Phase diagram of colloidal crystals of poly(methyl methacrylate) spheres in the exhaustively deionized dispersion

Phase diagram of colloidal crystals of poly(methyl methacrylate) (PMMA) spheres (100 to 300 nm in diameter) was measured precisely for the exhaustively deionized aqueous dispersion. Strong iridescent colors changed from bluish to reddish and metallic as sphere size increased at high sphere concentrations around 0.1 in volume fraction. Large single crystals, on the other hand, were recognized with the naked eyes at low sphere concentrations around 0.0005 to 0.001 especially for PMMA spheres smaller than 200 nm. The critical concentrations of melting (or crystallization), ϕ_c of all the PMMA spheres examined were quite low between 0.00025 and 0.00060 in volume fraction. The ϕ_c values were insensitive to kind (polystyrene, silica, and fluorine-containing spheres) and also to size (90~300 nm) of colloidal spheres. The ϕ_c value of the deionized dispersion with the coexistence of the ion-exchange resins was lowest, and that without resins was tenfold large. Addition of sodium chloride increased the ϕ_c values sharply. Extension of the electrical double layers forming around the colloidal spheres is most important to determine the ϕ_c values.