| Summary: | Barrier performance of continuous coatings is determined by the defect population in the barrier layer. In this paper we consider two classes of defects: those in which the permeant does not interact with the barrier material, ‘macrodefects’, and those in which there is an interaction between the permeant and the barrier material, ‘nanodefects’, thus leading to a greater activation energy for permeation than that of permeation through the underlying polymer. If a barrier is improved by reducing the density of macrodefects then typically the transmission rate will decrease, and eventually the activation energy will increase as the proportion of permeation that takes place through the nanodefect population becomes significant. Recent studies, however, have highlighted examples in which a greater activation energy is associated with a greater permeation. This is explored in terms of a simple double-Arrhenius model, which highlights the significant contribution from nanodefect permeation even in cases in which the overall barrier performance is not that high.
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