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 similar to 1%.