PSI - Issue 32

Ilya A. Morozov et al. / Procedia Structural Integrity 32 (2021) 131–136 I.A. Morozov et al./ Structural Integrity Procedia 00 (2021) 000–000

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changes affecting the wettability and free surface energy. A stiffness of the modified layer is usually greater than the initial polymer; in certain cases surface loses stability becoming wrinkled (Chung et al. (2011)). Close attention should be paid to strain behavior of such coatings under operating loads. The gradient of mechanical properties at the soft-substrate/stiff-layer interface can lead to cracking / delamination of the coating (Tsubone et al. (2007), Morozov et al. (2018)). Such defects may reduce the positive impact of plasma treatment (Morozov et al. (2019)) making the practical usage of these materials questionable. Polyurethanes have two-phase structure: hard domains are distributed in a softer matrix. Their treatment in low energy gaseous plasma (of argon, nitrogen, oxygen or ammonia) increases wettability and free surface energy (Wilson et al. (2003), Sanchis et al. (2008), Alves et al. (2011)) making the materials promising in the development of biomedical products. The most pronounced effect is reached with the plasma of argon or nitrogen (Wilson et al. (2003), Alves et al. (2011)). The two-phase structure of the polyurethane is hidden by a surface nanolayer of the soft phase (McLean et al. (1997)): the effect of polyurethane phase separation on the plasma-modified surface should be studied both in terms of surface and subsurface structure. Methods for analyzing the subsurface structure include microtomography and various microscopy techniques (Alekseev et al. (2014)). Atomic force microscopy (AFM) is of particular interest connecting the three-dimensional topography and physical and mechanical properties of the material surface giving certain information about the material below the surface level (Spitzner et al. (2011), Guerrero et al. (2019)). In this work, a soft elastic polyurethane with an internal fibrillar structure was treated with low-energy argon plasma. The structural-mechanical and deformation properties of the surfaces are studied, focusing on the changes of the surface and subsurface structure of the material. 2. Materials and methods Creating polyurethane . Polyurethane was made from prepolymer and crosslinking agent in a mass ratio of 100:47. The components were heated at 80  C and evacuated. The mixture was cured in a mold with a free top at 50  C for 24 h. Initial elastic modulus measured by macroscopic test in uniaxial testing machine is 6 MPa; elongation at break - 900%. Plasma treatment . Plasma chamber was evacuated to 5x10 -5 Torr. The pressure of argon was set to 2x10 -3 Torr; the accelerating voltage of the electron source – 100 V; the current of discharge – 1 A. The duration of treatment varied from 30 to 300 sec. A temperature of the surface during the experiment did not exceed 30  C. Atomic force microscopy . The materials were examined with a Ntegra Prima (NT-MDT BV, Netherlands) AFM by nanoindentation in fast and slow regimes. Probes with calibrated radius R of the tip and stiffness k of the cantilever were used. The initial surfaces as well as stretched materials were investigated. In the latter case, the materials were stretched to 50%, fixed and then scanned by AFM. In the fast indentation regime, the probe ( k = 0.5 nN/nm, R = 5 nm) presses (frequency ~1 kHz) the surface with a small load (3...4 nN) without damaging the argon-modified layer. Each point of the surface has its own interaction curve d ( z ): deflection d of the cantilever vs. the tip-sample distance z ; the interaction force is F = kd . The applied load changes the surface in a certain way (Morozov (2021)). Generally, the topography Z A at the initial tip-surface contact does not match with the topography Z B at maximal load; the relief Z B will be called subsurface. If the surface is covered with some soft nanolayer, than the difference  = Z B - Z A is the thickness of this layer. The slow indentation regime (frequency ~50 Hz) with stiff probes ( k = 4 nN/nm, R = 10 nm) was used to indent the treated surfaces (10x10 points of 5x5 μm areas) with a significant force (~150 nN). As a result, imprints remain on the plasma-treated surfaces (the surface of untreated polymer recovers elastically). Thus, the thickness of the coating was estimated the by depth of these marks (Morozov et al. (2019)). 3. Results and discussion The surface of the untreated polyurethane (Fig. 1a) is smooth and significantly differs from the subsurface (Fig. 1b). In the latter case, the stiff fibrilar structures are visible. The thickness of the outer soft layer (estimated from the value of  ) ~6 nm. The stiffness (Fig. 1c) of the fibrils is 15% higher than the surrounding matrix; according to