XPS Analyses of Plasma-Treated Silicone Rubber

Abstract

Silicone polymers exhibit good mechanical properties for a variety of biomedical and industrial applications. For instance, silicone rubber has been used for voice prostheses^1,2, urinary catheters³, contact lens material⁴, and icing coating material^s. However, their inherently high hydrophobicity limits certain applications of this material despite its favorable mechanical properties⁶. Plasma treatment of silicone polymers may affect their hydrophobicity and therewith their boundability to other materials without affecting the bulk properties. Plasma treatment often involves progressive oxidation of the surface and cross-linking of surface molecular groups which inhibits migration of low molecular weight oligomers to the surface. Various gases have been used to modify silicone polymers by plasma treatment, such as oxygen^6–8, helium^6,9, ammonia⁷, carbon dioxide⁷, nitrogen^6,10 and argon^6,7,10. Frequently a thin cross-linked, sometimes water washable, silica-like surface layer was produced by plasma treatment, but there is no consensus about the nature of the chemical groups produced at the outermost surface. The surface hydrophilicity created by plasma treatment is often lost over time ^6–8,11,12. This so-called hydrophobic recovery can be influenced by the storage conditions, whether in air or in liquid, temperature or subsequent adsorption of a surfactant¹³.