AbstractConcentration profiles of carbon tetrachloride in low‐density polyethylene pellets were analyzed mathematically to estimate the concentration dependence of the diffusivity. Profiles were measured by electron microprobe analysis and energy dispersive x‐ray spectroscopy (EDAX) and found to be sharper than expected. However, the diffusion front position was proportional todocumentclass{article}pagestyle{empty}begin{document}$ sqrt t $end{document}rather thantas might have been expected if case II diffusion applied. The mathematical model assumed a constant diffusivity behind the diffusion front and zero diffusivity in advance of the front. The resulting moving boundary problem was solved by integral methods, yielding concentration profiles consistent with the essential features of the experimental profiles. The difference in shape between experimental and calculated profiles was attributed to the concentration dependence of the diffusivity, which was estimated by a curve‐fitting procedure. The resulting integral average diffusivity was 1.28 × 10−6cm2/s, four times that estimated assuming standard fickian absorption from the mass uptake curves. This difference was attributed to the inapplicability of the standard relationships to this system in which a discontinuous diffusivity concentration relationship was presumed
展开▼
