Inorganic ''silicone oil'' microgels

We report investigations into the stability, morphology, internal microstructure and swelling behaviour of polydimethylsiloxane oil-in-water emulsions (1 vol.% monomer; 1 vol.% NH3 solution; 25 degrees C) prepared using mixtures of difunctional (dimethyldiethoxysilane (DMDES)) and trifunctional silane monomers. Both methyltriethoxysilane (MTES) and 3-aminepropyltriethoxysilane (APTES) are potential cross-linkers owing to their trifunctionality. The addition of MTES leads to major changes in various properties of the PDMS droplets. Transmission electron microscopy was used to observe the transition between liquid droplets ([MTES] < 50 vol.%) and solid particles ([MTES] > 50 vol.%). The particles showed significantly reduced swelling in n-heptane in comparison with the liquid droplets. This restriction on swelling indicates the formation of internally cross-linked particles. Si-29 nuclear magnetic resonance (NMR) and mass spectroscopy demonstrated increased incorporation of trifunctional material in PDMS polymers as the MTES concentration was raised. The chemical structure of the trifunctional monomer was shown to be important, as Si-29 NMR suggests that APTES does not polymerize with DMDES but may reside in the aqueous continuous phase as a cationic oligomer. The presence of ethanol during particle formation causes enhanced particle swelling. The particle size of the swollen particles, however, falls steeply as the concentration of ethanol is reduced. Overall the data suggest the formation of cross-linked microgels at high MTES concentrations. (C) 1997 Elsevier Science B.V.