Surface Modification Of Oxide Nanoparticles By Polymers And Its Effect On The Polyacrylate Nanocomposites Coatings

Dispersion of oxide nanoparticles in liquid media is the critical problem of their synthesis and applications. By means of adsorption onto the surface of oxide nanoparticles the added polymer to the colloidal dispersion offers an available method to prevent aggregation. The adsorption of macromolecules provides electrostatic or steric repulsion to stabilize nanoparticles. This dissertation investigated the stabilization mechanism of oxide colloidal dispersion and the effects of macromolecular type and dispersion conditions on the colloidal stability. The polyacrylate nanocomposite coatings were prepared by dispersing oxide nanoparticles in polyacrylate. The thermal degradation, photooxidative degradation and wear mechanisms under water lubrication of nanocomposite coatings were also evaluated.The results of FTIR analysis and adsorption experiments indicated that the non-ionic macromolecule, PEG or PAM, was adsorbed on the oxide surface through hydrogen bonding. Adsorption of anionic PAA and PMAN took place by hydrogen bonding and chemical interaction between the oxide surface and the carboxyl groups of the polymer, and the predominant adsorption attraction depended on the pH of solution. Cationic polymer adsorption of MPTMAC was caused by the strong attractive electrostatic interactions between the negative charges on the oxide particles and the positive ones on the polyelectrolyte. Experiment data and theoretical analysis also provided evidence in support of hydrogen bonding, chemical interaction and electrostatic attraction of in the case of polyampholyte PMAA-b-PDMAEMA and titanium dioxide system.Adsorption behaviours of polymer were discussed in detail, which depended on polymer concentration and its molecular weight, pH, ionic type and strength. Adsorbed amount and adsorption layer thickness presented distinct change as a function of the type of polymer, which may take into account of the change of macromolecular configuration. Polymers exhibited Langmuir adsorption behaviour. Saturated adsorbed amount was found to increase with an increase in polymer molecular weight. For different polymer types, pH values had distinct effects on theadsorption behaviour. Ionic strength had minor effect on the adsorption of non-ionic polymer, while polyelectrolytes were sensitive to the ionic strength and type. The adsorbed amount exhibited a maximum as a function of the electrolyte concentration. The enhanced adsorption in the presence of multivalent ions was ascribed to the bridge between macromolecular chains. The onset in the adsorption behaviours depended on the type of alkali metal counterions, which was reflected by assuming their attractive interaction with the oxide surface.The changes in surface charge density, zeta potential and the structure of electrical double layers were observed after polymer adsorption. The adsorption of non-ionic polymer led to few shift of pH_iep of oxide nanoparticles. However a pronounced shift of pH_iep, which depended on polymer concentration and its molecular weight, was observed to low or high pH value after anionic or cationic polyelectrolyte adsorption respectively. Polyampholyte adsorption represented distinguished influences on the zeta potentials in the different pH ranges divided by the pH_iep of polyampholyte.The stabilization of adsorbed polymer to the dispersed oxide nanoparticles in aqueous solution was elucidated. PEG and PAM provided steric repulsion to oxide nanoparticles. Polymer of high molecular weight in the scale of 104 g/mol tended to favor the colloidal stability. The dispersed samples achieved the best stability at the polymer concentration of saturated adsorbed amount. PAA and PMAN were found to stabilize oxide nanoparticles through electrostatic and steric repulsions. Polyelectrolytes at high pH of the solution provided more effective degree of sustained particle dispersion. For the polymer of molecular weight exceeding 10s g/mol, macromolecular chains were detrimental to the dispersed system due to the bridging flocculation under the saturated adsorbed amount. When MPTMAC was used to disperse oxide nanoparticles, the decrease in pH value brought about stronger electrosteric repulsion. At low polyelectrolyte concentrations unstable suspensions were observed from a stability test. At high polyelectrolyte concentrations higher particle coverage caused electrosteric stabilization of the dispersion. However, further increase in MPTMAC concentration after saturated adsorption would flocculate the dispersed system. In the presence of PMAA-b-PDMAEMA the pH value should be controlled to the one beyond its pH_iep for gaining good colloidal stability. PMAN was employed to fabricate polyacrylate nanocomposite coatings, and its initial concentration was kept at the concentration of saturated adsorbed amount. The pH value was controlled in the range of 9 + 0.5.The thermal degradation behaviour and mechanism of ZnO/polyacrylatenanocomposites were studied firstly by DSC-TG and FTIR. ZnO stabilized or destabilized the polyacrylate macromolecules according to the temperature region. The acceleration in the early stage of thermal degradation was due to the surface hydroxyl groups on the ZnO surface that catalysed the thermal degradation of polyacrylate, while the thermal stabilization in the high temperature was proposed for the formation of the complex by interaction of polyacrylate and zinc oxide particles. Zinc oxide samples of different content and particle size had different effect on thermal behaviour of polyacrylate/ZnO composites. Kinetics analysis of thermal degradation verified the catalytic effect of zinc oxide on the thermal degradation of polyacrylate. Compared to the filler-free polyacrylate the composites degraded at lower temperature, the decomposition rate of polyacrylate was slowed down, and the occurrence of major polymer backbone cracking was delayed.The effect of titanium dioxide on the photooxidative degradation of polyacrylate coatings was assessed and compared with typical organic ultraviolet light absorbers. As ultraviolet light irradiation proceeding, the breakage of C-H occurred and the amount of C=O and C-0 in oxidation products increased. UV analysis showed that titanium dioxide particles and organic UVA absorbed or scattered UV light. Over the duration of experiments the organic UVA decomposed with time, whereas the absorbance of titanium dioxide nanoparticles by virtue of their inherently stable inorganic nature remained constant. Anatase was a photocatalyzer, which accelerated and catalysed photochemical degradation of the polyacrylate coatings, while rutile was an effective stabiliser and prevented the photochemical degradation. Simultaneous addition of rutile and organic UVA in polyacrylate coatings was found to be more effective in anti-photooxidative degradation.Under water lubrication the tribological properties of polyacrylate coatings modified by S1O2 nanoparticles were investigated. The surface abrasion and the wear mechanism were discussed by assistance of FTIR, SEM and abrasion experiments. The addition of SiO2 nanoparticles favoured to the formation of lubricated membrane and absorbed water layer under water lubrication, which improved the friction-reduction performance. When the amount of nano-silica was low, main wear mechanisms of coatings were tribocorrosion abrasion and grain abrasion. As the coating containing 5wt% SiO2 nanoparticles, the favorable wear-resistance of the coating was obtained, which was provided with integrated lubricated membrane and absorbed water layer. Compared with micron SiO2 particles, silica nanoparticles offered more effective friction-reduction ascribed to their high specific area and...