PSI - Issue 40

I.A. Morozov et al. / Procedia Structural Integrity 40 (2022) 314–320

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I.A. Morozov et al. / Structural Integrity Procedia 00 (2022) 000 – 000

1. Introduction Plasma treatment of polymeric surfaces is a promising technique for improving the functional properties of materials, in particular biomedical characteristics. The modification of surfaces of soft polymers is of interest; the usage of the same treatment conditions as for stiff materials is often not applicable due to strong heating, buckling or cracking the surface. The stiffness of the obtained coating can significantly exceed the polymeric substrate. On the one hand, it leads to the loss of stability of the modified surface and the formation of a folded structure (Tsougeni et al. (2007)). On the other hand, mechanical deformations of the substrate damage the treated surface: defects occur (Tsubone et al. (2007), Morozov et al. (2018)) that are not acceptable for real material use. Low-energy treatment in gaseous plasma (Ozdemir et al. (2002), Wilson et al. (2003), Alves et al. (2011)) can be a solution of this problem. In particular, plasma chemical carbon deposition is of interest (Michelmore et al. (2015)). Depending on the choice of carbon-containing reaction gas, treatment regime and type of substrate, a variety of carbon structures are grown on the surface: diamond-like carbon films (Cuong et al. (2003)), fluorocarbons (Barz et al. (2019)), nanotubes, nanofibers, nanodiamonds (Carvalho et al. (2015)), etc. At the initial stage of coating growth, the nuclei (grains, islet structures) of forthcoming uniform coating are formed on the surface. This stage is of the greatest interest, as it potentially allows obtaining strain-resistant (heterogeneous) coatings with special properties. The structure of polymer is rearranged under the influence of plasma. This leads to changes in the free energy and wettability of the surface, which, in turn, affects the biocompatibility of the materials (López - García (2019)) . Polyurethanes are widespread polymers; their physical and mechanical properties are determined by the peculiarities of manufacturing. Polyurethane is a two-phase system consisting of hard domains in a softer matrix (Kojio et al. (2009)). The study of changes in the local properties of such a polymer after the plasma treatment is of undoubted interest. The aim of this work was to study the changes in the surface properties of polyurethane under the action of argon/acetylene plasma; as a result, carbon-containing nanostructures are formed on the surface (Carvalho et al. (2015)). The materials were investigated both in an undeformed state and after application of fatigue loading. The results are discussed in terms of local features of the structural-mechanical properties of the surface. Polyurethane was made from commercially available prepolymer (urethane prepolymer based on simple polyester and toluene diisocyanate) and crosslinking agent (hardener MOCA – 13.2%, plasticizer polyfurite – 84.7%) in a mass ratio of 100:47. The components were heated at 80 ℃ and evacuated. The mixture was cured in a mold with a free top surface at 40 ℃ for 24 hours. The initial elastic modulus measured by uniaxial mechanical test was 8 MPa, elongation at break – 900 %. 2.2. Plasma treatment The internal two-phase structure of polyurethane is covered by a nanolayer of soft fraction (Morozov (2021)). Treatment of such a surface with low-energy plasma will affect only this surface layer. In this regard, the surface was pretreated (activated) in argon plasma for 30 s: studies have shown that in this case the etching of the upper surface layer occurs. The surface prepared in this way was treated in argon and acetylene plasma. The whole treatment cycle consisted in the following: the chamber was evacuated to a pressure of 5*10 -5 Torr. Argon pressure of 2*10 -3 Torr was set and a glow discharge was ignited in a low-energy electron beam source. The accelerating voltage of electron source was set to 100 V and the discharge current of electron beam was set to 1 A. After treatment in Ar plasma, C 2 H 2 was pumped into the working chamber (the flow rate was 1 cm 3 /min). Samples were exposed in Ar/C 2 H 2 plasma for 30, 60 or 120 s. 2. Materials and methods 2.1. Creating polyurethane