Doctoral Degrees (Chemistry and Polymer Science)

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Browsing Doctoral Degrees (Chemistry and Polymer Science) by Author "Barnard, Elaine"

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    Synthesis, structure and crystallization behavior of amphiphilic hetero-arm molecular brushes with crystallizable poly(ethylene glycol) and n-alkyl side chains
    (Stellenbosch : Stellenbosch University, 2020-02) Barnard, Elaine; Klumperman, Bert; Pfukwa, Rueben; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: This study describes the synthesis of hetero-arm amphiphilic molecular brushes (AMBs) and their selfassembly into well-ordered nanostructures in the solid and solution-state. Firstly, a series of hetero-arm AMBs were prepared using the grafting through synthetic approach, via conventional free radical terpolymerization of macromonomers containing either poly(ethylene glycol) (PEG) or n-alkyl chains of different lengths, i.e., polar 4-vinyl benzyl-PEG methyl ethers (VB-PEG), tert-butyl diphenyl silyl poly(ethylene glycol) methacrylate (TBDPSi-PEGMA), and apolar N-alkyl maleimides (MI-Cm). By varying the PEG side chain degree of polymerization (DP = 12, 16 and 20) and n-alkyl chain lengths (C10, C12, C16 and C20), AMBs with varying combinations of hydrophilic/hydrophobic side chain lengths were produced. The self-assembly behaviour of these hetero-arm AMBs in arm-selective solvents was investigated in relation to the hydrophilic/hydrophobic side chain lengths, choice of common solvent, and method of solvent switching (dialysis versus evaporation). Well-ordered micro and nanostructures of various morphologies were obtainable, and the assemblies were characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Fluorescence Microscopy. Two fluorescent studies were employed with the first study investigating the possibility of core entrapment of hydrophobic guests, using a fluorescent dye perylene red. A second study investigated the capability of using the azide moieties (N3-PEGMA side chains) as reactive handles for attaching fluorescent conjugates on the hydrophilic chain ends which will subsequently form part of the coronas upon self-assembly in aqueous media. Furthermore, the effect of side chain length on AMB phase behavior, semicrystalline morphologies and crystallization kinetics in the solid state, was elucidated via differential scanning calorimetry, polarized light optical microscopy and x-ray diffraction experiments. SAXS results indicated that all materials exhibit a phase segregated lamellar structure in the melt. Most of the AMB materials prepared were double crystalline, i.e., contained crystals from alkyl and PEG chains. AMB crystallization was constrained by the AMB architecture, the frustration being most evident in AMBs with combinations of either low DPPEG, or short alkyl chain lengths. Large, well-developed spherulites, implying break-out crystallization, were only observed for the AMBs with the combination of the longest PEG chain (DP = 20) and longest alkyl chain length (C20). A peculiar behaviour was found when spherulitic growth rates and overall crystallization rates of the PEG chains, within this particular AMB sample, were determined as a function of crystallization temperature. In both cases, a distinct minimum with decreasing temperature was observed, probably caused by the challenges encountered in crystal packing of the PEG side chains, tethered to an amorphous backbone, which also contained already crystallized C20 chains. This minimum is analogous to that observed in the crystallization of long chain n-alkanes, or high molar mass polyethylenes with bromine pendant groups that has been attributed to a self-poisoning effect; this is the first observation of this phenomenon in AMBs.