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Synthesis, Characterisation and Applications of Colloidal Nanocomposite Particles

Fielding, Lee A. (2012) Synthesis, Characterisation and Applications of Colloidal Nanocomposite Particles. PhD thesis, University of Sheffield.

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Abstract

Relatively monodisperse poly(methyl methacrylate)/silica and poly(methyl methacrylate-co-n-butyl acrylate)/silica nanocomposite particles are prepared via in situ aqueous emulsion polymerisation using a 18 nm glycerol-functionalised silica sol and cationic azo initiator at 60 °C. These particles had typical mean weight-average diameters of 140-330 nm and silica contents of up to 39 wt. %. The importance of surface functionalisation of the silica sol is highlighted and systematic variation of parameters such as the initial silica sol concentration and initiator concentration are investigated. The nanocomposite morphology comprises a copolymer core and a particulate silica monolayer shell, as indicated by aqueous electrophoresis, x-ray photoelectron spectroscopy, small-angle x-ray scattering (SAXS) and electron microscopy. Moreover, films cast from poly(n-butyl acrylate)-rich copolymer/silica nanocomposite dispersions are significantly more transparent than those prepared from poly(styrene-co-n-butyl acrylate)/silica nanocomposite particles. A particle formation mechanism is proposed based on the various experimental observations made when periodically sampling such nanocomposite syntheses at intermediate comonomer conversions. Polystyrene/silica nanocomposite particles, which consist of a polystyrene core coated with a monolayer shell of silica nanoparticles, are shown to have an intrinsic density distribution superimposed on the particle size distribution. This density distribution leads to an artificial narrowing of their particle size distribution determined by disc centrifuge photosedimentometry (DCP). A mathematical method to account for this density distribution is developed by reanalysing the raw DCP data so as to calculate the true particle size distribution. Using the mean silica packing density calculated from SAXS, the real particle density can be calculated for each data point. The corrected DCP particle size distribution is both broader and also more consistent with particle size distributions reported for the same polystyrene/silica nanocomposite sample using other sizing techniques, such as electron microscopy, laser light diffraction and dynamic light scattering (DLS). A commercial glycerol-functionalised 18 nm silica sol is used to stabilise Pickering emulsions and form covalently cross-linked colloidosomes. Colloidal core-shell polymer/silica nanocomposite particles produced using this glycerol-functionalised silica are also used to produce colloidosomes comprising hybrid shells. Stable oil-in-water Pickering emulsions required both low pH and the addition of electrolyte and the formation of colloidosomes is achieved by the addition of a polymeric diisocyanate. The oil phase can be removed from the colloidosomes, resulting in microcapsules with either a 18 nm particulate silica shell or a 240 nm polymer/silica shell. These microcapsules can be imaged by optical microscopy in solution and by scanning electron microscopy in the dry state. The permeability of these colloidosomes with respect to small molecule release is also examined. Finally, control experiments performed with a non-functionalised silica sol confirmed that Pickering emulsions cannot be converted into colloidosomes in the absence of surface glycerol groups. Finally, four poly(2-vinylpyridine) (P2VP) latexes with intensity-average mean diameters ranging between 246 and 955 nm were prepared and the adsorption of nano-sized titania particles onto the surface of these latexes from aqueous solution was investigated. The titania particles adsorb strongly at pH 10 and the optimal loading of titania was investigated for each latex. The resulting P2VP-titania nanocomposite particles are characterised in terms of their titania contents, surface coverages, UV-visible absorption spectra and colloidal stabilities. UV-visible spectra were recorded for the titania nanoparticles, P2VP latexes and P2VP-titania nanocomposite particles. For the larger nanocomposite particles, UV-visible absorption was dominated by the latex core, whereas the smaller P2VP-titania particles exhibited UV attenuation to longer wavelengths compared to both the bare latex and the titania particles. On calcination of these nanocomposites, re-dispersion of the hollow titania particles proved ineffective and meaningful UV-visible spectra could not be recorded.

Item Type: Thesis (PhD)
Keywords: colloidal nanocomposite aqueous emulsion polymerisation morphology silica core-shell x-ray photoelectron spectroscopy small-angle x-ray scattering electron microscopy thin films polystyrene poly(methyl methacrylate) silica disc centrifuge photosedimentometry particle size density distribution packing density dynamic light scattering analytical ultracentrifuge Pickering emulsions colloidosomes polymer/silica oil-in-water release poly(2-vinylpyridine) latexes titania particles UV attenuation hollow particles
Academic Units: The University of Sheffield > Faculty of Science (Sheffield) > Chemistry (Sheffield)
Depositing User: Dr Lee A. Fielding
Date Deposited: 29 Oct 2012 16:00
Last Modified: 08 Aug 2013 08:50
URI: http://etheses.whiterose.ac.uk/id/eprint/2859

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