Permeability of multi-layer structures

In this paper we show that 'permeability' of a heterogeneous structure with mass transport and thermodynamic properties varying across its thickness is a misleading concept leading to incorrect results and design decisions while two structural transmission rate equations are recommended for practical applications. The notion of structural identity of multi-layer films is introduced to explain the apparent failure of the 'permeability' concept. Structural identity of two or more films means the same material sequence in the structure relative to separated environments with constant relative thickness of each corresponding layer. Structurally identical films indeed have the same 'permeability', however the notion of identity is shown to contradict the practical goals of multi-layer film design. Engineering examples are provided to demonstrate potential misuses of the 'permeability' concept in practical multi-layer design decisions. Correct problem statements and calculation procedures are included. Some general limitations of transmission rate equations are also discussed. These include the role of boundary conditions, temperature and concentration dependence of permeant diffusivity and solubility in a polymer matrix, the presence of co-permeants, surface sorption effects, film thickness, homogeneity of polymer matrix for permeation purposes and correct utilization of available data for predicting gas transport properties of multilayer films.