Microscopic Composition Maps of Poly(styrene-co-2-hydroxyethyl methacrylate) Colloidal Crystals and Interconnected Colloidal Arrays

Colloidal crystalline films were prepared from poly(styrene-co-2-hydroxyethyl methacrylate) (PS HEMA) latex particles by evaporative deposition. The hexagonally ordered surfaces of the colloidal crystals (CCs) were transformed with styrene vapor at room temperature to interconnected colloidal arrays (ICAs) that have a honeycomb-like ridge of polymer surrounding hexagonally ordered dimples in the surface. When the styrene vapor temperatures were increased systematically to 45 degrees C, the regularity of ICA structure decreased and finally disappeared. Images from transmission electron microscopy (TEM) and from atomic force microscopy (AFM) show that the surfaces of the PS HEMA particles and the ICAs have raspberry textures. Monolayer CCs and ICAs fabricated on TEM grids were analyzed by energy dispersive spectroscopy (EDS) to determine the elemental compositions of the different regions of the textured surfaces. Carbon, oxygen, and sulfur were distributed all over the surface of the CC. While carbon was distributed over the entire surface of the ICA, oxygen, sulfur, sodium, and potassium were concentrated mainly on the ridges of the honeycomb and not in the dimples of the ICA. The results are discussed in terms of a mechanism of transformation of the CC to the ICA in which styrene monomer swells the polystyrene-rich regions of the particles, and the swollen polystyrene rises to the surface. The polyHEMA-rich regions of the particles maintain the hexagonal periodicity, and liquid styrene evaporates to leave a more polystyrene-rich textured surface.