Nanometer-scale interface instability in silicon oxide/polymer sandwich structures detected after two years

Nonthermal plasma-deposited glassy silica is often used as a gas barrier film to protect polymer material in many applications. This study revealed that glassy silica is a slightly porous material (3 vol%) with small pores (2.5 nm) formed during thin film deposition. The infrared spectrum shows that the as-deposited plasma silica contains gaseous carbon dioxide, which is likely encapsulated in the pores. It can be assumed that these CO₂ molecules diffuse from the silica layer through the silicon oxide/polymer interface into the protected polymer material. The low crosslinked polymer material is then locally oxidized by CO₂, which changes its chemical and physical properties. This means that the silicon oxide/polymer interface gradually moves into the polymer material over time. CO₂ diffusion is therefore considered responsible for a shift of the silicon oxide/polymer interface by 30–35 nm after 27 months.