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ePaper created 2017-03-14, 10:25:37 | version 1.49.03
CHEMISTRY AND PROCESS INDUSTRY O O C O CH O HC electron impact +H substrate 1 STABLE PLASMA POLYMERS GENERATED BY RING OPENING J ako b B ar z , B e nt sian Elk in, M i cha e l H au pt , Chr is t ian O e hr Chemical surface functionalization also has a negative effect on the degree of polymerization Coating by plasma polymerization is an established and widely used process to equip surfaces with speciic properties. An ex- ample are various oil and water-repellent coatings developed at our institute. In addition to coatings where it essentially comes down to a macroscopic effect, coatings that have speciic chemical functional groups and therefore display par- ticular characteristics are also in demand. Chemical functional groups that enable the attachment of various substances to a surface include the amino, epoxide, carboxyl and thiol groups. They can be employed, for example, in the biotechnology sec- tor or in the ield of adhesive technology. Since the conventional methods are not eficient enough and can only be used to a limited extent, Fraunhofer IGB is investigating an alternative plasma process, characterized by surface functionalization by means of ring-opening plasma polymerization. Initial situation The usual way of producing a chemically functionalized plasma polymer layer is by employing a starting substance (precursor) that already contains the desired functional group. Frequently, a double bond is also included as a “break point”. Examples thereof are plasma polymers of allylamine (amino groups) or acrylic acid (carboxyl groups). The problem here is that the energy input from the plasma, which leads to the activation of the target break point, almost equals the energy threshold at which the functional group in the plasma can be destroyed. In order to retain the functional groups, the energy input must therefore be kept as low as possible. However, this and cross-linking of the generated layer, as well as on the deposition rate. Moreover, the precursors used in this way are toxic and / or unstable owing to their reactivity. Other methods for the production of functionalized plasma polymer layers, such as grafting or the use of chemically simple gases or gas mixtures, can only be used to a limited extent. Method An alternative way of generating functionalized plasma polymers is characterized by the desired chemical function not being present in the starting substance, but rather generated only after plasma has taken effect. Ring-opening polymeriza- tion by means of electron impact in plasma generates highly reactive intermediates that contain precursors for the desired chemical function. On the surfaces exposed to the plasma, they form a polymer layer which displays a high density of these functional groups. An illustrative example of this basic principle is the formation of aldehyde-functionalized layers of cyclopentanone (Fig. 1). The ring is opened (preferably at the bond adjacent to the keto group) by electron impact in the plasma. This generates a biradical, which reacts with a hydrogen atom from the plasma to form an aldehyde-containing radical intermediate and can be incorporated into the growing plasma polymer layer. In an analogous manner, plasma polymers with primary amino groups, hydroxyl groups, thiol groups and further functional layers were also produced at Fraunhofer IGB. 7 2