2 research outputs found
Composition and Stability of Plasma Polymer Films Exhibiting Vertical Chemical Gradients
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 −NH<sub>2</sub>, 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 CO<sub>2</sub>/C<sub>2</sub>H<sub>4</sub> 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
Response of Plasma-Polymerized Hexamethyldisiloxane Films to Aqueous Environments
Thin
plasma polymer films were deposited in hexamethyldisiloxane
(HMDSO) and HMDSO/O<sub>2</sub> low-pressure discharges and their
chemical structures analyzed using infrared (IR) spectroscopy and
neutron reflectometry (NR). The (plasma-polymerized) ppHMDSO film
exhibits hydrophobic, polyÂ(dimethylsiloxane)-like properties, while
the retention of carbon groups is reduced by O<sub>2</sub> addition,
yielding a more inorganic, hydrophilic ppSiO<sub><i>x</i></sub> film. Both films show a minor (vertical) density gradient
perpendicular to the substrate, where the exposed film surface seems
to be more oxidized, indicating oxidative aging reactions upon contact
with air. The hydration and water uptake abilities of the films in
aqueous environments were investigated in humid environments using
ellipsometry, NR in D<sub>2</sub>O, and multiple transmission-reflection
IR measurements after equilibration of the films in water