Tuning of surface properties of thin polymer ﬁlms by electron beam treatment Sina Burkert *, Marco Kuntzsch, Cornelia Bellmann, Petra Uhlmann, Manfred Stamm Leibniz Institute of Polymer Research Dresden, Hohe Strasse 6, D-01069 Dresden, Germany 1. Introduction Surface modiﬁcation is a key instrument for the design of new products and processes. To change the properties of polymers, various techniques like plasma etching, irradiation with photons, ions, and electrons are available. A very promising variable and fast method is the latter one, which was used in this work to understand the impact of electron treatment on physico-chemical surface properties of thin polymer ﬁlms. Contrary to the common plasma modiﬁcation, the surrounding media could be freely changed and is not limited to vacuum [1]. The intensity of the modiﬁcation could also be controlled by varying the absorbed dose. During the irradiation of polymers reactive products like excited atoms or molecules, ions, as well as radicals are formed. They induce complex chemical reactions, like cross-linking, decomposition and modiﬁcations, which result in a changed structure as well as changed properties of the polymer [2]. Typical industrial applications of this technology are curing of organic coatings, cross-linking of plastics, as well as sterilization of medical devices [3]. It is known that oxygen plasma incorporates oxygen containing functional groups in a polymer surface and increases its wettability [4,5]. Dole and Pyankow showed in their work, that similar oxidation mechanisms could be achieved by gamma and electron irradiation. These effects were found only at a penetration depth of a few microns [6,7]. Therefore it is expected that thin polymer ﬁlms of less than 15 nm will be completely modiﬁed by electron beam treatment, i.e. a perpendicular modiﬁcation gradient in the layer is not expected. We used polymer brushes, which represent a special case of polymer thin ﬁlms (in this case of less than 15 nm) with a conformation of the polymer chains stretched away from the surface due to the excluded volume effect. They are a versatile tool for easily reproducible surface functionalization and predestined to create stable (covalently ﬁxed) homogeneous or adaptive smooth thin polymer ﬁlms with a roughness of less than 1 nm [8a– e,9]. The electron beam treatment of the brushes was performed at ambient conditions. Hence, reactive oxygen was expected to penetrate the whole polymer during electron irradiation. The two chemically different polymers, polystyrene (PS) and poly-2- vinlypyridine (P2VP), were chosen to form the model polymers used in this study. They are both known to be degradation resistant. They mainly cross-link during irradiation [10,11]. Thus, besides the cross-linking between the brush chains, the radiation was expected to induce strong effects on surface properties preferably over the introduction of oxygen containing functional groups by electron beam treatment of these polymers. The effectiveness and the limitations of the electron beam treatment were speciﬁed by analyzing the physico-chemical surface proper- ties for varying irradiation doses. Accurate information about the chemical changes in the polymers was achieved using X-ray spectroscopy. Detailed results will be published soon and are only Applied Surface Science 255 (2009) 6256–6261 ARTICLE INFO Article history: Received 7 January 2009 Received in revised form 26 January 2009 Accepted 29 January 2009 Available online 11 February 2009 Keywords: Polymer brushes Polystyrene Poly-2-vinylpyridine Electron irradiation Surface modiﬁcation Gradient ABSTRACT The aim of our work was to understand the impact of electron treatment on polymer thin ﬁlms, particularly on their surface properties as well as the possibilities and limitations to tune these properties. Two different polymers, polystyrene (PS) and poly-2-vinlypyridine (P2VP), were chosen to form thin polymer ﬁlms by grafting of end-terminated linear polymer chains to a surface with sufﬁcient grafting density, forming so called ‘‘polymer brushes’’. We were able to identify the surface properties and specify ongoing physico-chemical changes after electron beam treatment by using zeta potential and contact angle measurements. By varying the absorbed dose it was possible to tune the surface properties over a wide range. The detailed knowledge about the latitude of functionalization of the tested polymers was a prerequisite for the creation of wettability gradients by electron beam treatment by adapting a special mask of known thickness and density. Hence, electron beam treatment opens an easy reproducible way to generate surface gradients in functionality. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +49 351 4658 271; fax: +49 351 4658 281. E-mail address: burkert@ipfdd.de (S. Burkert). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.01.096