The average information depth of the present XPS measurement is limited to approximately 8 to 10 atomic surface layers. One can see that with ongoing deposition, the concentration of silver increases, while the fluorine content decreases and becomes undetectable on the sample sputtered for 200 s. The decrease is due to the increasing masking effect of the growing Ag layer which at last becomes homogeneous and continuous. On the other hand, with decreasing thickness of Ag layer, its masking effect gradually declines, e.g., because of the appearance of cracks and discontinuities in the layer, and the chemical structure of the underlying PTFE becomes
visible in the XPS spectra. For the sputtering
time of 20 s, the measured fluorine concentration of 37.3 at.% FK506 is close to that of the pristine PTFE. The F/C ratio of silver-sputtered samples is markedly different from that of the pristine PTFE (F/C = 2:1) CP-690550 and may be due to the ability of silver to attract hydrocarbon contaminants from ambient atmosphere . The thicker the sputtered layer, the lower the F/C ratio. This effect is most pronounced in the case of the thickest Ag layer (200-s sputtering time), where fluorine is not detected because of the masking effect of the silver coating. However, the concentration of carbon is still notable (54 at.%) in this case. The origin of carbon may completely be attributed to the contamination with hydrocarbons and other carbon-rich compounds from ambient atmosphere. XPS data (Table 1) also elucidate the processes in the course of the sample relaxation. During the 14 days of relaxation, the surface chemical composition changes significantly. A gradual decrease of the detected silver content, compared to that of the as-sputtered samples, occurs as a consequence of the tendency to minimize surface energy at the metal-polymer interface. This phenomenon has been frequently observed especially in the case of plasma-treated polymers,
where oxygen-containing groups reorient towards polymer volume in order to reduce surface energy in the contact with Nintedanib (BIBF 1120) ambient atmosphere . Thus, the relaxation leads to segregation on the metal-polymer interface and boarding of cracks in the silver coating (Table 1, increase of fluorine content). This process favorably affects the surface wettability which finally stabilizes at a constant level (Figure 1). However, there are other concurrent processes that make the simple and straightforward explanation of the observed phenomena difficult (e.g., anomalous decrease of fluorine content for deposition time of 20 s, Table 1). This may particularly be caused by random, uncontrollable adsorption of hydrocarbons from ambient atmosphere during the relaxation process (see decrease of oxygen content at 100 and 200 s deposition times, Table 1).