Crystal Structures of Monodisperse Colloidal Silica in Poly(methyl acrylate) Films

Dispersions of 22.3 vol % of 153 nm diameter silica particles coated with 3-(trimethoxysilyl)propyl methacrylate in liquid methyl acrylate (MA) as 264- 528 μm thick films in glass cells spontaneously order into colloidal crystals and Bragg diffract visible light from lattice planes parallel to the film plane. Photopolymerization of the MA gives elastomeric poly(methyl acrylate) (PMA) films with retention of the crystalline order. Swelling the films with more MA and further polymerization give colloidal crystalline composite films with larger lattice spacings. The crystal structures are modeled as face centered cubic for the monomer dispersions and rhombohedral for the polymer films. Layer spacings observed in scanning electron micrographs (SEM) of microtomed PMA films are used to calculate the (111) spacings and the lattice parameter a of the model. The layer spacings correlate well with the diffraction wavelengths from optical spectra. Ultrasmall angle X-ray diffraction also was performed to demonstrate the three-dimensional crystalline order. The SEM and optical spectra also fit hexagonal close packed (hcp) or mixed rhombohedral and hcp structures.