Crosslinked polydimethylsiloxane exposed to oxygen plasma studied by neutron reflectometry and other surface specific techniques H. Hillborg a,b , J.F. Ankner c , U.W. Gedde a, * , G.D. Smith d , H.K. Yasuda e , K. Wikstro ¨m a a Department of Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden b ABB Corporate Research, Department of High Voltage Engineering, SE-721 78 Va ¨stera ˚s, Sweden c Reactor Research Facility, University of Missouri-Columbia, Columbia, MO 65211, USA d Department of Materials Science and Engineering and Department of Chemical and Fuels Engineering, University of Utah, Salt Lake City, UT 84 112, USA e Center for Surface Science and Plasma Technology and Department of Chemical Engineering, University of Missouri-Columbia, Columbia, MO 65211, USA Received 16 August 1999; received in revised form 9 December 1999; accepted 10 January 2000 Abstract Spin-coated specimens of crosslinked polydimethylsiloxane (PDMS) exposed to radio-frequency (RF) and microwave (MW) oxygen plasma were studied by specular neutron reflectometry, X-ray photoelectron spectroscopy (XPS), Wilhelmy balance, contact angle measure- ments, scanning electron microscopy and atomic force microscopy. Neutron reflectometry and XPS showed that the oxygen plasma led to the formation of a smooth (10 nm), oxidised surface layer with a thickness of 130–160 nm. The oxidised layer contained a mixture of the original polymer and silicon bonded to three or four oxygen atoms (SiO x ). The oxidised layer was thinner after longer plasma exposure, indicating a decrease in specific volume due to a conversion of the polymer structure to an inorganic SiO x -rich structure. The formation of the SiO x -containing layer with low segmental mobility was further confirmed by the small hysteresis in the Wilhelmy balance measurements. The similarity in the hydrophobicity recovery kinetics of specimens aged in dry air, dry argon and vacuum and XPS data showed that the hydrophobicity recovery is not due to contamination through adsorption from the atmosphere but due to migration of low molar mass PDMS species to the surface. Scanning electron microscopy also showed the presence of surface cracks in heavily oxidised specimens.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Oxygen plasma; Polydimethylsiloxane; Surface properties 1. Introduction Silicone rubbers are used as housing materials in outdoor high-voltage composite insulations, replacing the traditional porcelain insulation [1,2]. The polymer used in these rubbers is mostly polydimethylsiloxane (PDMS). The hydrophobicity of silicone rubbers provides a high surface resistivity and water repellence even in the presence of moisture and contaminants. The initially hydrophobic surface becomes progressively more hydrophilic after extensive exposure to electrical discharges [1,2] and it then loses its water repellence. This leads to wetting of the PDMS surface and trails of water can eventually lead to breakdown of the insulation. Exposure to oxygen plasma also causes a loss of hydrophobicity of PDMS [1,3] and this can be used as an accelerated method to simulate ageing. The original hydrophobicity is regained after several hours in the absence of electrical activity [1,2,4–8]. The ability to recover hydrophobicity is a very important property of silicone rubbers. Because of this much work has been done to reveal the underlying mechanisms. It has been suggested that external contamination of the polymer surface, transport of low molar mass PDMS species from the bulk to the surface, chain scission, and reorientation of polar species from the surface into the bulk are the respon- sible mechanisms [3,7–9]. Exposure of PDMS to corona discharges or oxygen plasma leads not only to oxidation and chain scission but also to crosslinking and the formation of an inorganic silica-like surface layer [3,10], which retards the recovery of hydrophobicity [11]. This oxidised layer consists partly of SiO x , i.e. silicon bonded to three or four oxygen atoms. This paper presents data obtained by specular neutron reflectometry together with a range of other surface assess- ment methods on oxygen-plasma-exposed silicone rubber specimens prepared by spin coating. Neutron reflectometry is a powerful tool for the investigation of the surface Polymer 41 (2000) 6851–6863 0032-3861/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(00)00039-2 * Corresponding author. Tel.: + 46-8-7907640; fax: + 46-8-7906946. E-mail address: gedde@polymer.kth.se (U.W. Gedde).