Improving the Stability of Amino-Containing Plasma Polymer Films in Aqueous Environments

Plasma polymer films that contain amine groups (NH₂-PPFs) are known to degrade over time, particularly in aqueous environments. To reduce such aging effects, a vertical chemical gradient regarding the amine group density was explored ranging over a few nanometers at the coating surface. The gradient-containing nanofilms were formed in low-pressure plasma by tuning plasma conditions while keeping the plasma "switched on". The coating process started with a more cross-linked NH₂-PPF (70 W, 4:7 NH₃/C₂H₄), followed by the deposition of a few nanometers of a less cross-linked yet more functional NH₂-PPF (50 W, 7:7 NH₃/C₂H₄). Characterization of the prepared gradient coatings showed that the chemical composition depends on the NH₃/C₂H₄ gas flow ratio, as observed by different analytical methods: plasma diagnostics during deposition and depth profiling analyses of the deposited coating. Finally, surface chemistry was analyzed during air and water aging, showing a similar aging process of the NH₂-PPF single layer and NH₂-PPF with a vertical chemical gradient in air, while the stability of the gradient coating was found to be enhanced under aqueous conditions maintaining an [NH₂]/[C] amount of ∼1%.