Diffusion of nonionic azo dyes in nylon‐6

Diffusion of 2,4‐dinitroaniline and three nonionic azo dyes in Nylon‐6 film was studied by analysis of the concentration‐distance curves (profiles) of penetrants in the polymer. Actual diffusivities D ( c ) of penetrants in polymer, diffusion coefficients as a function of the concentration C f of penetrant in polymer, were calculated from the profile. It was found that D ( c ) is almost constant or decreases gradually with decreasing C f in the range of high‐medium C f but decreases appreciably with decreasing C f at low C f . The change in D ( c ) with C f was explained in terms of the dual‐mode sorption‐diffusion model. The penetrants diffuse in the polymer as two distinct species, i.e., a dissolved species and an adsorbed species. The former is the penetrant taken up by the polymer by a partition mechanism (dissolved species) and the latter is that taken up by Langmuir sorption (adsorbed species). The actual diffusivity D P ( c ) of the dissolved species decreases with decreasing C f . While the actual diffusivity D L ( c ) of the adsorbed species normally increases gradually with decreasing C f . D P ( c ) is usually larger than D L ( c ) .