Einfluss von Morphologien, die unter Dehndeformation erzeugt werden, auf mechanische Eigenschaften von Polymermischungen
Zusammenfassung der Projektergebnisse
The research project is focused on the rheology and the morphology development in immiscible polymer blends under elongational deformation in the molten state. It was shown that elongational creep experiments are very well suitable for investigations of the morphology development in non-Newtonian polymer blends, which are of industrial interest, particularly. In creep experiments the dispersed particles are elongated under constant deformation conditions, i.e. at constant capillary numbers, contrary to the usually performed experiments at constant strain rate. Creep experiments could be used to extend the works on Newtonian materials for non-Newtonian systems which can provide new insights in relationships between elasticity of the materials and droplets deformation behavior. It was observed that the initial size of the dispersed droplets predetermines their deformation behavior. Whereas the small droplets undergo breakups during the elongation at moderate capillary numbers, the bigger particles are deformed homogeneously into highly stretched fibrils instead of flow induced breakups. Furthermore, a maximum deformability of the droplets was observed. Increasing the hydrodynamic stress applied above a certain level does not lead to a more pronounced deformation of the particles, although the affine deformation of the droplets was not reached. This behavior was found at stresses corresponding to the threefold of the critical capillary number and higher. The slip at the interface could be one of the reasons for this phenomenon. This maximum deformability was found to increase with the initial particle size. It was proved that the stress in the sample is the decisive parameter determining the morphology development after cessation of the flow, i.e. whether the fibrils break up due to Rayleigh instabilities or shape relaxation takes place and the fibrils regain their original spherical shape. Furthermore, the competition between the Rayleigh breakup and shape relaxation is influenced by the rate of a stress relaxation in the material. A blend with a compatibilizer preferentially located at the interface was prepared successfully. The incorporation of the efficient compatibilizer leads to the formation of interactions between matrix and dispersed phase, which results in pronounced increase in elongational viscosity and slower stress relaxation. Surprisingly, no pronounced differences were found between the behavior of uncompatibilized and compatibilized droplets during the elongation. However, the behavior of deformed droplets after elongation differs considerably. The presence of the compatibilizer led to a suppression of droplet breakups and the shape relaxation was preferred. It follows from the results, that not only the concentration and size of the dispersed particles but also their shape affects the mechanical properties of the blend. With increasing aspect ratio of the droplets the properties of the dispersed material influences the behavior of the whole blend. In the system studied, blends with PS matrix and LLDPE dispersed fibrils displayed markedly lower tensile modulus and higher toughness in comparison with blend containing spherical particles. During the project small-angle X-ray scattering was employed for the first time in order to get information about droplet behavior during the flow. It was found that this method provides additional valuable information about degree of anisotropy in the material which are not accessible by the commonly used electron microscopy. A new method for the quantitative evaluation of the homogeneity of the deformation during elongational experiments, which is a crucial issue in elongational rheometry, was developed within an interesting side activity of the project.
