XPS characterisation of core-shell silica-polymer composite particles synthesised by atom transfer radical polymerisation in aqueous media

Near-monodisperse, siloxane-functionalised silica particles are used as a colloidal substrate for the surface-initiated polymerisation of various hydrophilic methacrylates: oligo(ethylene glycol) methacrylate (OEGMA), 2-(N-morpholino)ethyl methacrylate (MEMA), and ammonium 2-sulfatoethyl methacrylate (SEM) by atom transfer radical polymerisation in aqueous media at room temperature. The bulk and surface compositions of the resulting composite particles were assessed using various techniques. Thermogravimetric analysis of the resulting silica-polymer composites indicated polymer loadings of 5.4-8.6%, depending on the nature, structure and target degree of polymerisation (D_p). Dynamic light scattering studies indicate increases in hydrodynamic diameter of 14-87 nm compared to the reference silica particles. FT-IR spectroscopy confirmed additional features characteristic of the carbonyl group and pendant end-chain functionalities of the methacrylic polymer chains. The elemental and chemical surface compositions of the initial silica particles and final polymer-grafted composite particles were extensively investigated by X-ray photoelectron spectroscopy (XPS). The composite particles had appreciably higher C/Si atomic ratios, compared to the original initiator-functionalised silica particles, and these ratios increased with increasing target D_p. In addition, close inspection revealed that the relative intensities of the various components of the peak-fitted C1s envelopes varied significantly, depending on the target degree of polymerisation and the chemical structure of the methacrylic monomer. Moreover, in the case of the MEMA and SEM polymerisations, new nitrogen (MEMA) and sulfur (SEM) XPS signals were detected. This XPS study confirmed the presence of a thin outer layer of grafted polymer chains surrounding the silica particles.