Low-Pressure Plasma Treated Polyetheretherketone for Biomedical Application- A Comparative Study on Surface Chemistry and Storage Stability

The surface chemistry of different plasma-treated PEEK was characterized by contact angle measurements and X-ray photoelectron spectroscopy and the results of the different methods were compared. In addition, the physicochemical changes on the surface were documented and evaluated within the course of the aging/recovery of the surface within one week. All plasma treatments had serious effects on the aromatic basic structure of the polymer, but at the same time increase the nitrogen and oxygen content of the surface up to a maximum of 28.2 % by means of oxygen plasma. Regardless of the process gas, the surface energy increases to >63 mN/m what results in a clearly hydrophilic surface. Overall, three phases of hydrophobic recovery were observed, in two of which the degradation of functional groups was shown by a decreasing surface energy. The changes in the acid properties could be quantified by Berger’s method. The treatment in the oxygen plasma leads to an acidic surface with a Dshort value of 1.6. In nitrogen plasma, on the other hand, a slightly alkaline surface is generated with a shortened Berger parameter of -0.3. Regardless of the process gas and the initial nature of the surface, a slightly more acidic surface is formed during storage compared to the reference. The elemental composition of the plasma-treated PEEK appears to be relatively constant over the storage time, but with significant changes in the binding states. The double bonds induced during the plasma treatment are degraded and with them also the induced carboxylic acid.