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A study of doubly crosslinked microgels and their composites
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
This thesis presents a study of pH-responsive doubly crosslinked microgels (DX MGs) and associated investigations to enhance their performance. The potential application of this material is soft tissue engineering, so the research concerns mechanical properties, other properties like swelling, microporous and conductivity are also discussed. The MG particles are based on poly(EA/MAA/x), where EA is the ethyl acrylate, MAA is the methacrylate acid and x represents the crosslinker. The particles were subsequently functionalised using glycidyl methacrylate (GMA) to introduce vinyl groups in the MG particles. The formation of DX MGs includes a pH triggered swelling of MG particles in the dispersion to form a physical gel and a heat-triggered free-radical reaction to form a covalent hydrogel. The starting point of this study was using graphene oxide (GO) nanosheets to prepare DX MGs composites with a high modulus. We mixed low concentrations of GO with MG particles and formed DX MG/GO gels. Both shear and compressive modulus linearly increased with the concentration of GO, but the ductility of gels was slightly reduced. The moduli for the DX MG/GO gels was increased by a factor of 5 - 6 when only 1.0 wt.% of GO was included.The next study used muti-wall carbon nanotubes (CNTs) which are widely used to prepare electrical conductive composites. A big challenge for applying CNTs is that they easily form large aggregates in water, which was solved by the space-filling and volume excluded properties of MG particles. The ductility of the composite DX gels increased with CNT concentration, as did the modulus. The conductivity of gels significantly increased with the concentration of CNT and they had a very low percolation threshold. The cytotoxic study for the composite gels showed that they were not toxic, so they may be suitable for soft tissue engineering.The effect of crosslinking monomers in MG preparation was studied in the last part of the research. Three types of poly(EA/MAA/x) MGs were studied and compared. The x value was 1 mol. % of divinylbenzene (DVB); 1,4-butanediol diacrylate (BDDA) or a 1:1 mixture of both DVB and BDDA. The MGs containing DVB demonstrated higher swelling and more ductile properties and could withstand ~76% of compressive deformation. Moreover, the effects of intra-MG crosslinking of the MGs on the swelling behaviour and the mechanical properties were investigated.