Synthesis of photo-cross-linkable microgel colloids for cluster formation studies

In many physical processes involving colloidal particles, transient structures (e.g., in transient particle gels, phase-separating suspensions) are created. Freezing these structures by chemically inter-cross-linking the particles could be an intriguing route to create network structures with special (e.g., mechanical) properties. Alternatively, photochemically fixing transient structures, thereby making them accessible to detailed analysis, could provide new insight in processes such as colloidal aggregation or crystal nucleation, which are of fundamental importance for soft matter physics as well as for applications. For this purpose we synthesized microgel particles, which carry photoreactive groups on the surface, and explored the possibility of creating chemical interparticle cross-links by UV illumination. Via seeded growth emulsion polymerization, we synthesized monodisperse polystyrene microgels with a radius of about 210 nm. In organic solvents, the particles swell to a hydrodynamic radius of 350 nm. These polystyrene microgels were functionalized by using a polymer analogous Friedel-Crafts benzoylation to introduce benzophenone groups onto the surface. The colloidal particles were characterized by static and dynamic light scattering, optical microscopy, and transmission electron microscopy. The interparticle cross-linking behavior of the modified microgels in concentrated dispersions was examined by dynamic light scattering and optical and scanning electron microscopy. To study the effect of short-range attractive forces between the particles, linear polystyrene chains acting as depletion agents were added. In time-dependent irradiation studies, we observe a coexistence of individual particles and clusters with a size range around 2 mum in radius already at shortest irradiation times upon addition of linear polymer. These (transient) clusters-introduced by depletion attraction-are permanently fixed by photo-cross-linking and, thus, on dilution of the system made amenable to analysis by light scattering.