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Significance Statement
Hydrophobic surfaces are needed for an array of applications, for instance, as water, dirt and oil repellant outfit of polymeric materials. Polyethylene is a common plastic with a range of uses. Therefore, the current study focuses on modifying polyethylene surfaces to become strongly hydrophobic. Above all, the hydrophobicity must be resistant to abrasion.
Polyethylene is an interesting polymer since it only consists of carbo-hydrogen and carbon-carbon bonds making it quite inert and challenging to modify. In case polyethylene surfaces are rendered permanently hydrophobic, then other plastics can be treated similarly. Perfluorinated alkyl chains possess high water and oil repellency making them good candidates for hydrophobic surfaces. The main aim, therefore, is to graft polyethylene with perfluorinated alkyl chains.
The desired oleophobic and hydrophobic effects can be attained by a highly water repellent perfluorinated carbon chain. This chain must be attached to the perfluorophenyl azide so that to obtain a photo reactive surface modifier. Therefore, researchers at Zurich University of Applied Sciences focused on developing strongly and permanently hydrophobic polyethylene surfaces. They chose polyethylene owing to its unique and model character. Photografting was adopted as a grafting method. Their work is now published in Applied Surface Science.
A photo reactive surface modifier consists of three segments: The azide as the reactive group, which upon activation splits off dinitrogen forming a highly reactive nitrene intermediate that is able to insert into a C-H bond of the polyethylene substrate. The aromatic moiety serves as antenna for the UV-light, activating the azide, and the perfluoroalkane is responsible for the hydrophobic effect. Synthesis of the photoreactive surface modifiers is described. In two steps, starting from commercially available compounds, the modifiers were prepared in high yield and purity. The synthesis is simple, efficient and clean. The preparation of the surface modifiers can easily be scaled up.
Steady-state photo physical investigations showed that the photoreactive surface modifiers possess a strong absorbance in the UV at 266 nm wavelength, and some of them crystallize when brought onto a quartz surface.
The authors were able to come with four fluorinated compounds that were characterized to serve as photo reactive surface modifiers when photografted to polyethylene. The four compounds were photografted to polyethylene. They also determined the water contact angle as well as the surface’s fluorine concentration on the grafted polyethylene and was found to correlate linearly for low coverage.
The researchers performed abrasion and solubility tests on the photografted polyethylene. Washing with acetone exhibited ambiguous outcomes. The surface concentration of the grafted surface modifier decreased with time and didn’t reach a steady state. Abrasion results indicated sufficient abrasion resistance of all the grafted surface modifiers when compared to results of surfaces coated with long chain perfluoroalkene. An abrasion model was developed. This study found that the larger the photo grafted surface modifier, the higher the abrasion resistance.
Reference
Konstantin Siegmann, Jan Inauen, Diego Villamaina, Martin Winkler. Photografting of perfluoroalkanes onto polyethylene surfaces via azide/nitrene chemistry. Applied Surface Science, volume 396 (2017), pages 672–680.
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