CRYSTAL-STRUCTURES OF MONODISPERSE COLLOIDAL SILICA IN POLY(METHYL ACRYLATE) FILMS

Dispersions of 22.3 vol % of 153 nm diameter silica particles coated w ith 3-(trimethoxysilyl)propyl methacrylate in Liquid methyl acrylate ( MA) as 264-528 mu m thick films in glass cells spontaneously order int o colloidal crystals and Bragg diffract visible light from lattice pla nes parallel to the film plane. Photopolymerization of the MA gives el astomeric poly(methyl acrylate) (PMA) films with retention of the crys talline order. Swelling the films with more MA and Further polymerizat ion give colloidal crystalline composite films with larger lattice spa cings. The crystal structures are modeled as face centered cubic for t he monomer dispersions and rhombohedral for the polymer films. Layer s pacings observed in scanning electron micrographs (SEM) of microtomed PMA films are used to calculate the (111) spacings and the lattice par ameter a of the model. The layer spacings correlate well with the diff raction wavelengths from optical spectra. Ultrasmall angle X-ray diffr action also was performed to demonstrate the three-dimensional crystal line order. The SEM and optical spectra also fit hexagonal close packe d (hcp) or mixed rhombohedral and hcp structures.