## 3 Results and Discussion
### X-ray Photoelectron Spectroscopy Analysis of Nitrogen-Treated BOPP Film  
The wide scans from 0 to 1400 eV to identify the elements present at the surface of the sample are shown in Figure 11 for an untreated sample and Figure 12 for sample treated with nitrogen. For the latter sample, the narrow scans of the 1 s peaks of carbon (282-290 eV), nitrogen (395-404 eV) and oxygen (528-537 eV) are shown in Figures 13-15, respectively.  
Figure 11: X-ray photoelectron spectroscopy wide scan of untreated BOPP sample.  
Figure 10: Aging behaviour of the BOPP samples treated in nitrogen with the laboratory DBD system.  
The XPS analysis of an untreated sample yielded a composition of 100% carbon, with no evidence of nitrogen or oxygen present in the surface. For the nitrogen-treated sample, the XPS measurements indicate that both nitrogen (7.2 at %) and oxygen (3.6 at %) atoms were incorporated in the surface of the treated BOPP films. Oxygen was detected on the treated samples despite the fact that the chamber was evacuated and backfilled with the process gases, which should only have contained oxygen in extremely low traces. However, comparison with the findings of other research groups indicates that their polypropylene films treated in pure nitrogen also contained oxygen [31; 32; 42; 43]. This is usually attributed to the much higher reactivity of oxygen species in the plasma than nitrogen species. None of these publications, though, discuss the source of the incorporated oxygen. In addition to low residual background levels of oxygen in the reactor chamber, which may be entrained between the  
Figure 12: X-ray photoelectron spectroscopy wide scan of BOPP sample treated in pure nitrogen.  
Figure 13: X-ray photoelectron spectroscopy narrow scan (Carbon 1 s) of the BOPP sample treated in pure nitrogen.  
electrodes, it is assumed that the oxygen atoms also originate from water or oxygen adsorbed on the surface of the BOPP film. The relatively high amount of incorporated oxygen on the surface of the BOPP films, in comparison to the amount of incorporated nitrogen, leads to the assumption that it is mainly the excited particles, which are close to the surface of the BOPP film, that react with the polymer surface. Excited particles created further away from the surface have a lower probability of reacting with the polymer surface, because they have to overcome a longer diffusion distance in order to undertake a reaction with the surface of the polymer. If the atmosphere in the vicinity of the polymer contains the particles that mainly react with the surface of the polymer, then it is easily imaginable that substances adsorbed onto the polymer surface influence the composition of the gas in this region in greater concentrations than a few hundred ppm. The adsorbed substances thus have a greater impact on the surface treatment than the gaseous admixtures in the nitrogen. This theory would also explain why no significant differences could be measured in the surface energies of the samples treated with concentrations of up to 500 ppm of nitrous oxide, carbon dioxide and acetylene.  
Figure 14: X-ray photoelectron spectroscopy narrow scan (Nitrogen 1 s) of the BOPP sample treated in pure nitrogen (7.2 at %).  
Figure 15: X-ray photoelectron spectroscopy narrow scan (Oxygen 1 s) of the BOPP sample treated in pure nitrogen (3.6 at %).