# Impact of Nitric Oxide on the Surface Properties of Selected Polymers
## 3 Results and Discussion  
### FTIR-ATR Stability Measurements  
The NO treatment of PMP produced shallow signals, as shown in Figure 2. The analyzed signals were located at 3377, 1712, and 1642 cm\({}^{-1}\). These signals originated from the adsorption of HNO\({}_{3}\) to the polymer surface in the case of the signals 3377 and 1712 cm\({}^{-1}\), while the signal at 1642 cm\({}^{-1}\) originated from the formation of nitrate ester groups [10,25,33]. Looking at the close-ups of the spectra shown in Figure 2a-d, it was qualitatively observable that all three signals decreased to the same level of the untreated sample after 120 h of immersion in either water or NaCl solution. The signal intensities were low overall compared to MABS (Figure 3e,f), which led to significant variations when analyzing the signal heights. Treating the PMP samples with Millipore water resulted in a decrease in signal intensity for both the signal at 3377 and 1712 cm\({}^{-1}\). The signal at 1630 cm\({}^{-1}\) stayed stable (Figure 2e). The spectra of the samples immersed in physiologicalNaCl solution showed a slightly different behavior. All three signals decreased steadily throughout the treatment at relatively similar rates (Figure 2f). The NO treatment of MABS resulted in several clearly visible signals. For this investigation, the signals at 3443, 1630, and 1549 cm\({}^{-1}\) were the focus, as they were well separated from the signals caused by the polymer. These signals corresponded to adsorbed HNO\({}_{3}\) in the case of 3443 cm\({}^{-1}\), nitrate ester groups in the case of 1630 cm\({}^{-1}\), and nitro groups in the case of 1549 cm\({}^{-1}\)[10; 25; 33]. The samples immersed in Millipore water showed relatively constant peak heights, with only the signal at 1630 cm\({}^{-1}\) decreasing slightly from 0.15 to 0.14. The other two signals remained constant (Figure 3a,b,e). The immersion of the samples in physiological NaCl solution resulted in substantial decreases in the signals at 3443 and 1630 cm\({}^{-1}\). The signal at 3443 cm\({}^{-1}\) decreased from 0.015 to 0.003, while the signal at 1630 cm\({}^{-1}\) decreased from 0.14 to 0.10. Like the samples treated with water, the signal at 1549 cm\({}^{-1}\) remained constant (Figure 3c,d,f). Figure 2: Results of the FTIR-ATR measurements of PMP. The samples were treated with NO (1000 ppm) for 17 h and subsequently immersed in Millipore water **(a,b)** and physiological sodium chloride solution **(c,d)**. The samples were immersed for 120 (green) and 240 h (purple). The spectra are depicted in comparison to an untreated sample (black) and the sample directly after the NO treatment (red). **(e)** Peak heights at 3377 (black), 1712 (red), and 1642 cm\({}^{-1}\) (green) plotted against the immersion time in Millipore water and **(f)** physiological sodium chloride solution. No peak height analysis was performed for PU, as the signals originating from the NO treatment and the PU signals overlapped too heavily (Figure 4). The signals' development at around 3480 and 1630 cm\({}^{-1}\) was observed qualitatively. These signals originated from the adsorption of HNO\({}_{3}\) in the case of 3480 cm\({}^{-1}\) and the formation of nitrate ester groups in the case of 1630 cm\({}^{-1}\)[25; 33]. In the case of water and NaCl solution treatment, the signal at 3480 cm\({}^{-1}\) decreased to the same level as the untreated sample after only 120 h, with no additional changes. The signal at 1630 cm\({}^{-1}\) stayed stable in both cases. As discussed in the previous publication [10], various possible mechanisms for forming the nitro and nitrate ester groups and the adsorption of HNO\({}_{3}\) have been discussed in the literature. The foundation for these mechanisms is the reaction of NO with oxygen forming nitrogen dioxide (NO\({}_{2}\)) and the subsequent reaction of NO\({}_{2}\) with the moisture in the air forming HNO\({}_{3}\) (Figure 5) [34; 35]. Figure 3: Results of the FTIR-ATR measurements of MABS.