Fracture of polymer nanocomposites and adhesives: current challenges for molecular simulations

Molecular dynamics (MD) simulations have developed to an astonishing degree over the last decades, becoming a central pillar for chemistry, molecular biology and materials science. In this talk, while summarizing my group’s efforts in the application of MD to polymer nanocomposites and adhesives, I will offer my views on current challenges for simulations of their fracture. Overcoming them will require both new ideas and algorithms and closer integration with experiments. Specifically: (1) Starting models. These materials can have complex morphologies and molecular structures that are “frozen” by cross-linking and grafting reactions. We need to make better use of often limited experimental data in the construction of molecular models. (2) Force fields. To account for fracture, these have to be “reactive”, allowing bond breaking and formation. Atomistic force fields are computationally demanding, and not always accurate. (3) Length- and time- scales. Those accessible to MD are still much smaller than those of interest for the fracture of soft materials. Coarse-grained force fields could be used, but their applicability to realistic, polymer-specific models of reactive systems remains largely unexplored. (4) Validation. These should be primarily against experiments, but also through comparison with more accurate simulations on model systems. While macroscopic mechanical properties such as elastic moduli and fracture toughness are obviously important, real-time, in-situ imaging data would be extremely beneficial. (5) Reproducibility. In addition to more sharing of raw simulation data, it would be important to plan community efforts whereby different groups undertake independent simulations of the same systems and compare the results.