Browsing by Author "Roman, Stacey Kelly"

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    Thermoresponsive hydrogels from poly(ethylene glycol)-based graft poly(dimethyl siloxane) amphiphilic copolymers with tunable lower critical solution temperatures
    (Stellenbosch : Stellenbosch University, 2021, 2021-03) Roman, Stacey Kelly; Mallon, Peter; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: This study aimed to investigate poly(oligo(ethylene glycol) methyl ether methacrylate)-graft-poly(dimethyl siloxane) copolymer materials as potential precursor materials to produce polymer nanofibre hydrogels. This study initially consisted of finding the optimal ratio between OEGMA and EGMA, as the OEGMA by itself was found unsuitable for this particular study. Various factors were taken into account when determining the optimal ratio, such as the physical state and the processing ability of the product. The optimal ratio was determined to be 40 OEGMA:60 EGMA. Thereafter, a comprehensive sample library of poly(OEGMA-co-EGMA)-g-PDMS was successfully synthesised using conventional free radical polymerisation to yield amphiphilic graft terpolymers. Three different molar mass PDMS macromonomers were used, namely 1000, 5000 and 10 000 g/mol. The sample library systematically varied the PDMS content by copolymerisation ratio and graft length. Thus, the terpolymers were investigated to determine the influence of PDMS incorporation as well as the PDMS chain length on the hydrogel properties. The PDMS acts as the physical cross-links between polymer chains via the formation of hydrophobic domains. The presences of these physical cross-links allowed the material to be physically stable in water, where it swelled and prevented complete dissolution. The sample library was characterised using 1H-NMR spectroscopy, ATR-FTIR spectroscopy, SEC, TGA and DSC. The results of this study show that although not all of these materials showed hydrogels with dimensional stability, there is nevertheless a relationship between the swelling ability of the material and the graft content. It was found that the swelling ability of the materials in both water and hexane is dramatically affected by the length of the PDMS graft chain lengths. The longer chain length samples show higher swelling ratios for a given weight percent of PDMS, due to the formation of larger hydrophobic domains. Remarkably, the results show that at a constant 40 OEGMA:60 EGMA ratio the lower critical solution temperatures of the terpolymers could be varied within a 23 °C range from 39 to 62 °C. The mechanical properties of the hydrogel materials were determined using rheology. Interesting results were obtained owing to the varied tunable properties across the entire sample library. A desired product can be tuned by their physical appearance, the mechanical strength, the LCST and the degree of swelling due to the complex amphiphilic solid state morphology. The terpolymer sample library was also electrospun to yield microfibres, however they were not found to be long continuous fibres, but rather moisture sensitive discontinuous fibres especially when compared to that of the hydrophobic copolymer PEGMA-g-PDMS. Unfortunately due to the lack of dimensional stability of these polymers this specific terpolymer series cannot be viably used to produce nanofibres as confirmed by rheology and preliminary investigations using the electrospinning technique.