Characterization of polyethylene terephthalate films coated with thin AlxSi1 − xOy layers using monoenergetic positron beams

• Positron annihilation was used to detect open spaces in ceramic/polymer structures.
• Free volumes in metal oxide coated polyethylene terephthalate were characterized.
• The size of open spaces in metal oxides correlates with molecular penetrations.
• Free volumes below the interface act as a barrier against molecular penetrations.

Open spaces in polyethylene terephthalate (PET) coated with aluminum silicate (AlxSi1 − xOy) were probed using monoenergetic positron beams. The energy distributions of the annihilation γ rays and the positron lifetime spectra were measured for 60-nm-thick AlxSi1 − xOy (x = 0–1) deposited on PET using electron-beam evaporation. A clear correlation was obtained between the line-shape S parameter of the annihilation γ rays, water vapor, and oxygen transmission rates. The results suggest that open spaces in the AlxSi1 − xOy layers play an important role in water/oxygen diffusion in AlxSi1 − xOy/PET, where the concentration/size of such spaces showed its minimum value at x = 0.2–0.3. The free volume fraction below the interface between AlxSi1 − xOy and PET (region width = 100–400 nm) was decreased by the deposition of the coating layer, and this region was also considered to act as a barrier against penetrations of gasses and molecules.