Effect of electrode geometry on the uniformity of plasma-polymerized methyl methacrylate coatings

Plasma polymerization is widely used to deposit functionalized coatings which properties can be tuned by different operational parameters. Although plasma-deposited coatings have many interesting properties, the influence of these parameters on the polymerization process is not yet well understood. In this study, plasma-polymerized methyl methacrylate (MMA) coatings are produced and carefully planned experiments are conducted to evaluate the influence of discharge power and electrode geometry on the chemical composition and thickness of the deposited films. For this purpose, polymerization processes are carried out in two plasma reactors with different electrode geometries (parallel plate versus mesh-to-plate) and the influence of discharge power is examined for both reactor types. This study reveals that the chemical composition of the deposited films is strongly influenced by the power of the discharge: at high powers, the coatings contain less ester functional groups, which is most likely due to monomer fragmentation at these elevated powers. It is also shown in this work that the reactor set-up strongly determines the uniformity of the deposited films: in the parallel plate reactor, film thickness, surface morphology and chemical composition depend on the location of the substrate in the discharge region, while in the mesh-to-plate reactor the deposited films are physically and chemically uniform. Taking into account the latter statement, the mesh-to-plate reactor might have important industrial applications.