The initial stages of Case II diffusion at low penetrant activities

Rutherford backscattering spectrometry (R BS) has been used to measure the volume fraction versus depth profiles of iodohexane vapour diffusing into glassy polystyrene. At penetrant activities of less than 0.40 the surface concentration increases slowly and approaches equilibrium only after very long vapour exposures. Since Case II diffusion is observed to begin only after a critical concentration of penetrant is reached, these kinetics create an induction time for the Case II process. However, the swelling kinetics of the surface are still slow after the induction time has elapsed even though the diffusion coefficient behind the front is much larger than that in the polymer ahead of the front. The agreement between these swelling kinetics and those predicted by the model of Thomas and Windle is relatively poor. On the other hand, if the Thomas and Windle model is modified (1) to allow the viscosity of the polymer to decrease exponentially with osmotic pressure, with the decrease set to match that observed in separate creep measurements as a function of tensile stress, and (2) to include an initial volume fraction of rapidly filling interstitial sites, which we take to be due to the initial free volume, the agreement between the model and the experiment is much better.