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

• Applicants: Professor Dr.-Ing. Bodo Fiedler; Professor Dr.-Ing. Robert Horst Meißner
• Subject Area: Computer-Aided Design of Materials and Simulation of Materials Behaviour from Atomic to Microscopic Scale
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 525597740

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.

• DFG Programme: Research Grants