Thesis (MSc)--University of Stellenbosch, 2006. / Bibliography. Also available via the Internet.

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Naphthonaphtho[2,1-b:3,4-b']dithiophene is a good electron-rich building block for p-type semiconducting materials and has been extensively studied in our group. Various donor-acceptor type copolymers based on naphtho[2,1-b:3,4-b']dithiophene were designed and synthesized for high-performance bulk heterojunction solar cell applications. The structure-property relationship of these copolymers with respect to the influence of alkyl side-chain position and chain length, fluorination substitution effect as well as acceptor group effect with different electron withdrawing ability were also investigated. The results of the PV device performance based on these copolymers indicated that the device efficiency was sensitive to the position of the alkyl side-chains attached, which could cause non-planarity of polymer backbone. The resulting naphthodithiophene-based copolymer with shorter side-chains afforded higher device efficiency. The incorporation of fluorine atoms into the copolymers leading to the poor solubility caused a decrease in the device performance compared to the non-fluorinated counterparts. The copolymer with thiadiazol[3,4-c]pyridine as an electron-deficient unit also showed promising device performance with efficiency up to 5.10%. Among all the copolymers designed and synthesized, three copolymers namely PNB-4, PNB-C2,6 and PNTP exhibited excellent preliminary device performance with a PCE more than 5.0% showing potential for further device optimization and development.

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Interest in producing hybrid organic-inorganic (HOI) materials has increased rapidly due to the unique combination of properties from the organic and inorganic components. The goal of the research described is to develop various HOI materials and explore their applications in corrosion protection over aerospace aluminum alloys, impact resistant materials, and surface protection over thermoplastic substrates. As a replacement to toxic chromate inhibitors, enrivonmentally friendly magnesium-rich primers (Mg-rich primers) have been investigated to provide corrosion protection over aerospace aluminum alloys. HOI binders were produced from an alkoxy silane and silica via sol-gel chemistry, where the combined organic and inorganic components provide flexibility, adhesion, and barrier properties. The derived topcoated Mg-rich primers showed promising corrosion protection in a salt spray exposure test and are competitive with chromate-containing primers. The condensation catalyst, tetrabutyl ammonium fluoride (TBAF), played an important role in the performance of the HOI binders and the derived Mg-rich primers. It enabled higher crosslink density and better barrier properties, however, reacted with Mg particles during salt spray exposure and caused the formation of blisters. A non-ionic condensation catalyst, dibutyltin dilaurate (DBTDL), had lower catalyst strength, but was expected to eliminate the blister formation of topcoated Mg-rich primers. Perfectly alternating polycarbonate-polydimethylsiloxane (PC-PDMS) multiblock copolymers were produced to create transparent impact resistant materials by confining the size of the rubber domains. The PC-PDMS block copolymers maintained high transparency at up to 62 wt% PDMS and shorter block length gave rise to larger partial miscibility. By incorporating the PDMS blocks to dissipate energy, the PC-PDMS block copolymers had much better impact strength than pure PC oligomers. Thermoset polycarbonate-polyhedral oligomeric silsesquioxane (PC-POSS) coatings were investigated to serve as surface coatings on PC substrate to provide abrasion resistance. The covalent bonding allowed high POSS loading at up to 18 wt% without sacrificing the transparency. The solvent composition and curing conditions largely determined the surface and bulk properties of the coatings. The incorporation of POSS molecules significantly increased the char yield and mechanical strength of the thermoset coatings, making them promising in surface protection applications.

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In this work, we created shear thinning block copolymers that could be potentially utilized as an artificial vitreous replacement. The materials were created using poly(ethylene glycol) (PEG) and silicone polymers, respectively, due to their high biocompatibility. Both the ABA and BAB geometry triblock copolymers were created and were characterized using parallel plate and cone-and plate rheometers. It was observed that the materials from both geometries exhibited a decrease in viscosity with increasing shear rates, thus fulfilling the criteria of being a shear thinning material. The materials were also characterized under different aqueous conditions. It was observed that the materials with a higher PEG composition were better able to retain their physical structure – did not disperse into aqueous solutions – at higher water content levels. The materials that retained their structure were also shown to retain their shear thinning properties. In the absence of solvent, the opacity of the materials increased with increasing PEG composition ratio per copolymer chain. When exposed to different aqueous conditions, the opacity of the materials was found to decrease at specific water concentrations. Materials with larger PEG blocks required a greater water content to exhibit optimal light transmission. / Thesis / Master of Science (MSc)

<P> Polymers are one of the most prevalent types of molecules in modern life. These long macromolecules make up everything from DNA to plastics to Jell-0™. An interesting class of polymers are block copolymers, which are composed of two (or more) chains, or blocks, of chemically distinct monomers covalently bonded end-to-end to form a single polymer. Different types of polymers tend to avoid each other, but since block copolymers are joined together the polymer species can not macroscopically phase separate. Instead, they separate on the scale of the size of the polymers, forming nanostructures. For a diblock copolymer melt, which is made from two types of polymers, these nanostructures can be, depending on the ratio of the length of one block to the other, spheres, cylinders, lamellae, or the more bizarre gyroid phase. </p> <P> Self-consistent field theory (SCFT) as formulated by Helfand in 1975 has in recent years been successfully applied to the study of the phase behaviour of diblock copolymers. However, most of the studies assume that the polymers are monodisperse, while almost all polymer melts are polydisperse. This work examines the effect of polydispersity in the block lengths on phase behaviour of diblock copolymer melts, by developing the SCFT for polydisperse block copolymers. The theory is examined using a perturbation method, as well as the random-phase approximation (RPA). The perturbation parameter is the ratio K of the weight-averaged molecular weight and the number-averaged molecular weight, which is a common measure of polydispersity. </p> <P> The results show polydispersity shifts the transition from a disordered phase to an ordered phase to a higher temperature, and increases the period of the nanostructures. It is also observed that polydispersity leads to larger non-lamellar phase regions in the phase diagrams. Results from the RPA also suggest that macrophase separation occurs for large polydispersities. </p> / Thesis / Master of Science (MSc)