# Impact of Nitric Oxide on the Surface Properties of Selected Polymers
## 3 Results and Discussion
### Zeta-Potential and Dynamic Contact-Angle Measurements  
The dynamic contact-angle measurements showed no significant changes for PMP (Figure 6a). Meanwhile, measurements with MABS and PU showed substantial decreases in advancing and receding angles. The advancing angle decreased from 103 to 74 degrees for MABS, while the receding angle decreased from 64 to below 10 degrees. The contact-angle hysteresis \(\theta_{A}-\theta_{R}\) increased from 39 to 64 degrees (Figure 6b). For PU, the advancing angle decreased from 106 to 68 degrees, and the receding angle decreased from 49 to 15 degrees. The contact-angle hysteresis showed a minor decline from 57 to 53 degrees (Figure 6c). The decrease in the contact-angles indicated an increase in hydrophilicity of the surfaces of both MABS and PU caused by the formation of the nitrate ester and nitro groups and the adsorption of the HNO\({}_{3}\) groups (Figure 6). The adsorption of HNO\({}_{3}\) was likely the major contributor to the changes in contact-angle, given the more polar nature of the HNO\({}_{3}\) molecules compared to the nitro and nitrate ester functional groups. This attribution was further supported by the streaming potential results obtained for PU (Figure 7c), which are discussed in the paragraphs following Figure 7. There are several known sources of contact-angle hysteresis. The important factors in this case are swelling, surface roughness, and chemical heterogeneity [29; 47; 48; 49]. Swelling and surface roughness could be neglected when comparing untreated and treated samples, as these factors were roughly constant between samples. Therefore, the most likely influencing factor was chemical heterogeneity. MABS is a copolymer consisting of four monomers (methyl methacrylate, acrylonitrile, butadiene, and styrene) (Figure 1b). As discussed in Section 3.1, the likely attack points for both NO\({}_{2}\) and HNO\({}_{3}\) were the double bonds of the butadiene monomers, the phenyl group of the styrene monomers, and the ester groups of methyl methacrylate monomers. No reactions occurred with acrylonitrile monomers due to their lack of suitable functional groups. This meant that nitro and nitrate ester groups only developed on three of the four monomers, increasing chemical heterogeneity as these functional groups were not formed evenly along the MABS molecules. The HNO\({}_{3}\) molecules likely preferentially adsorbed close to monomers with polar functional groups such as methyl methacrylate, again increasing chemical heterogeneity. With PU, the possible interaction sites were  
Figure 6: Dynamic contact-angle measurements of both the advancing contact-angle \(\theta_{\rm A}\) (gray) and the receding contact-angle \(\theta_{\rm R}\) (red) of (**a**) PMP, (**b**) MABS, and (**c**) PU. Each material was measured in its untreated state and after treatment with NO (1000 ppm) for 17 h.  
the hydroxy groups of trimethyllopropane and the aromatic rings of the MDI monomers (Figure 1b). The more strictly defined structure of the PU polymer, with alternating polyol and diisocyanate monomers, led to a more even distribution of the functional groups caused by the gas treatment across the PU molecules, leading to no minor increase in chemical heterogeneity. The same was true for the adsorption of HNO\({}_{3}\), which, for the same reasons, also likely occurred more uniformly across the surface of the PU samples. In contrast to MABS and PU, PMP possessed no functional groups or structural features that could interact with the treatment gas or facilitate the adsorption of HNO\({}_{3}\) (Figure 1b). This was confirmed by the FTIR-ATR results in Figure 3 and the previous publication [10]. This also meant that the surface concentration of HNO\({}_{3}\) was too low to affect the contact-angles of the PMP surface. Both sample materials reached the IEP at a pH of approximately three (Figure 7a,b). For PU, the zeta-potential measurements showed significant differences between the treated and untreated samples (Figure 7c).