Sustainable Polymers

Within constitutive modeling, we identify the relations between the loadings a material is exposed to and its associated response in terms of deformation. In particular, in the context of continuum mechanics, we aim at finding appropriate mathematical formulations which link the stress in each infinitesimal point of the material to the strain, classically based on experimental findings. With such constitutive models at hand, simulations of complex geometrical setups eventually allow for predictions of the macroscopic behavior of parts. In BIO ART, we focus on bio-sourced epoxies and analyze their elastic, viscous, and plastic mechanical properties, both generic and material-specific. Due to the complex material behavior under various, e.g., mechanical and thermal, loadings, we employ and further develop advanced viscoelastic-viscoplastic models and obtain their material parameters typically by inverse parameter identification using optimization techniques. Among others, we fit master curves from dynamic mechanical thermal analysis (DMTA) and use time-temperature superposition to determine essential material parameters for accurate representation of the material’s response, including creep and recovery behaviors. This process also incorporates refinements through iterative comparisons with experimental results, often highlighting areas for further investigation and model enhancement. Our approach not only aids in creating eco-friendly materials that compete with their petrochemical counterparts, but also lays the groundwork for broader applications, such as network model training in polymers. The pursuit of a deeper understanding of temperature-dependent behaviors and the detailed examination of phenomena like hardening effects are central to refining and validating the models, ultimately leading to the development of versatile, high-performance materials.

In contrast to continuum mechanics, a molecular description considers matter as discontinuously distributed in space. It typically uses an atomistic or molecular resolution, which is always a compromise between accuracy and computational cost: In case of a full-atom resolution, the chemical setup is reproduced excatly in terms of the individual atoms involved, whereas a molecular viewpoint requires coarse-graining techniques in order to reduce the degrees of freedom by subsuming groups of atoms into larger units, frequently referred to as superatoms. In view of fracture simulations, a purely atomistic description seems to be computationally prohibitive, since a minimum amount of materials is necessary to adequately capture the process zone in the vicinity of the crack tip. Typically, the process zone size of thermosets is in the range of micrometers, which would require hundreds of millions or billions of individual atoms. Here, coarse-graining techniques come into play which provide a compromise between chemical specificity and feasible system sizes. In our studies, we consider different levels of coarse graining, ranging from superatoms representing the repeat units of the polymer chains to those reflecting cross-linking points of the polymer network.

In our pursuit to explore the fracture behavior of bio-based epoxies, we employ a multiscale simulation approach that synergizes continuum and molecular description together with validation against experimental evidence. Here, the Capriccio method serves as our primary tool, facilitating the coupling of finite element (FE) and molecular dynamics (MD) domains within a unified computational framework. This method leverages the strengths of both continuum and particle-based approaches, allowing us to capture crack initiation and propagation on the molecular level. By doing so, we can consider the microscopic processes relevant for fracture and failure with chemical specificity, but are still able to provide realistic boundary conditions by the surrounding FE setup. By integrating experimentally-derived parameters into our models, we aim to accurately represent the mechanical responses and fracture characteristics of bio-based epoxy materials under a variety of conditions. The molecular frameworks offer a detailed view of molecular interactions and structural changes during fracture, thus providing insights that are often inaccessible through purely continuum approaches. By employing the Capriccio method’s bridging domain concept, we ensure a seamless exchange of forces and displacements between the FE and MD regions, allowing for a comprehensive investigation of fracture propagation. The method’s staggered solution scheme facilitates this integration, enabling coordinated updates between the domains within a coupling region. Our focus on bio-based epoxies is driven by their potential as sustainable alternatives to petrochemical epoxies; however, understanding their fracture behavior at multiple scales is essential for advancing their applications in demanding mechanical environments. By refining model parameters through systematic comparisons with experimental data, we aim to achieve a cohesive model that not only predicts fracture behavior accurately but might also guide the design and enhancement of new bio-based polymer composites.