Mechanisms of thermoset plasticity explained on the basis of spectroscopic analysis and atomistic simulations

环氧树脂的一个意想不到的特点是，在减少测试体积的情况下，延展性增加。这一点已经在不同的环氧树脂体系中以微观纤维的形式得到了证明。在机械载荷下，这些材料没有表现出典型的材料脆性破坏行为，而是具有明显颈缩的延性行为。随着试验体积的减小，延伸率和断裂伸长率增加。恒厚EP薄膜的可重复性生产具有挑战性，但另一方面它也允许用透射光红外光谱进行研究。在第一次对由环氧树脂制成的微观薄膜(d=50µm)的力学研究中，这些薄膜在载荷作用下形成剪切带并向下缩颈。到目前为止，对于这种效应在塑性方面还没有完全的解释。为此，本项目研究了环氧树脂在载荷作用下脆韧转变的物理、力化学和分子机制。选择具有可重复、恒定和可调节厚度的环氧树脂膜，以获得关于分子机制、它们的相互作用以及由此产生的宏观机械行为的信息。利用红外光谱分析，可以通过在载荷作用下的光谱变化和峰位移来获得在原位力学测试过程中分子间和分子内的作用机理的信息。分子动力学模拟可以用来推断分子的显式振动或构型。这三种方法，即使用理想化的分子动力学模拟模型对红外光谱进行AI中介的解释，并结合对薄环氧薄膜的实验，提供了理解小体积环氧化合物中塑性的分子过程的可能性。AI介导的模拟基于足够详细的从头计算，还包括非谐效应，是解释红外峰位移和可归因于特定分子相互作用的更一般光谱变化的关键，从而提供了与实验获得的环氧薄膜原位测试光谱的联系。有了这些技术诀窍，就有可能从更根本上了解基质在中间相(纤维复合材料)、薄涂层和粘合剂中的行为，并在新的技术应用中使用环氧树脂。从材料科学的角度来看，本文提出的研究可以为未来复合材料的结构设计提供新的见解。

An unexpected feature of epoxy resins is the increasing ductility with reduced test volume. This has already been demonstrated for different epoxy resin systems in the form of microscopic fibres. Under mechanical load, these did not exhibit brittle failure behaviour typical of the material, but ductile behaviour with pronounced necking. The ductility and elongation at break increased with decreasing test volume. The reproducible production of thin EP films with constant thickness is challenging, but on the other hand it allows the investigation by transmitted light infrared spectroscopy. In the first mechanical investigations of microscopic films (d = 50 µm) made of epoxy resin, these films formed shear bands under load and also necked down. So far, there is no complete explanation for this effect in terms of plasticity. For this reason, the physical, mechanochemical and molecular mechanisms of the associated brittle-ductile transition in the epoxy resin under load are investigated within this project. Epoxy resin films with reproducible, constant and adjustable thicknesses are selected to obtain information about the molecular mechanisms, their interactions and the resulting macroscopic mechanical behaviour. Using infrared spectrometry, it is possible to obtain information about inter- and intramolecular mechanisms acting during in-situ mechanical testing by spectral changes and peak shift under load. Molecular dynamics simulations can then be used to infer explicit molecular vibrations or configurations. This triad of methods, i.e. AI-mediated interpretation of IR spectra using idealised MD simulation models in combination with experiments on thin epoxy films, offers the possibility to understand molecular processes of plasticity in small volumes of epoxies. The AI-mediated simulations, which are based on sufficiently detailed ab initio calculations and also include anharmonic effects, are key to explaining IR peak shifts and more general changes in the spectra that can be attributed to specific molecular interactions, thus providing a link to experimentally obtained spectra from in situ tests on thin epoxy films. With this know-how gained, it will be possible to understand more fundamentally the behaviour of the matrix in intermediate phases (fibre composites), thin coatings and adhesives and to use epoxy in new technical applications. From a materials science perspective, the research proposed here could provide new insights into the structural design of future composites.