ABSTRACT
We discuss plasma surface modifications applied to perfluorinated polymers and polyolefins to achieve structural adhesive bonding or for biomedical purposes such as adhesion and proliferation of cells, and interfacial immobilization of biologically active molecules. We compare the properties of surface modifications performed in non-depositing plasma treatments with those of thin coatings produced in depositing plasma vapours (plasma polymerization), with particular emphasis on changes, on subsequent storage, to the properties and composition of the surface layers (‘ageing’). Such changes usually proceed for extended periods of time after plasma processing. Polymer surfaces treated in non-depositing plasmas generally are unstable, showing an increase in the air/water contact angles over days and weeks due to surface reorientation motions. Concurrently, the composition of the surface layers is also affected by post-plasma chemical reactions: originating from trapped radicals, oxidative chain reactions lead to the production of substantial amounts of oxygen-containing groups. These reactions also convert some of the groups originally incorporated into the surface layers by the plasma treatment; for instance, amine groups are converted to amide groups as evidenced by shifts in the XPS N 1s binding energy. Plasma polymer coatings analogously undergo oxidative compositional changes with time, and are capable of some surface reorganization. Thus, the nature and densities of the chemical groups on plasma-treated surfaces and plasma polymer coatings can change considerably with time. The relative contributions by concurrent reorientation motions and oxidative reactions to the compositional changes vary markedly between different plasma-prepared surfaces, but usually both processes contribute to the ageing of a surface. The generally long time constants of the reorientation of plasma polymer surfaces suggest that their limited, slow mobility may be neglected when interpreting interactions with adsorbing proteins.
