Harald; Stellenbosch University. Faculty of Science. Dept. Chemistry and Polymer Science.ENGLISH ABSTRACT: Branched polymers are a unique class of materials with broad applications in fields such as automobile parts, textiles, lubricants, packaging, medicine, construction, soil binders in agriculture, and so on. Much of their attractive properties when compared to their linear counterparts are a result of variations in chemical composition, molar mass and architectural distributions. Branching in polymers may exist naturally (e.g. polysaccharides, natural rubber) or deliberately induced (e.g. in synthetic rubber and packaging materials from polyolefins, rheology modifiers). Advances in the synthesis of branched polymers with complex architectures for various targeted applications have continued to add to the growing need of more advanced characterization methods for structure-property correlations. In this current work, the development of various advanced methods for separation and characterization of lowly and highly branched polymers with distributions that include molar mass, degree of branching and chemical composition were investigated. Monodisperse star-branched polystyrenes with varying numbers of arms (3, 4, and 6 arms) provided a clean model systems for the analysis of crude star-branched polybutadienes with 3 and 4-arms, miktoarm-polystyrene-polyisoprene, and then a more complex hyperbranched Bd-PS. The first experimental part described the preparation of 3-arm, 4-arm and 6-arm star polystyrenes from multi-armed RAFT agents. Here, for the first time, the comprehensive characterization of star functionality (number of arms) via triple detector SEC and correlating with average functionalities determined by offline 1H-NMR and comparing the resolution of different chromatographic techniques was presented. The second part extended the comparison between the separation capabilities of HPLC in both 1D and 2D modes to resolve highly branched block copolymers. The novelty of the work was highlighted regarding the comprehensive analysis of hyperbranched block copolymers using liquid chromatographic techniques in combination with spectroscopic techniques and remarkable correlations were established. The third experimental part of this work discussed, for the first time, the application of multidetector Thermal field-flow fractionation as a channel-based separation technique for determining polymer conformations from Mark-Houwink plots and the degree of branching using functionality plots for a set of 3-, 4-, and 6-arm star polystyrenes, and a more complex hyperbranched polybutadiene-polystyrene (PBd-PS) copolymer in a single experiment.
- ItemNovel barrier coatings based on nanoclay-polymer composites(Stellenbosch : Stellenbosch University, 2015-04) Murima, Douglas; Pasch, Harald; Hartmann, Patrice; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The investigation of the barrier properties of highly filled polymer-clay hybrid latex films is described. Montmorillonite (MMT) clay contents ranging from 10–30 wt.% were effectively incorporated into polystyrene-butyl acrylate (PSBA) random copolymers, via miniemulsion polymerization. The optical properties of the films were evaluated using UV-Vis spectroscopy. Compared to the neat films, the PSBA nanocomposites retained remarkable visual properties. The light transmittance for PSBA films with styrene/n-butyl acrylate (S/BA) comonomer contents of 40:60 and 50:50 (mol.%) only decreased from 70% in the neat films to 50% in the nanocomposite films containing 30 wt.% clay. The best optical properties were observed in the films with S/BA comonomer contents of 30:70 (mol.%), the light transmittance only decreased from 85% (neat film) to 60% in the nanocomposite films containing 30 wt.% clay. The improved optical properties for the PSBA-30:70 films (compared to the PSBA-40:60 and PSBA-50:50 counterparts) were attributed to an increase in the low UV-absorbing butyl acrylate component of the copolymer, which at the same time has a low Tg that probably facilitated dispersion of the rigid MMT platelets in the matrix. In this study, the overall water vapour transport behaviour was governed by the MMT clay presence and less affected by the copolymer composition variation. The lower diffusion coefficients in the polymer clay nanocomposites (PCNs) were a result of the impermeable clay platelets which forced the water vapour molecules to follow longer and more tortuous paths to diffuse through the nanocomposite films. The irregular shape in the PSBA-40:60 and PSBA-30:70 neat latex particles was lost in the hybrid particles and well defined, dumb-bell shaped particles were observed. This was because of the faceting effect of the rigid MMT clay platelets. The MMT clay platelets were predominantly adhered to the surface of the PSBA latex particles because MMT clay particles have a larger size than the effective size of the copolymer particles. The stable overall transport coefficients in the PSBA-30:70-MMT films were attributed to the morphological organization of clay platelets in the matrix. The storage modulus of the materials decreased with an increase in clay content. This was attributed to the dual role played by the organoclay, firstly as nanofiller and reinforcing agent leading to the increase in storage modulus, and secondly as a plasticizer leading to a decrease of storage modulus.