No abstract

The colloidal stability of dispersions comprised of magnetite nanoparticles coated with polydimethylsiloxane (PDMS) oligomers was investigated theoretically and experimentally. Particle-particle interaction potentials in a theta solvent and in a good solvent for the PDMS were predicted by calculating van der Waals, electrostatic, steric, and magnetic forces as functions of interparticle separation distances. A variety of nanoparticle sizes and size distributions were considered. Calculations of the interparticle potential in dilute suspensions indicated that flocculation was likely for the largest 1% of the population of particles. Finally, the rheology of these complexes over time in the absence of a solvent was measured to probe their stabilities against flocculation as neat fluids. An increase in viscosity was observed upon aging, suggesting that some agglomeration occurs with time. However, the effects of aging could be removed by exposing the sample to high shear, indicating that the magnetic fluids were not irreversibly flocculated.

Biocompatible, hydrophobic nanoparticles show great promise as biomaterials. This paper reports the synthesis, magnetic separation, and characterization of magnetite nanoparticles with polydimethylsiloxane (PDMS) adsorbed onto their surfaces. The particle size distributions were narrowed by employing a magnetic separation/fractionation technique to remove larger particles and aggregates from an original distribution. A probability averaging method that incorporates particle size distributions of the magnetite cores derived from TEM is proposed, together with implementation of a polymer brush model for calculating the thickness of the polymer surfactant, for predicting the sizes and size distributions of these complexes in suspension. The intensity, volume, and number average size distributions in solution were predicted, and the values were compared to sizes of the complexes measured by DLS. This approach provides a tool for a more precise characterization of the size distributions of polymer−nanoparticle complexes relative to previous methods that utilized only a mean (single) core particle size. The predicted sizes of the complexes in dispersion closely approximate measured values from DLS for particles with narrow size distributions. Agreement between the predicted and measured sizes improves as the particle size distribution becomes narrower.