Effect of crystal modification and superstructure on the static and dynamic creep properties of isotactic polypropylene

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Creep describes a significant phenomenon for materials like metals, concretes or polymers. This phenomenon occurs in most of the applications where they are exposed to a permanent load and a harsh environment. These external influences result in a deterioration in mechanical properties, a decrease in life time of materials and may even lead to premature failure. Static creep is defined as the time dependent, irreversible deformation by molecular rearrangement of polymers when they are subjected to stress. Dynamic creep describes a much more severe failure mechanism compared to static creep because of the applied stress containing a vibrational component. Good examples for vibrational stress loaded applications are underground drainages and sewerage pipes. Fundamental understanding of static and dynamic creep and predicting the life time of materials is crucial to develop materials, withstanding a significant level of stress and vibrations for extended periods of time. Isotactic polypropylene (i-PP) is a promising candidate to meet the requirements due to its low failure rate and superior mechanical properties. It is a polymorphic material showing various crystal modifications, such as alpha and beta, and crystal superstructures, such as spherulite and shish-kebab. The crystal structures are influenced by processing conditions and nucleating agents (NA) etc. We already reported on the influence of the crystal modification and superstructure of i-PP, nucleated by supramolecular nucleating agents on the toughness, tensile strength and fatigue crack propagation resistance. To the best of our knowledge, the influence of microstructure on static creep has been studied only to a limited extent in literature. Many research has been focused on improving static creep resistance of i-PP by using fillers and fibers such as glass fibers, carbon nanotubes and clay. However, a fundamental understanding of the correlation of creep resistance and the microstructure (crystal modification and superstructure) is completely missing. Therefore, the static creep behavior of i-PP with different microstructures will be analyzed at three different stress levels and temperatures. Furthermore, to best of our knowledge, no research has been done on the dynamic creep resistance of i-PP. Therefore, the objective of the research proposal is to establish structure-property relationships between microstructure and creep behavior of i-PP nucleated by commercially available supramolecular nucleating agents. The aim of the dynamic creep experiments is to investigate the effect of the i-PP crystal structure by additional superposition of a stress amplitude on the mean stress taken from the static experiments.