Anomalous moisture diffusion in viscoelastic polymers: modeling and testing

It is now well known that Fick's Law is frequently inadequate for describing moisture 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 moisture weight gain data for a polymer below its Tg. 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 model. Specimen thickness independence of the modeling parameters is established through comparison with test data. A finite element procedure that extends this methodology to more complex shapes and boundary conditions is also validated.