Formation of crystalline colloidal arrays by anionic and cationic polystyrene particles

Abstract

Cross-linked polystyrene microspheres bearing positive and negative charges have been prepared by surfactant-free emulsion polymerization by the use of various amounts of vinylbenzyltrimethylammonium chloride and sodium styrenesulfonate as comonomers, respectively. Increasing the amount of the ionic comonomers tends to decrease the particle size due to better surface stabilization, but a high concentration of an ionic comonomer leads to a competitive mechanism that increases the polydispersity of the particle size. The amount of cationic and anionic comonomers leads to differences in size, shape and uniformity of the particles. The cationic particles can self-assemble into crystalline colloidal arrays with intense visible light diffraction just like the anionic ones. It is particularly interesting to observe that good packing can be obtained even for particles not quite uniform in size. To better understand the packing behavior, the properties and stability of the colloidal crystals have been studied as a function of the particle concentration and ionic strength of the media. The presence of charges helps in the formation of periodic structure over a wide range of particle concentrations at low ionic strength.