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The surfaces of soft polyurethanes (with elastic moduli of 7 and 50 MPa) treated by plasma immersion ion implantation were studied; the materials became hard and wrinkled. The characteristics of the modified surface (modulus, roughness, wave and fractal parameters, linear deformation) and the cross‐sectioned layer were studied in relation to the treatment regime and the modulus of the polymer. A hypothesis that the wrinkling was solely caused by thermal expansion was checked but rejected. The cross‐section of the treated material had a hard part of a certain thickness and a transition zone. The elastic modulus of the hard part was estimated by the finite element method. A comparison of the obtained thicknesses with calculations by TRIM software indicated that the latter gave underestimated results. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45983.

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Plasma treatment of soft polymers is a promising technique to improve biomedical properties of the materials. The response to the deformation of such materials is not yet clear. Soft elastic polyurethane treated with plasma immersion ion implantation is subjected to fatigue uniaxial loading. The influence of the strain amplitude and the plasma treatment regime on damage character is discussed. Surface defects are studied in unloaded and stretched states of the material. As a result of fatigue loading, transverse cracks (with closed overlapping edges as well as with open edges deeply propagating into the polymer) and longitudinal folds which are break and bend inward, appear on the surface. Hard edges of cracks cut the soft polymer which is squeezed from the bulk to the surface. The observed damages are related to the high stiffness of the modified surface and its transition to the polymer substrate.