Durability of aminosilane-silica hybrid gas-barrier coatings on polymers

Aminosilane solutions with pH equal to 8 and to 11 were applied and cured on SiO2-coated poly(ethylene terephthalate) (PET) films. The permeability of the silane–silica hybrid coating on PET was reduced twofold compared to that of the SiO2/PET film, at solution concentrations as low as 1 wt.%, irrespective of the pH. This concentration level led to a dense silane monolayer crosslinked to the silica surface. The oxygen transport mechanisms in the hybrid coatings were determined based on the thermally activated rate theory. Permeation experiments were also performed under tensile loading, and the critical strain for loss of barrier performance was found to be improved by a factor of two, only in case of basic pH. The defect population and morphology of the hybrid coating subjected to hydrothermal aging were analyzed using a reactive ion etching method and atomic force microscopy, respectively. These experiments confirmed the defect healing action of the aminosilane at low concentrations in solution, through the formation of a densely crosslinked polysiloxane layer at the silane–silica interface for both pH8 and pH11. The influence of the silane treatment was emphasized in case of basic pH due to the dissolution of superficial oxide layers.