Anisotropic polystyrene nanoparticles of diameters below 0.5 microm were prepared by coating the surface of cross-linked polystyrene latex particles with a thin hydrophilic polymer layer prior to swelling the particles with styrene and then initiating second-stage free-radical polymerization. Conditions were found so that all particles had uniform asymmetry. The effect of surface chemistry on the development of particle anisotropy during seeded emulsion polymerization of sub-0.5 microm diameter particles was studied. The extent and uniformity of the anisotropy of the final particles depended strongly on the presence of the hydrophilic surface coating. Systematic variation of the degree of hydrophilicity of the surface coating provided qualitative insight into the mechanism responsible for anisotropy. Conditions were chosen so that the surface free energy favored the extrusion of a hydrophobic bulge of monomer on the hydrophilic surface of the particle during the swelling phase: the presence of a hydrophilic layer on the particle surface causes this asymmetry to be favored above uniform wetting of the particle surface by the monomer. Kinetic effects, arising from the finite time required for the seed to swell with the monomer, also play a role.

Methods are presented to synthesize suspensions of chemically and shape anisotropic colloids on submicrometer length scales. Particles are synthesized through seeded emulsion polymerization where a weakly cross-linked seed is swollen with monomer that phase separates at the reaction temperature resulting in a protrusion. The final particles can be considered to be composed of interpenetrating spheres. pH-sensitive anisotropy is created through the use of different surface coatings on each of the interpenetrating spheres. Dark-field imaging, dynamic light scattering, and scanning electron microscopy are used to characterize the particles.

Ultra-small-angle X-ray scattering was performed on suspensions of anisotropic polystyrene particles of varying degrees of anisotropy. The wave vector dependence of particle form factors is well described by a model developed by Debye for the scattering from fused spheres. As volume fraction is raised, all suspensions undergo a disorder/order phase transition. The scattering from disordered and ordered suspensions of anisotropic particles is the same as that of spheres up to volume fractions of 0.45, suggesting that, in the dilute crystalline phase, the anisotropic particles order into a rotator or plastic crystal phase, where the particle centers of mass are ordered, but the particle directors are randomly distributed. Further increase in particle volume fraction leads to differences in scattering between homonuclear dicolloids and spheres, implying that the homonuclear dicolloids form a body-centered tetragonal phase with both positional and directional order. This conclusion is supported by real-space imaging of dried films of the particles.

The microstructure of dense suspensions of anisotropic particles that are ordered at rest is explored under shear as a function of particle anisotropy and shear rate using small angle neutron scattering. For suspensions containing spheres and mildly anisotropic heteronuclear dicolloids, after preshearing, long-range order is present in the form of randomly stacked hexagonally close packed layers. As the shear rate is increased, long-range directional order is lost. At even higher shear rates, long-range order is re-established in the form of hexagonally packed layers sliding over one another. Suspensions of ordered homonuclear dicolloids are polycrystalline at rest, and as the shear rate is increased, sliding hexagonally packed layers develop while at larger shear rates long-range order is lost. These results demonstrate surprisingly large effects of shear on the microstructures of suspensions containing particles with small changes in anisotropy.