Diffusion of a small molecule through a swollen polymeric network is studied by two very different experimental techniques: the first is a microscopic one called forced Rayleigh scattering (FRS), and the second is the macroscopic method of time lag. The diffusant is methyl red, an acidhyphen;base indicator dye, and the matrices are swollen crosslinked gelatin networks. The two sets of diffusion coefficients deduced by the two techniques are found to be in complete accord with each other. Since the timehyphen;lag technique measures diffusion perpendicular to the gelatin slab and FRS measures diffusion in the plane of the gelatin sample, these experiments indicate structural isotropy and homogeneity of the matrix down to the scale of a few microns. It is also noted that the dependence of the dye diffusion coefficient on the solvent volume fraction does not obey free volume theory predictions.
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