Dynamic properties of semidilute solutions at the theta point

Here we report results obtained by neutron spin echo on semidilute theta solutions of polystyrene in deuterated cyclohexane. It is shown that the pertinent length for the diffusion coefficient is not the correlation length xi(theta) of concentration fluctuations but rather the distance between binary contacts. At length scales smaller than this distance, the dynamics is linked to the local viscosity, which is found to be surprisingly concentration and temperature dependent.