  We present a comprehensive investigation of polymer diffusion in the
semidilute regime by fluorescence correlation spectroscopy (FCS) and dynamic
light scattering (DLS). Using single-labeled polystyrene chains, FCS leads to
the self-diffusion coefficient while DLS gives the cooperative diffusion
coefficient for exactly the same molecular weights and concentrations. Using
FCS we observe a new fast mode in the semidilute entangled concentration regime
beyond the slower mode which is due to self-diffusion. Comparison of FCS data
with data obtained by DLS on the same polymers shows that the second mode
observed in FCS is identical to the cooperative diffusion coefficient measured
with DLS. An in-depth analysis and a comparison with current theoretical models
demonstrates that the new cooperative mode observed in FCS is due to the
effective long-range interaction of the chains through the transient
entanglement network.
