Mechanisms of vapor permeation through multilayer barrier films: Lag time versus equilibrium permeation

Multilayer, thin-film organic/inorganic composite barrier layers have recently been reported to achieve water vapor permeation rates of <10−5g∕m2∕day at 25°C∕40%RH on polyethylene terephthalate substrates. Using both transient and steady-state vapor permeation measurements combined with classical Fickian diffusion models, we determine the mechanism of vapor permeation through such barrier structures and show that results obtained to date are limited not by equilibrium diffusion but by lag-time effects caused by the extremely long effective path length for the diffusing gas. The implications for further improvement of flexible thin-film vapor barriers are discussed.