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Composition and Stability of Plasma Polymer Films Exhibiting Vertical Chemical Gradients
- Rupper, Patrick, Vandenbossche, Marianne, Bernard, Laetitia, Hegemann, Dirk, Heuberger, Manfred
- Langmuir 2017 v.33 no.9 pp. 2340-2352
- X-ray photoelectron spectroscopy, air, contact angle, crosslinking, drugs, electrodes, films (materials), filtration, hydrolysis, mass spectrometry, moieties, nanomaterials, oxidation, oxygen, pH, polymers
- Controlling the balance between stability and functional group density in grown plasma polymer films is the key to diverse applications such as drug release, tissue-engineered implants, filtration, contact lenses, microfluidics, electrodes, sensors, etc. Highly functional plasma polymer films typically show a limited stability in air or aqueous environments due to mechanisms like molecular reorganization, oxidation, and hydrolysis. Stabilization is achieved by enhancing cross-linking at the cost of the terminal functional groups such as −OH and −COOH, but also −NH₂, etc. To overcome such limitations, a structural and chemical gradient was introduced perpendicular to the surface plane; this vertical gradient structure is composed of a highly cross-linked base layer, gradually changing into a more functional nanoscaled surface termination layer. This was achieved using CO₂/C₂H₄ discharges with decreasing power input and increasing gas ratio during plasma polymer deposition. The aging behavior and stability of such oxygen-functional vertical gradient nanostructures were studied in air and in different aqueous environments (acidic pH 4, neutral pH ≈ 6.2, and basic pH 10). Complementary characterization methods were used, including angle-resolved X-ray photoelectron spectroscopy (ARXPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) as well as water contact angle (WCA) measurements. It was found that in air, the vertical gradient films are stabilized over a period of months. The same gradients also appear to be stable in neutral water over a period of at least 1 week. Changes in the oxygen depth profiles have been observed at pH 4 and pH 10 showing structural and chemical aging effects on different time scales. The use of vertical gradient plasma polymer nanofilms thus represents a novel approach providing enhanced stability, thus opening the possibility for new applications.