Anomalous Polymer-Penetrant Permeation

We review briefly and classify the so called ``non-Fickian anomalies'' associated with one-dimensional permeation (or sorption) of a penetrant through a polymer film of thickness l. We point out that ``Fickian'' diffusion controlled permeation can be readily distinguished from anomalous permeation by the l dependence of the experimental flux through the film for the former process, which satisfies a simple scaling law with respect to film thickness. Furthermore, we show that by varying the film thickness one can separate the time lag for ``non-Fickian'' diffusion into a purely ``Fickian,'' l-dependent contribution and an l-independent term due wholly to the anomalies. The latter can serve as a convenient figure of merit for the anomalous behavior, being essentially a mean relaxation time for the explicitly time-dependent process responsible for deviation from the simpler, ``Fickian'' behavior.