Diffusion of linear polymer chains in gels

Dynamic light scattering was used to investigate the probe diffusion of linear polymers in methyl methacrylate gels made in appropriate solvents. For the case where the probe polymer was polystyrene and the solvent toluene, StokesE-Einstein type scaling of the diffusion constant versus the molecular weight of the probe polymer at low molecular weights and a crossover to reptation behavior above a critical molecular weights were observed. Crossover occurs when the radius of the of gyration of the probe becomes comparable to the correlation length of the gel. In experiments with PMMA probes in MMA gels made in dioxane, composite correlation functions covering 4 decades were measured. A CONTIN analysis shows the fast collective diffusion of the gel as well as a broad mode due to the probe polymer diffusion. The average value of this broad mode scales with molecular weight according to the reptation prediction. Possible implications for the width of this mode are discussed.