# Impact of Nitric Oxide on the Surface Properties of Selected Polymers
## 3 Results and Discussion
### FTIR-ATR Stability Measurements
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 3443 (black), 1630 (green), and 1549 cm\({}^{-1}\) (purple) plotted against the immersion time in Millipore water and (**f**) physiological sodium chloride solution. These reactions leave both NO\({}_{2}\) and HNO\({}_{3}\) as the primary reactive compounds. The signals caused by HNO\({}_{3}\) were likely caused by excess HNO\({}_{3}\) molecules that did not participate in other reactions. Firstly, nitro groups can be formed through electrophilic addition of NO\({}_{2}\) to C-C double bonds, leading to nitro and nitrite ester groups forming in a 3:1 ratio (Scheme S1) [36; 37]. This reaction could have occurred at the double bonds contained in the butadiene monomers of MABS and unreacted monomers of PMP based on the structures shown in Figure 2. Chain scission and crosslinking reactions are also possible pathways. Here, NO\({}_{2}\) abstracts a hydrogen atom from the polymer chain, causing the formation of a radical site. The radical site can then react with NO\({}_{2}\) through a side reaction, forming nitro groups, as seen in the final equation of Scheme S2. Points of attack were likely to be the carbamate groups of PU (Figure 1b) [38; 39]. HNO\({}_{3}\) can lead to the formation of nitro groups through electrophilic aromatic nitration, where a hydrogen atom bound to an aromatic ring is substituted with a nitro group through a reaction with an intermediary  
Figure 4: Results of the FTIR-ATR measurements of PU. 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 to the sample directly after the NO treatment (red). Figure 5: Schematic overview of the different mechanisms of interaction of NO with the polymer surfaces. A version of this figure was first published in the _Journal of Polymer Research_, 31 (261), 2024, by Springer Nature, as part of the Graphical Abstract [10]. nitronium ion (Scheme S3) [40]. This reaction would have been possible at the aromatic rings of the styrene monomers of MABS or the MDI monomers of PU (Figure 1b). Nitrate ester groups can be formed through a secondary reaction of the electrophilic addition reaction. Here, the nitrite ester group decomposes, leaving an oxygen radical site with which NO2 reacts to form the nitrate ester group (Scheme S4) [36; 41]. The reaction of HNO3 with hydroxy groups is also a possible mechanism. An intermediary nitronium ion binds to the oxygen atom of a hydroxy group, leading to the separation of the hydrogen and, subsequently, the formation of a nitrate ester group (Scheme S5) [42; 43]. Potential attack points would have been the hydroxy groups of the trimethylolpropane monomers of PU (Figure 1b). This reaction could also occur as a follow-up reaction to an acid-catalyzed ester hydrolysis (Scheme S6). A potential attack point would have been the ester group of MABS. The formation of the functional groups discussed here was confirmed with both FTIR-ATR and XPS measurements in the previous publication [10]. PMP showed only a minor impact of NO (Figure 2). The small signals observable in the FTIR-ATR spectra disappeared quickly upon liquid contact (Figure 2c-f). PMP lacks any structural features mentioned in the discussion of the reaction mechanisms above. The polymer molecule consists of only carbon-carbon single bonds (Figure 1b), so the small signals visible in the FTIR-ATR spectra were most likely due to residual monomers in the samples. MABS and PU showed clearly visible signals in the FTIR-ATR spectra, with much higher intensities than PMP.