Brownian diffusion close to polymer brushes

Brownian diffusion of diluted colloidal particles of different sizes was investigated by evanescent wave dynamic light scattering in the vicinity of polystyrene polymer brushes grafted to a glass surface. The particles concentration profiles, resolved from the penetration depth dependence of the scattered intensities, evidenced an excluded region close to the glass hard wall with a characteristic size increasing with the brushes grafting density. The dynamic of large hard spheres particles (R=120nm), excluded from the brushes, was slowed down though slightly faster than the hard wall case. Smaller polystyrene microgels particles (R=16 nm and 42nm) that partially penetrated the brushes, presented a very slowed down dynamics, much more so for the smaller particles, reminiscent of size exclusion type of mechanism.