Anisotropic photonic crystal building blocks: colloids tuned from mushroom-caps to dimers

Nonspherical polystyrene particles tuned from collapsed spheres (mushroom cap and hemisphere shapes) to two-lobed dimers are synthesized by seeded emulsion polymerization. The presence of a swelling solvent and additions of divinylbenzene crosslinker facilitate shape control through directing phase separation of the polymer network and the monomer in the swollen particles as well as the solvent release behavior. Confocal microscopy of the particles, dyed in the initial swelling and polymerization stage followed by a second stage, differentiates the locations of effective seeds and second stage polymerized material which dictate the external particle morphology. The anisotropic particles are prepared in sufficient quantity and uniformity to self-organize into colloidal crystal and rotator structures, as is demonstrated with a mushroom cap-shaped particle system condensed under confinement. Structural transitions follow the drive to maximize packing efficiency at systematically varied confinement heights. Confocal microscopy confirms monolayer, buckled and bilayer thin films with unconventional orientational ordering.