Jonathan D. Goff

Stability of Polydimethylsiloxane-Magnetite Nanoparticle Dispersions Against Flocculation: Interparticle Interactions of Polydisperse Materials

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.

Synthesis and colloidal properties of polyether-magnetite complexes in water and phosphate-buffered saline.

Biocompatible magnetic nanoparticles show great promise for many biotechnological applications. This paper addresses the synthesis and characterization of magnetite nanoparticles coated with poly(ethylene oxide) (PEO) homopolymers and amphiphilic poly(propylene oxide-b-ethylene oxide) (PPO-b-PEO) copolymers that were anchored through ammonium ions. Predictions and experimental measurements of the colloidal properties of these nanoparticles in water and phosphate-buffered saline (PBS) as functions of the polymer block lengths and polymer loading are reported. The complexes were found to exist as primary particles at high polymer compositions, and most formed small clusters with equilibrium sizes as the polymer loading was reduced. Through implementation of a polymer brush model, the size distributions from dynamic light scattering (DLS) were compared to those from the model. For complexes that did not cluster, the experimental sizes matched the model well. For complexes that clustered, equilibrium diameters were predicted accurately through an empirical fit derived from DLS data and the half-life for doublet formation calculated using the modified Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Deviation from this empirical fit provided insight into possible additional interparticle hydrophobic interactions for select complexes for which the DLVO theory could not account. While the polymers remained bound to the nanoparticles in water, most of them desorbed slowly in PBS. Desorption was slowed significantly at high polymer chain densities and with hydrophobic PPO anchor blocks. By tailoring the PPO block length and the number of polymer chains on the surface, flocculation of the magnetite complexes in PBS was avoided. This allows for in vitro experiments where appreciable flocculation or sedimentation will not take place within the specified time scale requirements of an experiment.

Design of stable polyether-magnetite complexes in aqueous media: effects of the anchor group, molecular weight, and chain density.

The colloidal stability of polymer-stabilized nanoparticles is critical for therapeutic use. However, phosphates in physiological media can induce polymer desorption and consequently flocculation. Colloidal characteristics of PEO-magnetite nanoparticles with different anchors for attaching PEO to magnetite were examined in PBS. The effects of the number of anchors, PEO molecular weight, and chain density were examined. It was observed that ammonium phosphonates anchored PEO to magnetite effectively in phosphate-containing solutions because of interactions between the phosphonates and magnetite. Additionally, a method to estimate the magnetite surface coverage was developed and was found to be critical to the prediction of colloidal stability. This is key to understanding how functionalized surfaces interact with their environment.

Synthesis and Characterization of Novel Segmented Polyionenes Based on Polydimethylsiloxane Soft Segments

Novel polydimethylsiloxane (PDMS) based polyionenes were synthesized by a modified Menschutkin reaction involving reaction between bromo-terminated PDMS oligomers and various ditertiary amino compounds. In this study, the nature of the hard segment was varied by using various ditertiary amino compounds and in some cases by incorporating chain extenders, while the soft segment content was varied by changing the molecular weight of the PDMS oligomers. The mechanical properties of these materials were found to be dependent on both the nature and amount of the hard segments. These materials also showed distinct evidence of a microphase-separated morphology where under normal conditions, the hard segments formed in what are believed to be cylindrical ion-rich microdomains dispersed randomly in the soft PDMS matrix. When subjected to uniaxial deformation, the ionic cylinders were found to orient along their long axes in the stretch direction.