From Plasticity to Entropic Recovery: Investigating Reversible Deformation Mechanisms in Thermosets through Spectroscopy and Molecular Modeling

• Applicants: Professor Dr.-Ing. Bodo Fiedler; Dr. Julian Konrad
• 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

Epoxy resins generally behave rather brittle, but microscale samples show increased ductility. Thin epoxy films (<100 µm) exhibit pronounced ductility, localized shear bands and mechanically induced necking under tensile load. The formation of shear bands was identified as a cascade of macromolecular mechanisms involving polymer chain stretching, orientation and molecular bond stretching. This interplay of structural reorganizations under tensile loading characterizes the emergence of localized network adaptations and provides a central contribution to the understanding of ductile deformation. These findings raise fundamental questions about the physical prerequisites of plastic deformation in amorphous, close-meshed polymer networks. In particular, it must be clarified whether irreversible plastic network adaptations are involved or whether reversible viscoelastic relaxation processes predominate, which could be understood under thermal activation procedures. Epoxy films of different chemical compositions and reproducible sample thickness are investigated in terms of mechanical deformation using in-situ infrared spectroscopy. The focus is on spectral shifts of aromatic groups of the polymer backbones, especially within the para-substituted phenylene groups, which show stress sensitivity at the molecular level. The resulting peak shifts correlate with macroscopically visible deformation zones, which allow conclusions regarding intermolecular interactions, local orientation and potential network relaxations. Complementary to this, Molecular Dynamics Simulations, supported by AI-based spectrum predictions, allow a direct assignment of vibrations to intra- and intermolecular structural motifs. The elastic response of the polymer networks and thermally induced recovery processes are investigated. The targeted comparison of different network architectures allows an evaluation of the role of molecular connectivity and local mobility for the observed ductility. Temperature-controlled post-treatments are used to test the extent to which mechanically induced structural changes relax due to thermal activation and whether a complete regression of the initial state can be observed. The interplay of mechanical loading, molecular reorganization and thermal recovery facilitates to differentiate between reversible network mobility and irreversible plastic restructuring. The combination of experimental observations with simulation-based structure-property correlations enables a profound understanding of those molecular mechanisms. This knowledge creates the basis for the development of structurally adaptive, energy-absorbing and reusable thermoset materials for demanding technical applications.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr.-Ing. Robert Horst Meißner