title: Composition and Stability of Plasma Polymer Films Exhibiting Vertical Chemical Gradients.
abstract: 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/C2H4discharges 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.
authors.affiliation: Laboratory for Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology , Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland.
authors.forename: Patrick
authors.identifier: 0000-0001-5513-512X
authors.initials: P
authors.lastname: Rupper
authors.affiliation: Laboratory for Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology , Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland.
authors.forename: Marianne
authors.initials: M
authors.lastname: Vandenbossche
authors.affiliation: Laboratory for Nanoscale Materials Science, Empa, Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, 8600 Dübendorf, Switzerland.
authors.forename: Laetitia
authors.initials: L
authors.lastname: Bernard
authors.affiliation: Laboratory for Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology , Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland.
authors.forename: Dirk
authors.initials: D
authors.lastname: Hegemann
authors.affiliation: Laboratory for Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology , Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland.
authors.forename: Manfred
authors.identifier: 0000-0001-5799-3785
authors.initials: M
authors.lastname: Heuberger