Stable colloidal dispersions of magnetic nanoparticles in aqueous media and polymer melts.

摘要

The work presented as part of this thesis describes a process to obtain two types of conventionally used magnetic nanoparticles, magnetite and cobalt ferrite, with a narrow size distribution that can be suspended in aqueous media by surface modification using a functional poly (ethylene glycol). Suspensions of these functionalized magnetic nanoparticles using polymers of different lengths were used to study particle stability in water at physiological conditions using dynamic light scattering and zeta potential measurements. Saturation magnetizations, energy dissipation capacities, blocking temperatures, and the magnetic anisotropy constant were analyzed for magnetic nanoparticles with the same magnetic core and size and different polymer grafting densities.;Magnetic suspensions in poly(ethylene glycol) with different mechanical and rheological properties were studied using dynamic susceptibility as a function of temperature to explore its use as a mechanism to study transitions in thermo-responsive polymers such as gels and in crystalline polymers. These nanoparticles were used to extract the viscosity of polymer melts using oscillatory measurements as a function of frequency from the small-molecule liquid-like to the entangled polymer. The local viscosity, or so called "nano-viscosity", obtained using magnetic measurements was compared to the bulk viscosity measured using conventional techniques. To our knowledge, these results comprise the first study on rotational motion of nanoparticles (10-9 m) in polymer melts where evidence based on small molecules (10-10 m) suggests that conventional hydrodynamic models fail to predict the rotational friction.