Research
Overview
Although amorphous polymers are used in a large variety of applications, there are still aspects of their mechanical behavior and their underlying molecular mechanisms which are, from a continuum mechanical point of view, not well understood yet. Studying phenomena such as fracture of polymers or the impact of nano-sized filler particles within composites requires nano-scale experiments that are often either prohibitively expensive or simply impossible.
Moreover, for such specific fields of applications well established numerical methods cannot provide a solution either: On the one hand, classical continuum approaches rely on a precise constitutive model that cannot fully capture molecular mechanisms. On the other hand, particle-based simulation methods are computationally too expensive and thus prohibit the examination of sufficiently large models. Instead, the advantages of both are combined in concurrent multiscale approaches such as the Capriccio method [1]: The regions in which the effects to be investigated occur, e.g. filler particles and their immediate surrounding, are modeled in full detail with Molecular Dynamics, while areas of less interest are resolved computationally efficiently by continuum mechanics solved with the Finite Element Method. Thus, the Capriccio group aims to develop a general methodology to investigate the complex effects of amorphous polymers on the molecular scale utilizing the Capriccio method. The main steps are depicted in Fig.1:
Fig. 1: The research interests of the Capriccio group
Particle-based material descriptions, either in atomistic or coarse-grained resolution, are able to reliably model the structure of polymers and thus capture effects occurring on the molecular scale.
Using these for pseudo-experiments with Molecular Dynamics allows for a continuum mechanical material characterization, i.e. the identification of elastic, viscous and plastic parts of the material behavior.
The obtained data and gained insights facilitate the choice and calibration of a continuum mechanical constitutive law which represents the material behavior examined in the preceding particle-based simulations.
The Capriccio Method is a partitioned domain multiscale method where the regions of interest are modeled with the particle-based material descriptions by means of MD while the rest is resolved with FEM using the obtained constitutive law. The two domains are coupled via a so called bridging domain where both material descriptions overlap. The regions of interest are identified based on the insights gained through the pseudo-experiments.
Polymer nanocomposites are becoming increasingly important for modern engineering solutions. The Capriccio method allows for a detailed investigation of hardly accessible phenomena on the molecular level, e.g. the forming of interphases between particles and bulk.
For many applications it is necessary to fully understand the molecular mechanisms responsible for the fracture of amorphous materials and composites. The Capriccio method shows high potential as a tool to further study and comprehend these micro effects.
[1] Pfaller S., Rahimi M., Possart G., Steinmann P., Müller-Plathe F., Böhm MC., “An Arlequin-based method to couple molecular dynamics and finite element simulations of amorphous polymers nanocomposites”, Computer Methods in Applied Mechanics and Engineering 260 (2013), p. 109-129.