The effect of molecular size on non-Fickian sorption in glassy polymers

Rutherford backscattering spectrometry has been used to determine the concentration against depth profiles of n-iodoalkanes diffusing into a polymer glass photoresist. All the iodoalkanes smaller than iodohexane show strongly non-Fickian, or Case II, diffusion. After an induction time a sharp front forms, with almost no concentration gradient behind the front. Ahead of the front the concentration decreases exponentially with depth, a form predicted for Fickian diffusion ahead of a moving boundary. Values of the diffusion coefficientD extracted from this Fickian precursor decrease strongly withn, the number of carbon atoms in the iodoalkane. A similar decrease is observed for the front velocity, the magnitude of which is in qualitative agreement with that predicted by the Thomas and Windle model of Case II diffusion. For the larger values ofn, D decreases asn-2, prompting speculation that these longer chains diffuse into the glass by a reptation-like mechanism.