Espalhamento por dispersões diluídas de ferrofluidos em géis poliméricos

Abstract

Ferrofluids are colloidal solutions of nanometric particles with an associated permanent magnetic dipole, resulting in a liquid that responds to magnetic stimuli. The dispersion of ferrofluids in polymer gels leads to another kind of material known as magnetic gels or ferrogels. We present in this study measurements of scattering by dilute solutions of ferrofluids of magnetite in gels of poly(acrylamide) / methylenebisacrylamide using light scattering, small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS). The variation of the scattering intensity I(q) with the scattering vector q shows that the magnetic particles form small fractal aggregates with approximately 200 to 300 particles per aggregate and fractal dimension D 2.9. We interpret these results as a consequence of the transfer of the surfactant molecules from the surface of the particles to the gel, leading to an irreversible aggregation of the magnetite particles. We also investigate the effect on the scattering intensity ofapplying a magnetic field perpendicular to the incident beam. The experimental data using SAXS are in acceptable agreement with a magnetic ellipsoid aggregate model where the effective magnetic moment is a function of the gel concentration. Using light scattering the interaction effects are predominant and the scattering intensity anisotropy is the opposite of that observed with SAXS. In a second part we investigate the dynamics of ferrogels by photon correlation spectroscopy, where the magnetic particles act as tracers. The results are in acceptable agreement with the diffusion of a Rouse network model with an initial constant diffusivity followed by an anomalous self-diffusive regime. We find that the transition time between these two regimes varies with the scattering vector as q2.