Dynamics of probe particles in polymer solutions and gels

The dependence of the dynamics of probe particles in polyacrylamide solutions and gels as a function of crosslink content, scattering angle and size of the probe is studied using photon correlation spectroscopy. Several fitting methods were applied to the auto-correlation functions obtained from samples ranging through the gelation threshold. We observed a complex dynamical behaviour ranging from the purely translational diffusion of the probes in the medium to a relaxational behaviour associated with the local movement of the latex particles in the gel. In addition, in samples which are beyond the gelation threshold, where a large fraction of the latex particles are immobilized, we observe a coupling between the movement of the particle and the collective diffusion mode of the gel network itself. Our results show that the crossover from diffusional to relaxational behaviour depends on the length scale over which the movement is probed and the mesh size of the network.