## 3 Results and Discussion  
The surface of polyurethane is naturally covered by a relatively smooth, soft (elastic modulus--4 MPa) layer that is 4-6 nm thick, hiding the internal heterogeneous structure of the polymer (Figure 1a); stiff fibrils (elastic modulus--11 MPa) are embedded in a softer matrix. Figure 1: Surface and subsurface of untreated (**a**) and 30 s treated in argon plasma (**b**) polyurethane. The insets show maps of the elastic modulus of the subsurface structure. The lighter areas correspond to the stiff objects. The elastic moduli of the typical soft and stiff areas of the surface are marked. Treatment in argon for 30 s modifies the surface, removing the upper soft layer. The transition between the surface and the subsurface disappears (Figure 1b) and a mesh-like surface nanostructure is formed (see inset in Figure 1b). The stiffness of the surface increases three-fold, becoming more homogeneous; the elastic moduli of the soft and hard areas (see markers in Figure 1) are close. The contrast of structural-mechanical properties is relevant to the untreated polymer; stiffer elevations (agglomerates of the hard fibrils) surrounded by softer depressions (soft areas between hard blocks of the polyurethane) are visible. The surface changed insignificantly after a short treatment in Ar/C\({}_{2}\)H\({}_{2}\) plasma (Figure 2a); the cellular nanostructure is preserved, but the surface stiffness becomes higher and more heterogeneous. Longer treatment times lead to the formation of a coating with structural and mechanical inhomogeneities (Figure 2b,c), where mesh-like areas alternate with smooth areas. The mechanical uniformity of the coating grows; the stiffness of the soft areas increases and approaches the elastic modulus of the stiff areas. The mismatch between surface and substrate stiffness is the reason for the folded topography (see insets in Figure 2), due to the loss of stability of the hard layer on the softer substrate. Argon post-treatment (Figure 2d-f) affects the surface roughness, partially destructs the mesh-like structures, and slightly increases the elastic modulus of the surface. Figure 2: Surface topography and elastic modulus maps of polyurethane treated in Ar/C\({}_{2}\)H\({}_{2}\) plasma for 30 s (**a**), 60 s (**b**), 120 s (**c**) and after argon post-treatment (**d–f**). The average thickness of the obtained coatings (Figure 3a) grows fast during 30-60 s of treatment in Ar/C\({}_{2}\)H\({}_{2}\) plasma; this process slows down in the interval of 60-120 s, which is explained by the densification and homogenization of the coating. Subsequent argon treatment leads to leveling of the coating thickness to \(\sim\)3.5 nm. Significant scattering of the thickness values is associated with the heterogeneity of the coating at the initial stage of formation, which, in turn, is due to the complex structure of the polyurethane. The contrast of the elastic modulus of Ar/C\({}_{2}\)H\({}_{2}\)-modified surfaces is related both to inhomogeneities of the polymeric substrate and to the non-uniformity of the coating at the initial stage of its formation. The highest scattering of local stiffness is achieved after 60 s of Ar/C\({}_{2}\)H\({}_{2}\) treatment (Figure 3b). The argon post-treatment smooths the mechanical properties of the surface; the ratio of hard/soft modulus becomes less pronounced. Note that the modulus of the initially softer areas grows fast, and the modulus of the initially stiff areas increases insignificantly. In comparison with pristine polymer, plasma treatment increases wettability (Figure 4a) and free surface energy (Figure 4b). The values for Ar/C\({}_{2}\)H\({}_{2}\) are slightly worse than after initial argon activation, but the post-treatment by argon leads to a significant decrease in the water contact angle (and rise in free surface energy) for the materials with 30-60 s of Ar/C\({}_{2}\)H\({}_{2}\) treatment. The increase in free surface energy is due to the dispersion component of the energy (Figure 4b), which is determined by the action of van der Waals interactions.