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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Rare isolated cracks, also filled with a soft fraction, were observed on the stretched surface treated of up to 60 s (Fig. 4b). Cracks were not observed on the stretched materials with a longer treatment (Fig. 4c, d); in these cases the wrinkles oriented along the axis of deformation are formed – the result of compression of the material in the transverse to strain direction.

Fig. 4. AFM-images of stretched by 50% materials with plasma treatment for 30 s (a), 60 s (b), 120 s (c) and 300 (d). The surface (II) and subsurface (I) structures are shown for the cases (a) and (b). The insets give a representative subsurface structure. The axis of deformation is horizontal. Strain-induced cracking at short treatment times is associated with a sharp gradient of mechanical properties on the inner boundary of the modified layer and unchanged polymer. Longer treatments provide smooth transition from the stiff coating to the internal structure of the polymer, making the surface less sensitive to the fracture. 4. Conclusions The internal structure of the untreated polyurethane is a framework of stiff fibrils percolating into the soft matrix; it is hidden under the soft nanolayer. The plasma-modified surface is divided into outer softer layer and an underlying stiffer layer. The first one is caused by gradual altering of the soft phase of the polyurethane. The treatment does not completely destroy the softer layer due to evaporation and the deposition of volatile compounds back to the surface. The stiff internal layer is formed in the modified internal fibrilar structure of the polymer, which degrades, disappearing completely as treatment time increases. The overall thickness and stiffness of the modified layer stop changing after 120 s of treatment. Uniaxial deformation produce cracks on the surfaces treated for no more than 30 s. The defects have irregular shape which is related to the structure of the hard domains of the original polymer. Protrusions were observed on the surface of the cracks – the portions of the soft phase squeezed to the surface. That is, short plasma treatment affects only the upper part of the soft layer of the polyurethane without smooth gradient between the modified and unmodified subsurface structure. Materials with longer treatment times in the stretched state have no cracks. The coatings have improved wettability and increased free surface energy (not shown here). This makes the materials promising for further study as deformable biomedical products. Acknowledgements The work is supported by Russian Science Foundation, Grant number 17-79-20042. References

Alekseev A., Efimov A., Loos J., Matsko N., Syurik J., 2014. Three-dimensional imaging of polymer materials by scanning probe tomography. Eur. Polym. J. 52, 154–165.