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

蠕变描述了金属、混凝土或聚合物等材料的重要现象。这种现象发生在大多数暴露于永久负载和恶劣环境的应用中。这些外部影响导致机械性能的劣化、材料寿命的降低，甚至可能导致过早失效。静态蠕变被定义为当聚合物受到应力时，聚合物分子重排引起的时间依赖性的不可逆变形。动态蠕变描述了一个更严重的故障机制相比，静态蠕变，因为所施加的应力包含振动分量。振动应力加载应用的好例子是地下排水和污水管道。对静态和动态蠕变的基本理解以及预测材料的使用寿命对于开发能够长时间承受显著应力和振动的材料至关重要。等规聚丙烯（i-PP）因其低失效率和优良的上级力学性能而成为满足这些要求的理想材料。它是一种多晶型材料，显示出各种晶体变体，如α和β，以及晶体超结构，如球晶和串晶。结晶结构受加工条件和成核剂（NA）等的影响。我们已经报道了超分子成核剂对i-PP的结晶改性和超结构的影响，对韧性，拉伸强度和疲劳裂纹扩展阻力。据我们所知，微观结构对静态蠕变的影响在文献中仅在有限的范围内进行了研究。通过添加玻璃纤维、碳纳米管和粘土等填料和纤维来提高异相聚丙烯的抗静态蠕变性能是目前研究的热点。然而，蠕变阻力和微观结构（晶体改性和超结构）的相关性的基本理解是完全缺失的。因此，本文将分析不同微观结构的i-PP在三种不同应力水平和温度下的静态蠕变行为。此外，据我们所知，没有对i-PP的动态抗蠕变性进行研究。因此，研究建议的目的是建立微观结构和蠕变行为之间的结构-性能关系的i-PP成核的市售超分子成核剂。动态蠕变实验的目的是通过在静态实验的平均应力上附加叠加应力振幅来研究i-PP晶体结构的影响。

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.