Polymeric surfaces have been increasingly plasma-activated to adopt adequate chemistries, enabling their use in different applications. An unavoidable surface oxygen insertion upon exposure to non-oxygen-containing plasmas was always observed and mainly attributed to in-plasma oxidation stemming from O2 impurities in plasma reactors. Therefore, this work investigates exclusive in-plasma oxidation processes occurring on polyethylene surfaces by purposely admixing different O2 concentrations to medium-pressure Ar and N2 dielectric barrier discharges (base pressure: 10-7 kPa). Hence, distinctive optical emission spectroscopy and in-situ X-ray photoelectron spectroscopy (XPS) data were carefully correlated. Pure N2 discharge triggered an unprecedented surface incorporation of large nitrogen (29%) and low oxygen (3%) amounts. A steep rise in the O-content (10%) at the expense of nitrogen (15%) was detected upon the addition of 6.2 × 10-3% of O2 to the feed gas. When the added O2 exceeded 1%, the N content was completely quenched. Around 8% of surface oxygen was detected in Ar plasma due to high-energy Ar metastables creating more surface radicals that reacted with O2 impurities. When adding only 6.2 × 10-3% of O2 to Ar, the surface O content considerably increased to 12%. Overall, in-plasma oxidation caused by O2 impurities can strikingly change the surface chemistry of N2 and Ar plasma-treated polymers.
Keywords: Ar/O2 plasma; N2/O2 plasma; OES; in-plasma oxidation; in-situ XPS; polyethylene.