Plasma grafting — a method to obtain monofunctional surfaces

Abstract

Tailor-made surfaces are needed for many applications concerning medical equipment or life-science polymer material. The polymers should only reveal one type of functionality (carboxyl groups), homogeneously distributed with a defined density over their total surface. Many attempts have been made to obtain those monofunctional surfaces on polymers via plasma treatments. The commonly used plasma treatment (with oxygen) results in a host of different functionalities often with a low stability. The latter is caused by damaging processes also occuring during the plasma treatment (fragmentation by charged particle bombardment or radiation damage). Therefore, it is desirable to minimize or, if possible, to completely avoid these effects. In principle, two kinds of strategies are used: first, minimising the applied energy (e.g. use of low power or pulsed powered plasmas) and minimising the kind and density of damaging particles during treatment in the plasma; second, separating substrate functionalisation from plasma in space (down stream) or in time (grafting). Both methods lead to a more homogenous distribution of functionalities and a better retention of the precursor structure. The aim of this contribution is to give a comparison of several approaches to produce homogeneously finished polymer surfaces. Attention is focused on plasma grafting. As substrate, polypropylene is chosen because of its widespread applications and its simple chemical composition, which makes observation of grafting yields easy to analyse. As functional groups, hydroxyl, carboxyl and epoxy groups are chosen. These are introduced by direct plasma treatment and also by grafting methods. Functional groups are observed by ESCA and IR measurements. The influence of the gases used to activate the polymer surface for grafting carboxyl groups is also discussed.