Tracer diffusion of polyorganosiloxane nanoparticles in solution: Effects of tracer topology and particle concentration

Spherical polyorganosiloxane nanoparticles containing photoreactive dye lab els have been used to study self-diffusion in concentrated colloidal disper sions by forced Rayleigh scattering (FRS). Samples studied contained about 0.5 wt. % of these tracer particles and the order of 50 wt. % nonfunctional host particles dissolved in cyclohexane. To avoid any influence of the lab el molecules on the particle mobility, the tracers have been equipped with a nonfunctional protective shell. The effect of the thickness of this shell as well as of the label content on particle diffusion has been examined ca refully. A much lower tracer mobility has been found for particles with pro tective shell of thickness less than 1 nm. In some cases, even complementar y grating effects have been found in the FRS signals, indicating a differen t tracer mobility of bleached and nonbleached particles. By diffusion studi es using tracers with protective shell sufficiently thick to avoid such unw anted effects, the single particle mobility for polyorganosiloxane microgel dispersions in the highly concentrated regime has been determined. Experim ental results show that polyorganosiloxane nanoparticles in organic solvent do not exhibit strong volume swelling as most colloidal model systems but are porous rigid spheres filled with only about 25 wt. % solvent. Therefore , they behave like a very realistic hard sphere system. (C) 2000 American I nstitute of Physics. [S0021-9606(00)51306-3].