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Suppression of Hydrophobic Recovery by Plasma Polymer Films with Vertical Chemical Gradients Dirk Hegemann, Emanuela Lorusso, Maria-Isabel Butron-Garcia, Noemi Elisabeth Blanchard, Patrick Rupper, Pietro Favia, Manfred P. Heuberger, and Marianne Vandenbossche Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03913 • Publication Date (Web): 30 Dec 2015 Downloaded from http://pubs.acs.org on January 8, 2016
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Langmuir
Suppression of Hydrophobic Recovery by Plasma Polymer Films with Vertical Chemical Gradients Dirk Hegemann*,†, Emanuela Lorusso†,‡, Maria-Isabel Butron-Garcia†, Noémi E. Blanchard†, Patrick Rupper†, Pietro Favia‡, Manfred Heuberger†, and Marianne Vandenbossche† †Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory of Advanced Fibers, Lerchenfeldstrasse 5, 9014 St.Gallen, Switzerland. ‡University of Bari, Department of Chemistry, Via Orabona 4, 70126 Bari, Italy.
KEYWORDS. plasma deposition; surface functionality; cross-linking; aging; restructuring.
ABSTRACT. Vertical chemical gradients extending over a few nanometers were explored. The gradients are based on plasma-polymerized oxygen-containing ethylene (ppOEt) films. Using plasma conditions with low CO2/C2H4 ratio and high energy input, cross-linked films were deposited as base layer, while increasing CO2 and lowering energy input resulted in less crosslinked, yet highly functional films as applied as top layer. Aging studies indicate that, in particular for very thin gradient structures, the cross-linked sub-surface zone effectively hinders reorientation of the surface functional groups thus restricting hydrophobic recovery and oxidation effects.
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INTRODUCTION. A surface treated in a plasma is typically driven away from its thermodynamic equilibrium due to the formation of near-surface gradients. After the treatment the surface tends to return to a state of equilibrium which comprises surface reactions such as adsorption and chemical reactions, but also restructuring to minimize its surface energy which is thus called “hydrophobic recovery”. These aging phenomena involve migration/diffusion of polar groups, reorientation as well as electrical charge leakage from the surface beside oxidation processes and thus depend on the surface treatment conditions, material properties and storage conditions.1-5 Most of all, hydrophilized polymers show hydrophobic recovery, since plasmainduced polar chemical groups are bound to motile polymer chains.6 Such effects can partly be suppressed by cross-linking of the near-surface layer or even more by deposition of a crosslinked plasma polymer film (PPF).7-9 The latter, however, is a trade-off between stability, i.e. cross-linking, and functional group density, since polar groups are terminal groups. The controlled deposition of PPFs comprising nano-scaled vertical chemical gradients from highly functional (at the outside) to highly cross-linked (towards the inside) might thus be a way out to obtain stabilized hydrophilic surfaces. Many applications actually require ultrathin, wettable, yet stable surfaces such as microfluidics, contact lenses, (bio)sensors, tissue engineering, blood filtration, textile-reinforced composites, electrodes, lithography, heat exchangers etc. The investigation of nano-scaled vertical chemical gradients has been stimulated by two observations in our laboratory: i) The deposition of very thin (
