Anomalous moisture diffusion in viscoelastic polymers: Modeling and testing

It is now well known that Fick's Law is frequently inadequate for describin g moisture diffusion in polymers or polymer composites. Non-Fickian or anom alous diffusion typically occurs when the rates of diffusion and viscoelast ic relaxation in a polymer are comparable, and the ambient temperature is b elow the glass transition temperature (T-g) 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 constru cting such a model. In this paper, a simple yet robust methodology is propo sed that would allow characterization of non-Fickian diffusion coefficients from moisture weight gain data for a polymer below its T-g. Subsequently, these diffusion coefficients are used for predicting moisture concentration profiles through the thickness of a polymer. Moisture weight gain data at different temperatures for an epoxy adhesive is employed to calibrate the m odel. Specimen thickness independence of the modeling parameters is establi shed through comparison with test data. A finite element procedure that ext ends this methodology to more complex shapes and boundary conditions is als o validated.