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一种重要现象。这种现象发生在它们暴露于永久负载和恶劣环境的大多数应用程序中。这些外部影响会导致机械性能恶化，材料寿命缩短，甚至可能导致过早失效。静态蠕变被定义为受应力作用时聚合物分子重排引起的时间依赖性、不可逆变形。与静态蠕变相比，动态蠕变描述了更严重的破坏机制，因为施加的应力包含振动成分。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寿命对于开发能够承受长时间显著水平的应力和振动的材料至关重要。等规聚丙烯（i-PP）因其低故障率和优异的力学性能而成为满足要求的有希望的候选者。它是一种多晶材料，具有各种晶体修饰，如α和β，以及晶体超结构，如球晶和羊肉串。晶体结构受加工条件和成核剂（NA）等因素的影响。我们已经报道了经超分子成核剂成核的i-PP的晶体改性和上层结构对其韧性、抗拉强度和抗疲劳裂纹扩展性能的影响。据我们所知，文献中对微观结构对静态蠕变的影响的研究非常有限。利用玻璃纤维、碳纳米管和粘土等填料和纤维提高i-PP的静态抗蠕变性能是目前国内外研究的热点。然而，对蠕变抗力与微观结构（晶体改性和上层结构）之间的关系的基本理解是完全缺失的。因此，本文将分析具有不同微观结构的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，还没有对i-PP的动态抗蠕变性能进行过研究。因此，本研究计划的目标是建立经市售超分子成核剂成核的i-PP的微观结构与蠕变行为之间的结构-性能关系。动态蠕变实验的目的是通过额外叠加应力幅值来研究i-PP晶体结构对静态实验平均应力的影响。

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.