Polyorganosiloxane nanoparticles as optical tracers

Abstract

Polyorganosiloxane microgels have been synthesized by polycondensation in a microemulsion of trimethoxysilanes. Highly crosslinked rather monodisperse particles of radius about 10 nm are obtained.

Using silanes with special functional groups as comonomers, model particles suitable for studies in colloid physics are available; photoreactive and fluorescent dyes have been chemically incorporated into the microgels to prepare tracers for diffusion studies using forced Rayleigh scattering and fluorescence correlation spectroscopy. Microgels containing small gold clusters have been developed as tracers for dynamic light scattering (DLS). Using laser light of wavelength 514 nm, light absorption of those tracers causes convectional flux which can be probed by distinct oscillations in the DLS signals. Using laser light at 647 nm, far from the absorption band of the gold-containing colloids, at low light intensity the convection ceases and particle self-diffusion can be measured in concentrated dispersions. By mixing a small amount of strongly scattering gold tracers into a matrix of refractive index matched colloidal particles, particle mobility has been probed as a function of particle concentration. Combining various optical methods, self-diffusion of polyorganosiloxane microgel tracers in a matrix of organic solvent and non-labeled microgels has been studied as a function of particle concentration over a wide concentration regime. Data analysis according to free volume theory yielded a particle concentration of 63 wt% for the zero-mobility limit. Therefore, it has been concluded that polyorganosiloxane microgels dispersed in organic solvents behave as hard spherical particles without volume swelling by the solvent. Opposite to previously studied PS microgels, the effective hard sphere volume fraction of polyorganosiloxane microgels is identical to the analytical volume fraction determined from the particle concentration.