  We discuss the manner in which the dynamics of tracer polystyrene chains
varies with the concentration of matrix polystyrene chains dissolved in
toluene. Using fluorescence correlation spectroscopy and theory, it is shown
that the cooperative diffusion coefficient of the matrix polystyrene chains can
be measured by fluorescence correlation spectroscopy in the semidilute
entangled concentration regime. In addition the self-diffusion coefficient of
the tracer polystyrene chains can be detected for arbitrary concentrations. The
measured cooperative diffusion coefficient is independent of the molecular
weight of the tracer polystyrene chains because it is a characteristic feature
of the transient entanglement network.

Soft Condensed Matter