ИСТИНА

Polymer films are widely used in medical applications as packing materials and membranes. The permeability and mechanical properties of films are important for both applications. To facilitate microfracture analysis and investigation of micromechanical properties an approach using atomic- force microscopy was proposed. The surface morphology of a polymer film changes during stretching. The purpose of our experiments was to obtain series of images showing a particular surface region under different strain values. Processing of such series allowed us to trace distinct microscopic defects and describe their changes during deformation. We have developed a stretching device that could be exploited with a stand-alone AFM. After scanning a particular surface region the cantilever was moved away from the surface and the film was stretched. Such cycle of measurement was repeated several times. Two types of films have been studied in order to test the proposed method: biaxially-oriented polypropylene (PP) and not-oriented plasticized polyvinyl chloride (PVC). Both of them have been stretched without necking but the stretching mechanism was different. It was found that surface roughness decreases slightly at low strain and increases at high strain. The increase of PP surface roughness is due to slip bands formation. The increase of PVC surface roughness is a consequence of defects fragmentation. The advantage of the proposed method is the possibility of visualization of crack nucleation at early stages. Considering medical applications, the method can be used to control the quality of various films.