Abstract
It is now well known that Fick's Law is frequently inadequate for describing penetrant diffusion in polymers or polymer composites. Non-Fickian or anomalous diffusion typically occurs when the rates of diffusion and viscoelastic relaxation in a polymer are comparable, and the ambient temperature is below the glass transition temperature (Tg) of the polymer. As a result, it is necessary to take into account the time-dependent response of a polymer, analogous to viscoelastic relaxation of mechanical properties, in constructing such a model.
In this paper, a simple yet robust methodology is proposed that would allow characterization of non-Fickian diffusion coefficients from penetrant weight gain data for a polymer below its Tg. Reduced absorption plots are used to verify the thickness independence of the diffusivity data. Subsequently, these diffusion coefficients are used for predicting penetrant concentration profiles through the thickness of a polymer. Penetrant weight gain data at different temperatures for an epoxy and a urethane adhesive specimen is employed to test the model.