Masters Degrees (Chemistry and Polymer Science)

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Browsing Masters Degrees (Chemistry and Polymer Science) by Subject "Acrylamide"

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    The synthesis and characterization of films and nanofibers from novel poly(N,N'-dimethyl acrylamide)-graft-poly(dimethyl siloxane) amphiphilic hydrogels
    (Stellenbosch : Stellenbosch University, 2017-03) Wagenaar, Stefan; Mallon, P. E.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: A series of graft copolymers comprised of poly(N,N’-dimethyl acrylamide) (PDMAA) and poly(dimethyl siloxane) (PDMS) were synthesized for the first time by way of free radical polymerization. The copolymer compositions were found to closely match the feed ratios of each reaction. HPLC separation showed that the samples were composed of a hompolymer and copolymer fraction, with the homopolymer fraction rapidly diminishing as the amount of PDMS monomer in the feed was increased. Nanofibres were successfully produced from the material by single needle electrospinning to produce fibres with an average diameter of around 700 nm. Films and nanofibres of the material were then analysed by way of DSC, FTIR, HPLC, SEM, Solid state NMR, TGA and WAXD, as well as conventional swelling studies. The presence of the hydrophobic PDMS grafts was effective in producing a physically crosslinked hydrogel network, as the materials were able to absorb up to 1.6 times their own weight in water and were held together by the hydrophobic clusters. The nanofibres were able to absorb as much as 10 times their weight in water, with water retention undergoing a sharp reduction with increasing PDMS content. Swelling occurred rapidly, with the films reaching around 90% of their equilibrium swelling in less than 5 minutes when immersed in water. SEM images of the nanofibres before and after water exposure revealed that they lacked dimensional stability and lose their fibrous structure when wet. The stability improved with increased PDMS content. The effective crosslinking density of the samples was calculated in order to investigate the effect of the PDMS content on crosslinking density, and it was shown that a non-linear relationship existed between the PDMS content and the resultant swelling of the material. DSC analysis was used to elucidate the water structure inside of the films and fibres, and the relative fractions of freezing and non-freezing water was determined. DSC analysis of dry films showed that the material’s glass transition occurred around 90°C, which is near the expected value for poly(N,N’-dimethyl acrylamide). However, the physical properties more closely resembled that of a rubber-like compound due to the large weight percentage of included PDMS. Determination of the material’s morphology showed that it underwent a morphological transition above a threshold PDMS content, changing from a highly phase segregated system to one with a higher degree of homogeneity. T1ρ relaxation data from solid state NMR showed that the respective values for the PDMAA and PDMS segments differed significantly in the low PDMS content samples, but converged with increasing PDMS content. Additionally, in the nanofibre state it was also found that the samples showed a lower degree of phase separation and that the PDMS grafts were more isolated, which would result in a lower effective crosslinking density. This correlated well with other observed data. SEM images of fracture surfaces from swollen samples revealed island-like structures which are believed to be primarily associated with the PDMS domains. This correlated well with other evidence from solid state NMR and swelling studies. Finally, tensile tests illustrated that the new material had improved physical properties over the homopolymer material.