Nanostructure of PEO-polyurethane-PEO triblock copolymer micelles in water.

Novel hydrophilic triblock copolymers which form micelles in aqueous solution were studied by static and dynamic light scattering (SLS and DLS), small angle neutron scattering (SANS) and densitometry. The polymers were symmetric A-B-A block copolymers having two poly(ethylene oxide) (PEO) tail blocks and a polyurethane (PU) center segment that contained pendant carboxylic acids. The aggregation number of the micelles decreased with increasing PEO mass content. When attempting to fit the SANS data it was found that no single model was suitable over the entire range of block lengths and PEO mass concentrations investigated here. For the polymer with the highest aggregation number, the data were fitted with a triblock model consisting of a homogeneous core with a corona of non-interacting Gaussian chains for which only two free parameters were required: the radius of the core and the radius of gyration of the corona. In this case, the core was found to be effectively dry. At lower aggregation numbers, a star polymer model generated significantly better fits, suggesting the absence of any identifiable central core structure. Good agreement was found between the sizes measured by DLS, SANS and theoretical predictions of micelle size from a density distribution theory. These results show that when significant changes in aggregation number occur, the nanostructure of the micelle can change substantially even for polymers that are remarkably similar.

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.

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.

Aqueous dispersions of magnetite nanoparticles complexed with copolyether dispersants: experiments and theory.

Magnetite (Fe3O4) nanoparticles have been synthesized and complexed with carboxylate-functional block copolymers, and then aqueous dispersions of the complexes were investigated as functions of their chemical and morphological structures. The block copolymer dispersants had either poly(ethylene oxide), poly(ethylene oxide-co-propylene oxide), or poly(ethylene oxide-b-propylene oxide) outer blocks, and all of them had a polyurethane center block that contained pendent carboxylate groups. The complexes were formed through interactions of the carboxylates with the surfaces of the magnetite nanoparticles. The magnetite cores of the magnetite-copolymer complexes were near 10 nm in diameter, and the particles were superparamagnetic. Complexes with mass ratios of polymer to magnetite varying from 50:50 to 85:15 were studied. One of our objectives is to design complexes that form stable dispersions of discrete particles in water, yet that can be actuated (moved together) upon exposure to a uniform magnetic field. DLVO calculations that accounted for magnetic attractive interparticle forces, as well as van der Waals, steric, and electrostatic forces are presented. Compositions were identified wherein a shallow, attractive interparticle potential minimum appears once the magnetic term is applied. This suggests that it may be possible to tune the structures of superparamagnetic nanoparticle shells to allow discrete dispersions without a field, yet weak flocculation could be induced upon exposure to a field.