Atomic force microscopy (AFM) morphological surface characterization of transparent gas barrier coatings on plastic films

The relationship between the surface morphology and water vapor diffusivity of barrier coatings on polymeric polyethylene terephthalate (PET) films is studied by atomic force microscopy. The coatings, composed of aluminum oxide and/or silicon oxide, were grown by various deposition methods. It is observed that no correlation exists when the surface roughness is measured over large scanned areas owing to the inhomogeneous coating morphology. However, for scan sizes restricted to representative zones 0.5 mu m wide, free from bumps and pinholes, it is found that those films smoother than the PET substrate show a low permeability. This result would support the idea that some other mechanism, different from the existence of surface irregularities (i.e. bumps, pinholes, etc.) and related to the form of growth, should also determine the barrier diffusivity.